A Criteria- and Case Study-Based Approach to Evaluate Adaptability in Buildings

Buildings and urban environments that are occupant-oriented, recyclable, and adaptable to changing demands are essential for a sustainable future. A thorough examination of the rate and scope of change, as well as an effective assessment of the existing building stock based on alternate uses, facilitate the design of adaptable buildings and urban environments. The need for spatial change is caused by a variety of internal and external factors. Internal factors are defined as the building's inability to meet the initially defined requirements or a decrease in the building’s capacity to meet these requirements due to aging and performance changes, even though the occupants’ requirements did not change [15,18]. External factors are defined as a building’s inability to meet changing demands and needs and increasing spatial and functional requirements [12]. Although a building’s physical life is long, once it cannot meet the demand for change and revisions, the building will inevitably be subject to obsolescence and abandonment, which is a design problem for all stakeholders, particularly property owners [19].

To prevent or delay building aging, the factors should be analyzed, and the necessary measures should be implemented on time. According to Lemer, [20] as cited in [15], technological changes affecting infrastructure services and requirements, shifts in regulatory processes, economic and social change, and changes in occupant requirements and behaviors are the factors that cause obsolescence. Graham [21] categorizes these factors based on service and value requirements. Service requirements are related to poor design of building components, lack of maintenance and repair of these components, inadequate indoor environmental quality, spatial dimensions that do not meet ergonomic conditions, alterations in regulations, and changes in occupant requirements. Value requirements, on the other hand, are the factors related to cost and financial value, such as a decrease in the building’s financial value, an increase in operation and maintenance costs, changes in aesthetic quality perceptions due to current trends, and the emergence of comparatively more valuable alternatives [21].

Building modifications for reuse, functional and physical upgrades, and other alterations are carried out by altering the building's function, capacity, performance, and operation throughout its service life. In terms of building adaptability, functional change refers to assigning diverse functions to the building rather than using it with the designated function [12].

The volumetric change of the building due to factors such as occupant and functional load is referred to as the change in building capacity [15, 19]. Performance change is carried out through refurbishment and/or rehabilitation or renovation and/or restoration [12]. Changes in occupant density and movement, furnishing/equipment use in or around the building, and climatic or physical environmental conditions may occur over time. For instance, replacing a fixed window with a retractable window may result in a change in the building's indoor environmental conditions due to the changes in occupant control. Similarly, a building expansion may necessitate additional circulation elements to improve circulation [15, 19].

Overall, an effective assessment of building change and types of change is becoming increasingly important for the adaptive reuse of existing buildings and the design of new buildings that achieve a certain level of adaptability. Hence, a building that fails to meet the demand for change and potential revisions is likely to become obsolete and abandoned. Therefore, adaptive reuse and functional and physical adaptability in buildings necessitate a certain potential for change in the building's function, capacity, performance, and operation. A shift from rigid functionalism to the use of capacity-for-change as a guiding principle, from centralized to distributed control, is nonetheless becoming evident.

Adaptability discourse in literature is founded on diverse perspectives in different disciplines. Schnädelbach [22] defined adaptability as an interdisciplinary concept encompassing architecture, art, engineering, and computer science. Due to terminological differences based on context, there is no universal definition that applies to all disciplines. The ambiguity in the definition and scope of adaptability increases the possibility of a misconstrued concept [23]. Friedman [24], as cited in [23] also mentioned the wide range of definitions as a factor that contributed to misconceptions about adaptability. Such ambiguous definitions of adaptability in architecture could be attributed to – what adaptability means to different stakeholders in disparate contexts and at varying points in time [25]. Therefore, it is essential to briefly review different definitions to clarify the relative meanings of adaptability in architecture based on the context. Adaptability was defined as an extended benefit obtained from a product within the context of product/service development [26]. Given Tschumi’s [27] discourse on the inclusion of uncertainties of function, event, and movement in architecture as an ability to accommodate social change, such a definition partially applies to architecture. The architectural product must somehow transcend the defined design boundaries, extending the benefit (or utility) it provides to its occupants. Gu, Hashemian, and Nee [26] distinguished two types of engineering/product development approaches: design adaptability and product adaptability. Design adaptability refers to a set of design principles that allow for product modification (adaptation) [26]. Product adaptability, on the other hand, enables the same product to be used for multiple functions by modifying its existing features [28]. In the architecture domain, design adaptability entails modularity and mass customization approaches [28] with limited building functionality (warehouses, factories, etc.). As a result, the former, design adaptability, may be irrelevant for unique architectural functions and contexts where design decisions are different from other designs. The latter, product adaptability, is more relevant to architectural discourses, where adaptability is defined as the ability of a space to be modified for uses other than the one originally designed for [29].

Definitions of adaptability in architecture have been broadened to include designs that are appropriate for changing social uses [30] or that can change function, capacity, or performance to adapt to changing conditions and needs [12]. Schmidt III et al. [5] define adaptability in architecture as the ability of a building to meet new requirements. Furthermore, current research on adaptive facades is linked to building adaptability, which occurs when parts of a building manually or automatically respond and adapt to environmental stimuli and/or occupant activities [22]. Orhon [31] similarly defines adaptability, as the ability to change the features of a building to adapt to the environment, occupants, and social context. Given the various approaches to defining adaptability in architecture, the present study focused on the AIA’s definition [29], which is related to product adaptability [26, 28] and focuses on a space’s ability to transform a new function. Even though adaptability and flexibility are closely related concepts in architecture and often used interchangeably, in the present study, we refer to adaptability as a governing concept that includes flexibility. Several studies indicated that expressions such as adaptability, flexibility, and polyvalence had multiple and often overlapping definitions that led to confusion and ambiguity [5, 23, 32, 33, 34, 35]. Flexibility in architecture refers to the ability of a space or building to accommodate change through the modification of physical elements such as movable and/or modular partitions, dry connections, etc. [36]. Flexible designs allow for different configurations or uses over time, providing options for users to adapt the space to their needs [9, 37, 38]. Adaptability, on the other hand, encompasses a broader range of factors beyond physical flexibility. It refers to the capacity of a building or space to respond effectively to changing conditions [30], requirements [39], or contexts over time [23, 40]. This may include changes in function, occupant needs, technological advancements, environmental considerations, or social dynamics. Adaptability involves not only the physical attributes of a building but also its ability to evolve and remain relevant in different circumstances [23, 35, 39, 41]. Therefore, the view of adaptability we focus on in the present study extends beyond mere physical flexibility, addressing a wide range of changes that may emerge over the course of a building's lifetime.

Historically, adaptability became prevalent during the modernist movement through the work of architects such as Le Corbusier, Frank Lloyd Wright, and Mies van der Rohe, as a result of increased social change following the mid-nineteenth-century Industrial Revolution [28]. The modernist movement also pioneered a functionalist approach, emphasizing the new objectivity [42], which resulted in simple forms that expressed a building's structural and mechanical systems [10, 28, 32]. A certain degree of freedom in spatial design was introduced as a result of the adaptability approaches facilitated by frame construction [43]. Studies in literature that focused on adaptability strategies based their conceptual frameworks on the Open Building approach of Habraken [44], as a viable alternative to the prevailing conventional practice of adopting a single program based on unsubstantiated projections through time, wrapping the result in built form and then knitting mechanical and structural systems into and around the functions, [45] and the Shearing Layers approach of Brand [46], which assumed that the buildings were composed of several layers (site, structure, skin, services, space plan, and stuff) with different hierarchies and lifespans [32]. Schneider and Till [30] also proposed a governing principle of “hard” and “soft” elements in the building, with the former referring to the use of services and technology and the latter to the space itself, as the foundations for integrating adaptability strategies into the building. These three main approaches, which underpin the adaptability in buildings, in other words, the shift from rigid functionalism to the capacity-for-change as a guiding principle [47], were briefly discussed in the following subsections.

2.3.1.

Open Building Approach

Mass housing constructed after the Second World War failed to adapt to social, economic, and technological changes due to their standard and rigid designs [48]. According to Dutch architect N. John Habraken [44], such a design approach stemmed from the was due to the perspective that the building industry needed to adopt methods based on standardization and repetition similar to the automobile industry, which led to the development of the alternative approach, that supports theory or the open building by Habraken in 1961 with the non-profit research group SAR (Stichting Architecten Research). They argued that the housing problem could be solved only when buildings accounted for change over time and inhabitant control was taken into consideration as a separate design task [44]. Later, the open building approach, which succeeded the supports theory, became an international movement based on the organization of levels in buildings and their technical and decision-making processes (Figure 1) [45]. Habraken argued that there existed a connection between the physical layers of the building and the hierarchical order of the environment [38]. The open building approach identified distinct levels of intervention in the built environment [49], from the macro to the micro levels, namely land use, fabric, support, support infill, layout, and planning (Figure 1). The changes at the macro levels impacted the lower levels of the hierarchy, however, a certain level of flexibility and change potential existed at the micro-levels [48]. The concept of levels distinguished control between support, which was not determined by occupants, and infill, which was determined by individual building occupants [50]. In other words, occupant control, which could be more flexible and responsive at the micro level (spaces), accommodated a greater potential for change (thus, adaptability), but had no influence on administrative decisions at the district, area, or block level [6], and vice versa. Therefore, the open building concept was considered a transformation mechanism that addresses adaptability through individualized characteristics [51].

Figure 1.

Principle of Levels (Source: Authors. Adopted from Open Building Research Group, TU Delft, [50])

The levels in the open building approach consider not only control and decision-making capacities but also long and short service life as a principle [6]. Building elements/components with long service lives, such as the structural system, building envelope, service systems, shafts, and circulation elements, were included in the support level and were required to be simple to construct, economical, and to allow flexibility at the infill level. Support level (with a lifespan of 100 to 200 years) is also common to all building occupants and any change at the infill levels should not affect the support level [43]. Higher levels (i.e., support) also offer the degree of freedom for implementing adaptability at the infill level, which includes elements with shorter life spans, such as partitions, doors, finishings, circulation, and horizontal installations [39, 52]. Aside from the structural system and the options it provides for organizing the infill layers, other decision-making processes are involved in the building construction process. Examining the relationship between occupant control and the physical environment allows for the inclusion of the relationship between boundaries and occupant behavior [5] via decision-making at multiple levels, from collective to individual [6]. Contrary to the conventional approach, the open building approach seeks a balance between supply and demand [6, 10].

Since the 1970s, the open building approach has been popular in Japan, due to the shift in the construction technique from the traditional Japanese wooden structure to the reinforced concrete skeletal systems [53]. Several open building systems, such as the Kodan Experimental Project (KEP) [4, 53], Century Housing System [54], and open building schemes such as ‘skeleton-infill’ (SI) have been developed in Japan since the adoption of the open building approach [4]. Today, the CIB W104 working group and MANUBUILD maintain the open building approach on a global scale [48]. Although many projects have been carried out in various parts of the world using the approach, Schmidt III et al. [5] criticized the fact that open building was more focused on the design and utilization phases and did not take uncertainties such as time and change into account. As a result, despite its limitations in constraining adaptability primarily to the infill level [5], Habraken’s method had a significant impact on adaptable architectural design [38].

2.3.2.

The “Shearing Layers” Approach

A building is a collection of systems comprised of layers with variable service lives [21] and the individual building components are only partially interconnected [15]. Habraken’s open building approach only considers levels of support and infill. Other approaches were also developed, such as categorizing building elements based on their degree of deterioration and assessing buildings hierarchically through layers. Frank Duffy [55] pioneered the “shearing layers” approach, arguing that the economics of a building change drastically over its service life. According to Duffy [55], building components should be classified based on their service life, as physical and temporal layers: shells (50 years), services (15 years), scenery (5 to 7 years), and sets (every day). Later, Brand [46] expanded on the shearing layers concept and identified six layers, namely the Site, Structure, Skin, Services, Space Plan, and Stuff (Figure 2), based on their differences in service life and overall effects. Of these six layers, site refers to the boundaries and context, which are eternal Duffy [55], as cited in [46], structure included foundation and load-bearing elements with a service life between 30 and 300 years, skin referred to the elements of the building envelope with an approximate service life of 20 years, services include the building installations with a service life between 7 and 15 years, space plan refers to the interior spatial layout with a service life between 3 and 30 years depending on the building function and stuff refers to furnishing with no definite service life (Figure 2) [46].

Figure 2.

The shearing layers approach (Source: Authors. Adopted from [46])

Sub-components within layers were also considered as building elements that could be changed without affecting the related layer. Layers can be physically separated or intersected through certain components. For instance, a roof as a building envelope component (part of the layer, skin) can also be a part of the structure layer or rely on the structural framework [19]. The characteristics and hierarchical order of the components and layers can be used as data in the analysis of a building’s capacity to change.

The shearing layers approach was used and transformed by other researchers. The literature indicates varying scopes, systems, and subsystems for the layers approach, depending on the context and framework of the studies. Table 1 presents examples of the alternate uses of the shearing layers approach in literature. Slaughter, Sause, and Pessiki [56] investigated and characterized the interactions between building layers and components in 1997 to propose structural floor framing systems to accommodate nonstructural requirements such as installations. They included structure, enclosure, service, and spatial/functional layers (Table 1) as critical criteria concerning their performance indicators, such as strength, stability, serviceability, capacity, and versatility [56]. Ashbolt [57] adopted Brand’s layers [46] with the inclusion of circulation as an additional layer. Abdullah and Al-Alwan [58] approach adaptability in architecture from the standpoint of smart material systems and the adaptive response of building components to the architectural environment, hence, the authors included “ambient” as an additional layer to Brand’s shearing layers approach [46]. Ambient did not refer to a physical component but rather referred to internal environmental conditions such as illumination levels, thermal comfort, etc. [58].

Table 1.

Example studies that interpreted Brand’s shearing layers approach [46]

Duffy [55]	Brand [46]	Slaughter, Sause, and Pessiki [56]	Ashbolt [57]	Abdullah and Al-Alwan [58]
	Site		Site
	Structure	Structure	Structure	Structure
Shell	Skin	Enclosure	Skin	Outfit
Services	Services	Service	Services	Services
			Circulation
Scenery	Space Plan	Spatial/Functional	Space Plan	Infill
Sets	Stuff		Stuff	Interior
				Ambient

The changes that a building goes through over its service life are related to the organization of these layers and the level of flexibility and durability of the building elements. Elements with shorter service life are expected to have a higher flexibility level, whereas the elements with longer service life (i.e., structure) are expected to have a higher durability level [21]. Brand’s shearing layers [46] were intended to increase a building's adaptability potential starting from the design phase. Given the potential for adaptability to contribute to the sustainability of the built environment [16], design decisions that facilitate building adaptability and flexibility towards change become critical.

Despite a short overview presented in this section, building adaptability is highly studied within the context of the 21^st century’s social, environmental, and economic changes, with a more recent emphasis on sustainability [66]. However, Habraken questions why “after more than a century of attempts by architects to design with flexibility in mind, the issue is still marginal to the profession at large” [33]. The abovementioned approaches were well-known attempts to frame adaptability and flexibility in buildings, especially in housing. However, there is still a lack of applied frameworks for adaptability and flexibility in existing buildings. Commonly, adaptability approaches propose a theoretical framework through which they assess the fixed and flexible parts of a building [33]. Despite their differences in categorizing parts/components of a building based on their adaptability potentials, the abovementioned approaches also bear a strong similarity in their descriptive nature. Most approaches consider adaptability on the layout level and do not scrutinize the adaptability potentials three-dimensionally. Given the scope above, the present study presents an attempt to create a quantitative approach via a scoring system that can be used in evaluating the adaptability potential of existing buildings. The latter sections explain the methodology and the related case studies.

The present section explains the approaches in selecting the adaptability strategies and parameters from the literature and in establishing their associations based on an expanded version of the shearing layers approach of Brand [46]: location (site), structure, layout (space plan), services, envelope (skin), elements, and reinforcements (stuff).

The scope of Manewa et al.’s [14] analysis was expanded based on the 33 selected studies, and a total of 45 strategies, specified 199 times, were identified (Table 2). The selected strategies for the evaluation criteria set were determined using histogram/Pareto analysis, which revealed the distribution of the frequency of use for the identified adaptability strategies (Figure 4). The cumulatively analyzed values revealed that ten strategies explained more than 60% of the total variance, and these adaptability strategies were chosen for the evaluation criteria set. Since there existed no discriminating data to choose between the equally frequent strategies, the lower rate was determined as 60%. Strategies selected for the evaluation criteria set were Flexibility/Versatility (21; 10.6%), Expandability/Scalability (19; 9.5%), Dismantlability/Separability (14; 7.0%), Movability/Mobility (13; 6.5%), Overcapacity/Redundancy (11; 5.5%), Convertibility (9; 4.5%), Reusable/Recyclable (9; 4.5%), Independence (8; 4.0%), Modularity (8; 4.0%) and Refitability (6, 3.0%), respectively (Table 2, Figure 4).

Figure 4.

Histogram/Pareto diagram for the frequency distribution of the identified adaptability strategies (Source: Authors)

Flexibility/Versatility was defined as the ability to transform the interior space [79, 80, 81] and to make small-scale changes to the interior space [2] for different uses. Schmidt III et al. [5] used flexibility in a similar sense to the concept of versatility. The ability of a building to expand horizontally or vertically [76], as well as the potential for change in building dimensions [57, 79, 80]were defined as Expandability/Scalability. The ability to quickly and safely disassemble a building [12, 70] was referred to as Dismantability/Separability, with an emphasis on the reuse of building components and the reduction of construction wastes [2, 75]. Movability/Mobility refers to the mobile or portable building components [57, 70, 78, 79, 80]. Overcapacity is defined as the excessive design of structural systems [75], floor heights, circulation, and service spaces [17] that are unlikely to be renewed. The continuation of a building with a function other than its original function is referred to as Convertibility [2, 5, 14, 57, 78]. Reusable/Recyclable are terms that refer to the reusability and recyclability of building components and elements [5,79]. The physical and functional separation of building components and systems in such a way that other components are not damaged during replacement is referred to as Independence [19, 75]. In a building, Modularity refers to independent modules, and clustering methods are used to systematize the module hierarchy and sub-relationships [26]. Refitability refers to the interchangeability of building parts [70]. Parameters include the spatial and structural features of a building and refer to tangible building elements, compared to the conceptual descriptions of adaptability strategies. The scope of Manewa et al. [14] was expanded for 19 selected studies, and a total of 17 parameters, specified 140 times, were identified (Table 3). Histogram/Pareto analysis was used to select the frequently used parameters for the evaluation criteria set (Figure 5). The cumulative analysis of the values revealed that 11 parameters explained more than 80% of the total expressions, and these were selected for the evaluation criteria set: Structural design/slabs/loads (14; 10.0%), plan organization (14; 10.0%), floor height (12; 8.6%), technical span (11, 7.9%), core/vertical circulation (11, 7.9%), service spaces (10; 7.1%), envelope design (10; 7.1%), building dimensions and height (9; 6.4%), mechanical, electrical, IT and HVAC installations (8; 5.7%), location/accessibility/orientation/proximity (7; 5.0%), and partitions/walls (7; 5.0%).

Figure 5.

Histogram/Pareto diagram for the frequency distribution of the identified adaptability parameters (Source: Authors)

Structural design/slabs/loads are significant for the adaptability of the buildings based on their type, dimensions, and layout. The capacity to compensate vertical and lateral attachments [12,13]; connections that prevent collapse, electrical and mechanical service distribution schemes to adapt to different uses [2]; dry connections, partitionable structural design [13]; and the establishment of new service layouts [67] are all related to the structural design, slabs and loads. Multifunctionality, horizontal and vertical overcapacity of spaces [2, 13], increased spatial efficiency and continuity of spaces, buffer zones, modular design [13], loose fit approach in interior design [2, 59], and locating main functions around service spaces [67] are issues considered in plan organization.

Function, structural system, service spaces, and height limitations in the building codes are the factors [65]that are related to floor height. Buildings with increased floor height and structural spans are easier to adapt to different functions [2, 59, 76]. According to Kincaid [74], increased floor height may be uneconomical in the short term but beneficial for adaptability in the long term. Technical span refers to the axial arrangement in the structural system. In terms of providing subdivisions and responding to different uses [65], grid structural designs without uneconomical long spans [2] are preferable. Stairs, service cores, and entrances are all part of the core and vertical circulation design. Service and circulation areas, as well as core locations, should be designed so that the structure can expand vertically or horizontally and be divided into various functional units [13]. Locating cores at building ends provides an uninterrupted and spacious interior [15], whereas the central core allows spatial transformations along the façade line [59] and makes spatial changes while maintaining structural integrity [2]. The capacity of circulation spaces is determined by the density of occupants and the function of the building [67]. Service spaces accommodate building service systems. Modular and/or detachable installation systems [13], ducts for vertical service elements [59], raised floors, and/or suspended ceilings for the horizontal distribution of service elements [15, 59] increase the adaptability level of buildings. Envelope design should ensure that changes in the interior do not affect the façade (such as double façade systems), allow for different uses, properly control the interaction between façade modules and physical and visual access [13], should be accessible from the interior and exterior space, independent from the structural system [2, 13], and allow retrofit and maintenance [15]. Building dimensions and height influence the number of units and occupant density, as well as a building’s adaptability in terms of function and density [67]. Overdesign of vertical shafts to accommodate additional mechanical and electrical services [15], sizing of drainage and pipelines to meet additional uses [12], mechanical ventilation in deep-plan buildings [67], and decentralized distribution of service spaces to accommodate system control in the event of further spatial divisions [79] are mechanical, electrical, IT, and HVAC installation decisions that contribute to the adaptability of buildings. Building location, accessibility, orientation, and proximity include spatial requirements around the building to accommodate future facilities such as parking, pedestrian and vehicle access [12], and to prevent interruption during major works [13]. To support building adaptability, interior partitions/walls should be non-loadbearing [59], removable, reusable, and lightweight [2, 13].

The literature explains the relationships between adaptability strategies and parameters based on a building's spatial and structural features [14]. The present study focuses on the relationships between the 10 adaptability strategies and the 11 adaptability parameters included in the evaluation criteria set. Therefore, 5 of the shearing layers [46], respectively, location, structure, layout, services, and envelope, were used as the grouper of the relationship matrix, and the 11 physical parameters were classified under the layers (Table 4). The location layer included building location/accessibility/orientation/proximity, the structure included building dimensions and height, structural design/slabs/loads floor height, technical span, and core/vertical circulation, the layout layer included plan organization and partitions/walls, services included service spaces and installation elements, and the envelope layer included double skin facades, modular/panel systems, independent, universal, and irregular systems.

Parameters were studied to delineate their physical equivalents as building features, which were covered in the literature. The physical attributes of a building were then associated with the strategies (Table 4). Steel structural systems (9), modular and panel facade systems (8), prefabricated reinforced concrete frame systems (7), steel stairs (7), raised floors (7), and suspended ceilings (7) were the physical attributes that were frequently associated with the selected 10 strategies (Figure 6). Movable/rearrangeable and fixed partitions and double skin facades were also other physical attributes associated with the strategies (Figure 6), while shafts and independent facades were associated with 5 strategies. Table 4 indicates the strategies that were highly associated with the physical attributes; Flexibility/Versatility (26/33), Convertibility (26/33), Scalability (15/33), and Independence (15/33), respectively.

Figure 6.

Frequency of physical attributes of buildings within the scope of adaptability strategies (Source: Authors)

The score matrix presented in Table 4 was obtained by a simple weighted scoring system based on the frequency of adaptability and strategies in the literature. The frequency of adaptability strategies and parameters (X) was converted into a score of X/10. For instance, Flexibility/Versatility, with a 10.6% frequency in literature was assigned as k_strategyFlex = 10.6/10 = 1.06 points, or location/accessibility/orientation/proximity, with a frequency of 5%, was assigned as 0.5 points. The physical attribute/parameter score was calculated by multiplying the parameter scores separately with the associated adaptability strategies’ scores. Total attribute scores were obtained via the sum of values for each attribute/strategy score. The maximum adaptability score for a building is 65.63, but this is only possible if the building meets all of the sub-parameters/physical attributes. If a building has an open plan, modular organization, and multifunctional spaces, the total floor plan/space organization parameter score is the sum of the scores of 1.91, 2.96, and 1.91, respectively. Flexibility/Versatility (a total of 21.84 points) explained 33.28 percent of the total score, Scalability (a total of 11.25 points) explained 17.14 percent of the total score, and Convertibility (a total of 8.81 points) explained 13.42 percent of the total score.

The main goal of the evaluation criteria set is not to score structures and assign them adaptability levels such as low, medium, or high; rather, the goal is to reveal the relationship between strategies and parameters through scoring by assessing physical parameters and determining which strategies contribute to adaptability and to what extent. In this respect, the adaptability evaluation criteria established in the present study is a qualitative assessment tool rather than a quantitative scoring system.

The Schröder House was designed by Gerrit Rietveld in 1924 in Utrecht and was considered one of the iconic examples of the De Stijl movement and modern architecture with its spatial organization, furniture design, and façade [91]. The house was built next to traditional three-storey row houses [92].

The Schröder House accommodates spatial transitions via distinct planar elements, and the interior space’s continuity was articulated and adapted to different uses via sliding and pivoted surfaces (movable partitions, Figure 7), which allowed the occupants to perform spatial transformations [91]. The lower level has a traditional layout, whereas the upper level of the Schröder House (Figure 7) was designed as a relatively open and multi-purpose space with movable partitions situated between the fixed partitions and the exterior walls and structural elements [93]. Hence three different spaces could be combined during the day and used in an open plan and the closing of the partitions allowed spatial privacy when required [93]. Non-load-bearing and movable partition walls contribute to the Flexibility/Versatility, Independence, Separability, Scalability, Convertibility, and Refitability of the Schröder House.

Figure 7.

The layers that augment the adaptability characteristics of the Schröder House (Source: Authors)

Vertical circulation was centralized on the layout and the main spaces were situated along the façade line, thus, there was no need for mechanical ventilation. Vertical shafts and the horizontal distribution of the installation elements ensured via the suspended ceiling facilitated maintenance and repair without affecting other building elements (Figure 7). The façade was designed with an independent structural system from the structure of the building and with a modular arrangement of the transparent envelope surfaces, which facilitates change in the facade (Table 5). The structural system of the Schröder House consists of reinforced concrete slabs and steel profiles [92, 93]. The Schröder House was scored based on its physical attributes (Table 5). For instance, the low-density score is a function of the adaptability parameter “location/accessibility/orientation/proximity” and adaptability strategies “scalability” and “convertibility” (Table 3). Therefore, the open space on three sides of the building corresponded to the low-density score, while the low-rise score was assigned based on the two-storeys of the building. Structure scores were based on the steel and reinforced concrete slabs used in the structural design of the building, and floor height was scored based on the three-meter height. The technical span was seven meters, and the centralized layout of the vertical circulation was scored as well. The open plan approach and spatial multifunctionality were scored separately to address both the spatial and the functional aspects of adaptability; hence, the partition elements, fixed and movable, were scored within the same scope. For services and installations, the suspended ceiling system, shafts, and natural ventilation were scored. The glazing and opaque elements of the envelope were scored for their modularity, independence, and universality (Table 5).

Figure 8 presents the percentage of score distribution for individual adaptability strategies and the overall adaptability of the Schröder House. The overall adaptability score of the Schröder House was 33.24 over a total score of 65.63 (50.65%, Figure 7). For individual adaptability strategies, the highest score was identified for Independence, as 3.12 over a total score of 4.80 (65.00%, Figure 8), based on the following physical attributes: steel and reinforced concrete construction, open plan and multifunctional spaces, movable/rearrangeable and fixed partitions, suspended ceiling and shafts, modular, panel, and independent envelope systems. Refitability and Convertibility were scored as 1.29 (55.13%, over a total score of 2,34) and 4.85 (55.05%, over a total score of 8.81), respectively (Figure 8, Table 3). Flexibility/Versatility was scored as 11.49 over a total score of 21.84 (52.16%, Figure 8). The adaptability strategy with the lowest score was Modularity, with a score of 1.16 over 3.24 (35.80%, Figure 8).

Figure 8.

The overall adaptability score and strategy scores of the Schröder House (Source: Authors)

The Farnsworth House was built in Illinois by Mies van der Rohe between 1945 and 1951 and became one of the most prominent symbols of 20th-century modern architecture due to its structural clarity and minimalist approach [94]. Two rectangular planes of different sizes, one of which is an open terrace and the other enclosed by transparent surfaces, are offset from the ground by steel columns (Figure 9). The interior space was designed as an open plan around a central core (Figure 10) and the fixed interior partitions between the core and the main space were used as furniture [95]. The structure of the Farnsworth House was designed with a precast concrete floor and roof supported by steel frame elements [96]. A suspended ceiling is attached to the steel beams that support the roof slab [97]. The steel frame structure, centralized service spaces, and the limited use of partition elements increased the Flexibility/Versatility, Convertibility, and Independence of the building. The envelope is designed with transparent elements which were arranged independently of the building function (Figure 10).

Figure 9.

The Farnsworth House (Source: Author, Basak Gucyeter)

Figure 10.

The layers that augment the adaptability characteristics of the Farnsworth House (Source: Authors)

The Farnsworth House was scored based on its physical attributes (Table 5). The freestanding building was scored for low-density and low-rise based on its contextual and massing decisions. The steel frame and reinforced concrete slabs were scored for structure, and the floor height score was based on the three-and-a-half meters height. The technical span was nine meters and allowed open plan, modularity, and spatial multifunctionality, which were separately scored to address the spatial adaptability potentials of the Farnsworth House. The partition elements were limited in number and fixed to enclose the service spaces. Suspended ceiling was the main design decision to accommodate installations and the building was naturally ventilated. The envelope was fully glazed and was scored for independence and universality (Table 5).

The percentage of score distribution for individual adaptability strategies and overall adaptability of the Farnsworth House is presented in Figure 11. The overall adaptability score of the Farnsworth House was 26.23 over a total score of 65.63 (39.97%, Figure 11). The highest score for individual strategies was identified for Independence, 2.36 over a total score of 4.80 (49.17%, Figure 11), and the scored physical attributes were steel and reinforced concrete construction, open plan, and multifunctional spaces, fixed partitions, suspended independent facade elements. Convertibility and Flexibility/Versatility were scored as 4.07 over a total score of 8.81 (46.20%) and 9.68 over a total score of 21.84 (44.32%), respectively (Figure 11). The adaptability strategy with the lowest score was Overcapacity, with a score of 0.98 over 3.29 (29.79%, Figure 11). The Farnsworth House is a single-storey building; hence, the adaptability score is lower due to the lack of shafts and vertical circulation elements. The steel frame structure and the suspended ceiling support multiple adaptability strategies such as Flexibility/Versatility, Convertibility, Independence, and Refitability.

Figure 11.

The overall adaptability score and the adaptability strategy scores of the Farnsworth House (Source: Authors)

Otto Steidle’s Genter Strasse houses were constructed between 1967 and 1972 in a densely populated area of Munich. In the first phase of the project, Steidle collaborated with Doris Thut, Ralph Thut, and Jens Freiberg; in the second phase, he collaborated with Eckardt Böck and Gerhard Niese; and in the third phase, he collaborated with Roland Sommerer and Jens Freiberg. The project employed a variety of methods and solutions, with lightweight construction and prefabrication being the most important design approaches [98]. Load-bearing and non-load-bearing elements are visually distinguished in the building. Occupants can transform the spaces to meet their changing needs [99]. The adaptable organization of the prefabricated structural elements and the fixed partitions were partially presented in Figure 12.

Figure 12.

Layers that augment the adaptability characteristics of the Genter Strasse (Source: Authors)

The structural system is a prefabricated modular reinforced concrete frame. Mezzanine floors and 1.5-story spaces are possible due to the distribution of console beams in the columns at half-story height [100]. The frame structure is completely or partially filled with spaces of varying floor heights and layouts, depending on the occupants’ requirements (Figure 12). The structure’s rigidity is provided by the in-situ concrete core spaces with service areas. The free-standing structural elements left during construction allowed the house to be more adaptable to potential changes [101]. Dry connections allow for simple and quick changes to the prefabricated structural system and facade, floor, and ceiling panels without causing damage to other building elements [102]. The modular facade design also allows for different use of envelope materials.

Figure 13 presents the percentage of score distribution for individual adaptability strategies and the overall adaptability of the Genter Strasse houses. The overall adaptability score of the Genter Strasse houses was 33.67 over a total score of 65,63 (51.30%, Figure 13). For individual adaptability strategies, the highest score was identified for Reusable/Recyclable, as 1.58 over a total score of 2.35 (67.23%, Figure 13), based on the following physical attributes: prefabricated reinforced concrete structure and dry connections, use of steel stairs and modular panel systems in the façade. Expandability/Scalability and Refitability were scored as 6.85 (60.89%, over a total score of 11.25) and 1.41 (60.26.05%, over a total score of 2.34), respectively (Figure 13, Table 3). Movability/Mobility was the least scored adaptability strategy, with 1.16 over a total score of 3.06 (37.91%, Figure 13). Genter Strasse houses received aboveaverage scores for several individual adaptability strategies based on the adaptability evaluation criteria. The majority of the adaptability parameters related to façade design, structural design, core design, and spatial organization were fulfilled by Genter Strasse. As a result, the aforementioned parameters significantly improved the adaptability of the building.

Figure 13.

The overall adaptability score and the adaptability strategy scores of the Genter Strasse houses (Source: Authors)

In 2003, Alejandro Aravena and the Elemental Group designed Quinta Monroy Housing in Chile as a social housing project. The row house-style housing units were constructed in an unfinished state, a result of budget constraints and parcel limitations, thereby allowing occupants to undertake essential future extensions. The formal construction included the ground floor and upper floor apartments designed to 36 m² each, with different sizes of future extension possibilities of 9 and 18 m² (Figure 14) [103, 104, 105]. The formal construction of 36 m² incorporated structural system elements, sanitary spaces, and circulation elements that necessitated professional construction decisions [106, 107, 108]. Quinta Monroy Housing was constructed using a reinforced concrete structural system, featuring a total height of 8 meters across three storeys [109]. Independent units arranged within a three-meter structural grid exemplified an open plan design, enabling occupants to modify spatial characteristics according to their specific requirements [110].

Figure 14.

Layers that augment the adaptability characteristics of the Quinta Monroy Housing (Source: Authors)

The Quinta Monroy Housing project was scored based on its physical attributes for low-density and low-rise based on its contextual and massing decisions (Table 5). For structural design/slabs/loads, the building complex was scored for reinforced concrete and increased load-bearing capacity due to the possibility of future extensions. The building complex was also scored for floor height (fh < 3 m) and technical span (ts < 6 m). For core/vertical circulation, the centralized layout of the vertical circulation and steel structure stairs were scored as well. The open plan approach, modular organization, standard size, spatial multifunctionality, and overcapacity were the parameters that the Quinta Monroy Housing project received scores for plan organization (Table 5, Figure 14). For the parameter, partitions/walls, the building complex was scored for the physical attributes, fixed and movable. The building complex received no score for service spaces and received only a score for natural ventilation for the parameter, installations. The glazing and opaque elements of the envelope were scored for their independence, universality, and irregularity (Table 5).

The percentage of score distribution for individual adaptability strategies and overall adaptability of the Quinta Monroy Housing is presented in Figure 15.

Figure 15.

The overall adaptability score and the strategy scores of the Quinta Monroy Housing (Source: Authors)

The overall adaptability score of Quinta Monroy Housing was 30.20 (Table 5) over a total score of 65,63 (46.02%, Figure 15). Convertibility was identified as the individual strategy with the highest score, 5.26 (Table 4 and 5) over a total score of 8.81 (59.70%, Figure 15), and the scored physical attributes were low-density, low-rise, increased load-bearing capacity, reinforced concrete construction, cent realized core, open plan, modular, standardized, multifunctional plan organization, spatial overcapacity, moveable/rearrangeable and fixed partitions, natural ventilation, independence, universality, and irregularity of the envelope elements. Flexibility/Versatility was scored as 12.11 over a total score of 21.84 (55.45%) (Figure 15). The adaptability strategy with the lowest score was Dismantlability/Separability, with a score of 1.05 over 4.65 (22.58%, Figure 15).

The Quinta Monroy Housing project accrued fewer adaptability scores than expected, mainly due to the floor height being below 3 meters (2.6 m) and the technical span below 6 meters (maximum 5.80 m). The incorporation of structural steel circulation elements elevated the Independence, Movability/Mobility, Flexibility/Versatility levels [106]. As the building lacks service volumes such as shafts and raised floors, which typically segregate systems from other layers and facilitate repairs, it accrued lower scores in the Flexibility/Versatility, Convertibility, Independence, and Refitability strategies. Due to the Quinta Monroy structural system not being steel, prefabricated, or prestressed, and featuring low technical clearance and floor height, along with the absence of dry connections, the building layer received low scores. Additionally, the lack of service volumes led to diminished scores in the services layer. The facade layer similarly received lower scores due to the absence of a double facade system and modular layout. However, the design is quite prominent in terms of facilitating the spatial expansion of housing units in multiple directions, proving crucial not only for individual development but also for collective progress, as it fosters neighborhood interaction, selfregulation, and diversity in collective spaces and applications [111].

Constructed in Amsterdam in 2018 and designed by ANN Architecten, Het SchetsBlok encompasses 25 apartments with varied floor areas ranging from 46 m² to 150 m² (Figure 16) [112]. Tailored to specific occupant demographics, each apartment features unique sizes, floor plans, and facade designs. The residential complex emphasizes individual occupant preferences by incorporating modular layouts and facade designs for the apartments [113, 114]. The ground-level apartments are configured as duplexes, while two penthouse apartments occupy the 6^th floor [112]. Habraken’s open building was embraced in the design process, characterized by distinct layers for land use, fabric, support, support infill, layout, and planning [113]. The structural framework integrates a central reinforced concrete core, perimeter columns, and reinforced concrete slabs. Shafts are positioned at the corners of the core, adjacent to the apartment walls. Thus, a flexible floor area of 350 m² exists between the core and the facade, allowing unrestricted modifications. Non-load-bearing partition walls were designed to be demountable and/or movable, facilitating future adjustments based on the demand for different apartment sizes (Figure 16). The facade design is independent of the load-bearing structure and consists of a primary grid and a secondary grid adaptable to various apartment layouts, facilitating interior reconfigurations [114]. The opaque elements of the facade feature an inclined concrete composite grid and are clad with expanded aluminum panels [113].

Figure 16.

Layers that augment the adaptability characteristics of Het SchetsBlok (Source: Authors)

Het SchetsBlok was scored based on its physical attributes for low-density and high-rise based on its contextual and massing decisions (Table 5). The building complex was scored for reinforced concrete and increased load-bearing capacity for the structural design/slabs/loads parameter since it allows future modifications for the number and size of apartments in the building. The building complex was also scored for floor height (3 m < fh < 5 m) and technical span (ts ≥ 6 m). The centralized layout of the vertical circulation was scored for the core/vertical circulation parameter. The open plan approach, modular organization, and spatial overcapacity were the parameters that the Quinta Monroy Housing project received scores for plan organization (Table 5, Figure 16). Het SchetsBlok was scored for the physical attributes, fixed and movable, for the parameter partitions/walls, for the shafts for the service spaces parameter, and natural ventilation for the installations parameter. The glazing and opaque elements of the envelope were scored for all physical attributes within the envelope design parameter (Table 5).

Figure 17 presents the percentage of score distribution for individual adaptability strategies and overall adaptability of Het SchetsBlok. The overall adaptability score of the building was 33.66 (Table 5) over a total score of 65.63 (51.29%, Figure 17). Convertibility and Overcapacity/Redundancy were identified as the strategies with the highest scores, respectively, 6.07 and 2.27 (Table 4 and 5) over total scores of 8.81 and 3,29 (68.90% and 69.00%, Figure 17). Expandability/Scalability was the other strategy that stood out for Het SchetsBlok with a score of 6.85 over a total score of 11.25 (60.89%, Figure 17). Flexibility/Versatility was scored as 12.24 over a total score of 21.84 (56.04%) (Figure 17). The adaptability strategy with the lowest score was Movability/Mobility, with a score of 0.33 over 3.06 (10.78%, Figure 17).

Figure 17.

The overall adaptability score and the strategy scores of the Het SchetsBlok (Source: Authors)

Het SchetsBlok accrued higher adaptability scores for Flexibility/Versatility, Expandability/Scalability, and Convertibility strategies compared to the other studied examples, mainly due to its technical span larger than 6 meters and the use of a double skin façade. Given that Het SchetsBlok is situated in a low-density area there is a chance for horizontal expansion and the building, spanning a total of 7 floors, maintains an average floor height of 3.30 meters and a technical span of approximately 8 meters. The primary focus of Het SchetsBlok was to achieve flexibility that accommodates a wide range of unit sizes and to allow freedom in layout, addressing certain adaptability characteristics in meeting the requirements of the varied demographic groups of urban residents [112]. Adopting an open plan approach around the central core enhanced the building's flexibility, convertibility, independence, and expandability. Furthermore, the building received scores for spatial overcapacity due to the potential closure of the terrace floor for future inclusion in the flat. The modular organization parameter also contributed to the scores, given the modular layout of the flats within the interior.

Het SchetsBlok attained higher adaptability ratings in Flexibility/Versatility, Expandability/Scalability, and Convertibility strategies compared to other examined examples. This was primarily attributed to its technical capabilities, including a span exceeding 6 meters and the implementation of a double-skin facade. Situated in a low-density area, Het SchetsBlok presents opportunities for horizontal expansion. With a total of 7 floors, the building maintains an average floor height of 3.30 meters and a technical span of approximately 8 meters. Its primary objective was to achieve flexibility accommodating various unit sizes and allowing layout freedom, addressing adaptability characteristics to meet the needs of diverse urban resident demographics [112]. Embracing an open plan approach around the central core bolstered the building’s Flexibility/Versatility, Convertibility, Independence, and Expandability/Scalability. Additionally, the building acquired a spatial overcapacity score due to the potential closure of the terrace floor for future inclusions in flats. The modular organization parameter also contributed to its scores, reflecting the modular layout of flats within the interior.