Spatial planning axes are an important instrument of spatial planning to help connect urban areas and ensure the accessibility of rural areas and their development. The planning of such axes can steer population and traffic flows, decisions on the locations of economic developments as well as infrastructures such as cable networks. In this paper, current spatial planning axes and their continuity are analysed regarding their suitability to promote sustainable cross-border European development. Two neighbouring regions are investigated as examples, namely the Regional Planning Authority Oberes Elbtal/Osterzgebirge (Germany, Saxony) and the Ústí nad Labem Region (Czechia). The overarching research question is how transboundary spatial planning axes can be harmonised? This can be broken down into the following three sub-questions: (1) Which differences exist between the Saxon and Czech planning systems? (2) How is the need for harmonisation assessed by planning practitioners? (3) Which transboundary recommendations can be given from a scientific perspective? To answer these research questions, expert interviews were conducted with relevant stakeholders from spatial planning authorities as well as scientific and political institutions.