Implementing Sustainable Development Through Municipal Spatial Planning in Slovenia: A Case Study of Four Municipalities

Sustainable development is a central objective of contemporary spatial planning; however, empirical evidence on how sustainability principles are implemented through municipal planning instruments remains limited. This study examines how sustainable development is embedded in Municipal Spatial Plans (MSPs) and reflected in spatial development practice in four Slovenian municipalities—Gornja Radgona, Hrastnik, Kostanjevica na Krki, and Lenart. A qualitative, indicator-based comparative framework was applied, structured around five thematic areas, twelve sub-themes, and thirty-one indicators. The analysis triangulated statutory planning documents, ten-year official statistical data, and five-year municipal investment reports, deliberately avoiding composite indices to prevent false precision in cross-municipal comparison. The results show that all MSPs formally incorporate sustainability as a guiding principle; however, significant differences emerge in how concretely these principles are translated into spatial provisions, investments, and observed development trends. Lenart demonstrates the strongest alignment between planning objectives and implementation, while Hrastnik and Gornja Radgona exhibit persistent gaps related to demographic decline and mobility patterns. Kostanjevica na Krki illustrates a protection-oriented sustainability approach shaped by flood risk and constraints relating to cultural heritage. The study concludes that MSPs primarily function as strategic and coordinating instruments, while effective implementation of sustainable development depends on complementary governance arrangements, investment alignment, and monitoring mechanisms beyond statutory spatial planning. The findings provide transferable insights for municipalities facing similar sustainability challenges.