This thesis examines why long-standing spatial planning instruments become sites where structural tensions accumulate over time. Using the English Green Belt as its central case, it explores how instruments designed to provide spatial stability interact with changing governance conditions. Rather than reading persistent conflict as policy failure, the thesis interprets it as a symptom of deeper institutional dynamics. The study develops a conceptual framework showing how spatial instruments accumulate pressure through institutional entrenchment, functional expansion, and governance displacement. Over time, boundaries intended to contain urban growth become focal points for wider conflicts over housing, infrastructure, environmental governance, and regional coordination. Comparative insights from Frankfurt and Seoul show how different institutional arrangements shape planning systems’ capacity to manage change. The thesis argues that long-term spatial protection depends less on permanent boundaries than on governance capacity for institutional recalibration under uncertainty.