The illegal urbanization of rural areas near cities has unveiled failures in urban management. In many cases, urban policies have ignored this fact until the spaces have consolidated. This is the example of the Sierra de Santa Bárbara (Plasencia, Spain), where legalization becomes one of the most feasible solutions. The present work analyses its residential evolution during the last four decades through historical orthophotos review. Along with this, it evaluates public–private conflicts (homeowners vs municipal government) using regional newspaper archives. The results indicate that the strategy of ignoring illegal development increases these problems, leading to legalization as the only possible urban policy. In conclusion, the administration’s response is delayed and forced by critical consequences, which prevents learning in urban policies and new solutions that join legality and sustainability.