This document is on how to utilize of unused land for affordable housing World Bank. The poorest inhabitants of many cities are often unable to afford a minimum-size dwelling with safe water and sanitation as if the wealth created by cities was part of a zero-sum game where the poor will be at the losing end. The main challenge for urban planners and economists is reducing cities’ negative externalities without destroying the wealth created by spatial concentration. Each city’s priorities would depend on its history, circumstances, and political environment. But maintaining mobility and keeping land affordable remains the main urban planning objective common to all cities.