The rapid expansion of cities poses a very hard challenge to provide adequate housing for the poor, while its social and environmental pressures demand alternative approaches to low-cost housing. An answer to the challenge of housing for poor citizens can come from cities themselves. Urban centers are concentrated nodes where vast quantities so material sand energy are consumed in constant cycles of construction, operation, and demolition. Strategically intervening in the urban metabolism can help to secure very low-cost or even no-cost construction materials while reducing the environmental stresses due to resource extraction, leading to more sustainable cities.