Barcelona developers “La Casa por el Tejado” (House on the roof) have created lightweight, prefab apartments that can be craned into place to grow cities up, not out, in a matter of months.

“We believe that the growth of a city should start with using up the remaining buildable space in already-existing buildings,” write the developers.

Their team spent four years cataloguing the buildable “air rights” in Barcelona. They found hundreds of buildings that could still be built up by one or two floors totaling 800,000 square meters of available apartment space.

“In most historical cities, extensions of the old city were developed through sprawling. This meant that the city expanded outward, not upward. In many cases, the projected buildable area of a project was never exhausted and, thus, remains today as potential buildable space right in the city center.”