To provide more housing in dense cities, the only real way to go is up. Now, developers are adding units on top of older buildings - what Feargus O'Sullivan for CityLab calls "parasite homes."

Stephane Malka Architecture is working on a new social enterprise Les Toits Du Monde, or Roofs of the World. Under a new property law aiming to build 70,000 new homes a year in France, the city is relaxing zoning laws and allowing developers to build extra structure on top of existing ones.

If the site is suitable, the owner isn’t necessarily given money, however. Instead, he or she surrenders their rooftop in return for substantial renovations, such as façade repairs, better insulation, or new lifts.

This arrangement enables Les Toits Du Monde to keep costs low, as does the development’s structure, which uses prefabricated units that are constructed off-site in a workshop then bolted into steel supports. When completed ineight-to-12 months time, the three apartments at 3BOX will have cost €680,000 ($738,500) to build total, which isn’t bad for a prime inner city site.

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