Moshe Safdie's Habitat 67 (designed for Expo 67) challenges the basic dilemma of mass housing: while raising density — that is, stacking many dwellings on top of one another — how can each family be given what the suburban house offers, namely open air, a garden, and privacy? Safdie's answer is to build the block not as a pile of boxes but as a shifted cluster.

The building is made of 354 identical reinforced-concrete box modules precast in a factory. These modules are lifted by crane on site and stacked one on another, but each time shifted back. This shift is crucial: the roof of one module becomes the garden terrace of the shifted module above. So even in a dense block rising up to twelve storeys, every flat has its own private open terrace and windows facing more than one direction — no one is sealed directly under or over another. The gaps between modules let light and air into the building.

Habitat 67 uses industrial prefabrication — the mass production of identical parts — to produce not monotonous, gloomy blocks but variety and individuality. The same box, stacked differently, gives each flat a unique form. The experiment proved costly and did not spread widely, but it remains the most ambitious and influential prototype of prefabricated modular housing; it is a direct forerunner of today's modular and volumetric construction movements.