The Spatial City is the most significant application of "mobile architecture". It is a spatial, three-dimensional structure raised up on piles which contains inhabited volumes, fitted inside some of the "voids", alternating with other unused volumes. It is designed on the basis of trihedral elements which operate as "neighbourhoods" where dwellings are freely distributed.
This structure introduce a kind of merger between countryside and city compare to (Paolo Soleri's Arcology concept) and may span:
- certain unavailable sites,
- areas where building is not possible or permitted (expanses of water, marshland),
- areas that have already been built upon (an existing city),
- above farmland.
This spanning technique which includes container structures ushers in a new development in town-planning. Raised plans increase the original area of the city becoming three-dimensional. The tiering of the spatial city on several independent levels, one on top of the other, determines "spatial town-planning" both from the functional and from the aesthetic viewpoint. The lower level may be earmarked for public life and for premises designed for community services as well as pedestrian areas. The piles contain the vertical means of transport (lifts, staircases). The superposition of levels should make it possible to build a whole industrial city, or a residential or commercial city, on the same site. In this way, the Spatial City forms what Yona Friedman would call an "artificial topography". This grid suspended in space outlines a new cartography of the terrain with the help of a continuous and indeterminate homogeneous network with a major positive outcome: this modular grid would authorize the limitless growth of the city.
The spaces in this grid are rectangular and habitable modular "voids", with an average area of 25-35 square meters. Conversely, the form of the volumes included within the grid depends solely on the occupant, and their configuration set with a "Flatwriter" in the grid is completely free. Only one half of the spatial city would be occupied. The "fillings" which correspond to the dwellings only actually take up 50 % of the three-dimensional lattice, permitting the light to spread freely in the spatial city. This introduction of elements on a three-dimensional grid with several levels on piles permits a changeable occupancy of the space by means of the convertibility of the forms and their adaptation to multiple uses.
In Yona Friedman's own words "The city, as a mechanism, is thus nothing other than a labyrinth : a configuration of points of departure, and terminal points, separated by obstacles".