Floor Area Ratio

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The Floor Area Ratio (FAR) or Floor Space Index (FSI) is the ratio of the total floor area of buildings on a certain location to the size of the land of that location, or the limit imposed on such a ratio.

The Floor Area Ratio is the total building square footage (building area) divided by the site size square footage (site area).

As a formula: Floor Area Ratio = (Total covered area on all floors of all buildings on a certain plot)/(Area of the plot)

Thus, an FSI of 2.0 would indicate that the total floor area of a building is two times the gross area of the plot on which it is constructed, as would be found in a multiple-story building.

[edit] Floor Area Ratio in zoning

The Floor Area Ratio can be used in zoning to limit the amount of construction in a certain area. For example, if the relevant zoning ordinance permits construction on a parcel, and if construction must adhere to a 0.10 floor area ratio, then the total area of all floors in all buildings constructed on the parcel must be no more than one-tenth the area of the parcel itself.

A builder can plan for either a single-story building consuming the entire allowable area in one floor, or a multi-story building that rises higher above the plane of the land, but which must consequently result in a smaller footprint than would a single-story building of the same total floor area. By combining the horizontal and vertical limits into a single figure, some flexibility is permitted in building design, while achieving a hard limit on at least one measure of overall size. One advantage to fixing this parameter, as opposed to others such as height, width, or length, is that floor area correlates well with other considerations relevant to zoning regulation, such as total parking that would be required for an office building, total number of units that might be available for residential use, total load on municipal services, etc. The amounts of these things tend to be constant for a given total floor area, regardless of how that area is distributed horizontally and vertically.

Japan has extensively adopted the Floor Area Ratio in the zoning system since 1970. The evaluation of the adoption is, however, controversial: some say that it has deteriorated the skylines and building lines in Japanese cities; others claim that it has protected the residential environments.