Use in Zoning
Floor area ratios are used as a measure of the intensity of the site being developed. The ratio is generated by dividing the building area by the parcel area, being sure to use the same units.
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 FAR, 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.
An architect 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. Thus, many jurisdictions have found it unnecessary to include hard height limitations when using floor area ratio calculations.
Read more about this topic: Floor Area Ratio