Floor area ratio, floor space index, site ratio and plot ratio) is the ratio
of a building's total floor area to the size of the piece of land upon which it
is built. The terms can also refer to limits imposed on such a ratio.
As a formula: Floor area ratio = /
Thus, an FSI of 2.0 indicates 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. Terminology
The terms most commonly used for this measurement vary from one country or
region to the next. In Australia floor space ratio is used
in New South Wales and plot ratio in Western Australia.
In India floor space index and floor area ratio are both used.
In the United Kingdom and Hong Kong both plot ratio and site ratio are used.
In Singapore the terms plot ratio add gross plot ratio are more commonly used.
In the United States and Canada, floor space ratio and floor area ratio are
both used. Use in zoning
Floor area ratios are used as a measure of the density of the site being
developed. The ratio is generated by dividing the building area by the parcel
area, using 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.
Impact on land value Edward Ptacek noted that the allowable
FAR has a major impact on the value of the land. Higher allowable FAR yields
higher land value. Criticism
Andres Duany notes: Abdicating to floor area ratios is the
opposite of aiming a community toward something more than the sum of its
parts. FAR, a poor predictor of physical form,
should not be used when the objective is to conserve and enhance neighborhood
character; whereas traditional design standards enable anyone to make
reasonably accurate predictions, recognize violations, and feel secure in
their investment decisions. If FAR is carelessly combined with
traditional setbacks, assembled lots have a considerable advantage over
individual lots, which has a negative effect on fine-grained cities and the
diversity of ownership. = Japan =
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.
Footnotes References
Meriam, Dwight. The Complete Guide to Zoning. McGraw-Hill. ISBN 0-07-144379-7
External links Blog post explaining floor area ratio
and Space Index An explanation of the floor area ratio
by J.H. Crawford