Two numbers shape almost every early plan for a plot: how much floor area the rules allow, and how much of the ground the building may cover. They are called floor area ratio (FAR) or floor space index (FSI) for the first, and site coverage or ground coverage for the second. They are related but separate, and a design has to satisfy both.
Floor area ratio (FAR or FSI)
FAR is the total built-up floor area divided by the plot area. If the plot is 5,000 ft² and the permitted FAR is 1.5, the largest total floor area is 5,000 × 1.5 = 7,500 ft², which is about 696.8 m². That area can be spread over any number of floors, within the height limit.
To get a first idea of the number of floors, divide the permitted area by a typical floor plate. With a floor plate of 2,000 ft², 7,500 ÷ 2,000 = 3.75, so three full floors fit and the rest is a partial fourth floor. You can try this with the FAR / FSI Calculator.
Site coverage (ground coverage)
Site coverage is the area of the building at ground level (the footprint) divided by the plot area, as a percentage. With a footprint of 2,200 ft² on a plot of 5,000 ft², the coverage is 2,200 ÷ 5,000 = 44%. The open area is 5,000 − 2,200 = 2,800 ft², which is 56% of the plot. If the permitted coverage is 50%, the largest footprint is 2,500 ft², so this design is within the limit. The Site Coverage Calculator gives all of these numbers at once.
How the two limits work together
A building can be within the FAR limit but over the coverage limit, or the other way round. A wide, low building uses coverage quickly. A tall, narrow building uses FAR. Designers usually test both numbers while they sketch the massing, before they commit to a floor plate.
What counts and what does not
This is where most mistakes happen, because the rules differ between cities and countries. Check the local development rules for each of these:
- Balconies, terraces and open porches. Some rules exclude them from FAR or from coverage; some count part of them.
- Stair cores, lift shafts and service ducts. Some authorities exclude them from FAR.
- Basements and parking floors, which are often treated differently.
- Projections, canopies and overhangs.
- Area given up for road widening, which may or may not count towards the plot area.
Setbacks and height
FAR and coverage are only two of the limits. Setbacks from the plot boundary, maximum height, parking requirements and open space rules also limit the building. A plot can have enough FAR on paper and still not allow the building you want because of setbacks.
A short checklist
- Measure the plot area from the title documents, not from a drawing.
- Find the permitted FAR and coverage for the plot’s zone.
- Calculate the maximum floor area and the maximum footprint.
- Check what the rules count for balconies, shafts, basements and projections.
- Test your massing against both numbers and against the setbacks.
The figures from a calculator are a first check. The authority’s own rules, and your drawings as submitted, decide what is approved.