Authorities & RERA
What is FSI and FAR?
The number that decides how much building fits on a plot. And the reason a new tower can appear in your open space, ten years later, entirely lawfully.
The short answer
FSI (Floor Space Index) — also called FAR (Floor Area Ratio) — is how much total floor area may be built on a plot, as a multiple of the plot size.
FSI of 2 on a 10,000 sq ft plot = 20,000 sq ft of building permitted.
And here is the part that matters to you: if the builder doesn't use all the FSI, they keep it. And they can build it later — in your open space — entirely lawfully.
What FSI is
The formula
FSI = Total permissible built-up area ÷ Plot area
- Plot area
- 10,000 sq ft
- Permitted FSI
- 2.0
- Total floor area they may build
- 20,000 sq ft
They can build it as two floors covering the whole plot, or ten floors covering a fifth of it. FSI governs the total, not the shape — that is what ground coverage, setbacks and height limits are for.
FSI and FAR mean the same thing. Different states use different words. Maharashtra says FSI. Delhi says FAR.
The unused FSI problem — and this is the whole reason to read this page
A builder is permitted 20,000 sq ft of FSI. They build 16,000.
The remaining 4,000 sq ft of FSI does not disappear. It is a real, valuable, tradeable asset — and it belongs to whoever holds title to the land.
Until CONVEYANCE, that is the BUILDER.
Which means they can come back — five years later, ten years later — and build another tower in your open space. Lawfully. Because the FSI was always theirs and the land still is.
This has happened. Repeatedly. Societies have watched a new building go up in the garden they thought they had bought, and discovered they had no legal basis to object.
Once the land is conveyed to the society, the residual FSI goes with it. The builder cannot use it, because they no longer own the land.
That is a large part of why builders delay conveyance for years. It is not an administrative oversight. It is an asset they are holding on to.
Before you buy, ask two questions:
1. “How much of the permitted FSI has been consumed?”
2. “Has the land been conveyed to the society?”
If a lot of FSI is unused and conveyance has not been done, the open space you are paying for is not yours, and it may not stay open.
How FSI varies — and why cities look the way they do
- By city. Mumbai's FSI has historically been low by international standards — one reason it is so dense and so expensive. Some cities allow far more.
- By zone. Commercial zones typically get more than residential.
- By road width. A plot on a wide road usually gets more FSI than one on a narrow lane — because the road has to carry the traffic.
- By premium. Many cities sell premium FSI — pay the authority, build more.
- By TDR. Development rights bought from elsewhere can be loaded onto a plot, above the base FSI.
- Special categories — redevelopment, slum rehabilitation, affordable housing — often get enhanced FSI as an incentive.
How to check
- The sanctioned building plan states the permitted and consumed FSI. It should be in the RERA filing.
- Ask the builder directly: "What is the permitted FSI, and how much has been consumed?"
- Look at the master plan and the local building bye-laws for the permitted FSI in that zone.
- Check the approved layout — does it show any future phase, or a reserved plot?
- Ask about conveyance. Unused FSI plus unconveyed land is a live risk to your open space.
“Is there any unused FSI on this plot, and if so, what does the builder intend to do with it?”
Ask it. Get the answer in writing.
A builder who has consumed the FSI has nothing to build. A builder with 4,000 sq ft in hand, and the land still in their name, has an option — and options get exercised.
FSI vs ground coverage vs setback
| What it controls | Why it matters to YOU | |
|---|---|---|
| FSI / FAR | How much total floor area may be built, as a multiple of the plot area | Unused FSI can be built later. By the builder. In your open space. |
| Ground Coverage | How much of the PLOT the building may sit on, as a percentage | The rest is open space — which is what you're paying for when you buy 'green area' |
| Setback | The open distance that must be left between the building and each boundary | Light, air, fire access. And the thing builders most often encroach on. |
| Height limit | How tall the building may be — often driven by road width, and by airport proximity | Whether the tower blocking your view can lawfully exist |
These four numbers, set by the local authority in the building bye-laws, decide what a plot can hold. Together they are why one project has a tower and open lawns, and the one next door has three towers and a car park.
The Real Estate (Regulation and Development) Act, 2016 is a central law. But it is administered by a separate authority in each state, each with its own portal, its own rules, its own forms, and its own fee schedule.
Which means: the principles below apply everywhere. The procedure does not.
Always check YOUR state's RERA portal for the current rules, forms and fees. Search for it by name — MahaRERA, K-RERA, TS-RERA, TNRERA, UP RERA, HARERA — rather than following a link a builder or a broker sends you.
Frequently asked questions
What is FSI in real estate?
Floor Space Index — the ratio of total permissible built-up floor area to the plot area. An FSI of 2 on a 10,000 sq ft plot permits 20,000 sq ft of building. FAR (Floor Area Ratio) means the same thing; different states use different words.
What happens to unused FSI?
It doesn't disappear. It is a real, valuable, tradeable asset, and it belongs to whoever holds title to the land — which, until conveyance, is the BUILDER. So they can come back years later and build another tower in your open space, entirely lawfully. This has happened repeatedly.
Why do builders delay conveyance?
Unused FSI is one of the main reasons. Once the land is conveyed to the society, the residual FSI goes with it and the builder cannot use it. So delaying conveyance keeps an asset in their hands. It is not an administrative oversight — it is a decision.
How do I check the FSI of a project?
The sanctioned building plan states the permitted and consumed FSI, and it should be in the RERA filing. Ask the builder directly: what is the permitted FSI, and how much has been consumed? Then ask whether conveyance has been done. Unused FSI plus unconveyed land is a live risk to your open space.
Is FSI the same as FAR?
Yes — the same concept under different names. Maharashtra and much of western India say FSI; Delhi and much of the north say FAR.