Fungible FSI: the 35% that decides how big your redeveloped flat will be
Every Mumbai redevelopment negotiation turns on one number most society members have never heard of. Here is what fungible FSI is, who pays for it, and why the free part is yours.
Rules as of 2 November 2026, under Mumbai's Development Control and Promotion Regulations (DCPR) 2034.
Start with FSI
Floor Space Index is the ratio of built-up area to plot area. A plot of 1,000 sq m with an FSI of 1.33 allows 1,330 sq m of construction. In a redevelopment, the developer's entire business model is the gap between the FSI the old building used and the FSI the rules now allow. That gap comes from three sources: base FSI, Transferable Development Rights (TDR) bought from elsewhere, and fungible FSI.
What fungible FSI is
Fungible FSI was introduced in 2012 and carried into DCPR 2034. It is an additional allowance, over and above the base and TDR, of up to 35% for residential buildings (20% for commercial and industrial). "Fungible" means it can be used for anything: extra carpet area, balconies, flower beds, niches, the lift lobby, whichever the design needs. Before 2012 those elements were "free of FSI" and routinely abused; fungible FSI replaced that with a single, countable allowance.
Who pays for it
This is the part that decides your flat. Fungible FSI comes in two forms:
- Free-of-premium fungible FSI. In a redevelopment, the portion of fungible FSI that corresponds to the existing members' rehabilitation area is granted without a premium. It exists so that members can be rehoused at the new standard. It is, in plain terms, yours.
- Premium fungible FSI. Everything beyond that, which the developer uses for the flats it will sell, is bought from the Municipal Corporation at a premium pegged to the Ready Reckoner rate for the land. For residential use the premium has been set at 60% of the land rate, with higher rates for industrial and commercial.
The practical consequence: a developer who tells a society that fungible FSI is "being given to you as a concession" is describing the free portion that the rules already guarantee. The negotiation is over how much of the total the members get, not whether the free portion exists.
The caps that limit all of it
Fungible FSI does not stack without limit. Base FSI, TDR and fungible together must stay within the zonal cap for the plot, which depends on the width of the road it abuts. Narrow-road plots get less; plots on wider roads, and in the island city, get more. This is why two neighbouring societies with identical plots can receive very different offers: one of them is on a 9-metre road and the other on an 18-metre road.
What to ask before you sign
Ask for the FSI computation sheet, with the free and premium fungible components separated. Ask who bears the premium (it should be the developer). Ask how the fungible area is distributed in the member flats, because 35% applied to the building is not the same as 35% applied to your carpet area. And ask whether the premium component has been paid up front or is being deferred, since a deferred premium is a lien on a building you will live in.
What this means for you
If your society has received a redevelopment proposal, ask the developer's architect for a single sheet showing: base FSI, TDR, fungible FSI (split into the free-of-premium portion and the premium portion), the zonal cap, and how the total is divided between members' rehab area and the developer's sale area. If they cannot produce that sheet, you are not ready to vote. If they can, take it to an independent project management consultant before the special general meeting.
