FSI vs FAR: what is the difference?
FSI and FAR both express counted floor area relative to the relevant plot or lot area; the important difference for a real project is which local definition and exclusions apply. For example, 1,000 sq. m. of plot area multiplied by a ratio of 2 gives 2,000 sq. m. of counted floor area, not a promise that this much area can physically fit or be approved.
The ratio in that example is an assumption, not a permitted FSI for a specific site. Keep floor area, footprint and saleable area separate, then check the actual rules for the city and project.
Ask “what is the FSI on this plot?” and you will often get one figure back. That figure is the start of the calculation, not the answer.
Floor Space Index (FSI), often called Floor Area Ratio (FAR), connects permitted built-up area to plot area. But a feasibility model has to ask more than the base number: which area is counted, which additional rights are optional, what can actually be loaded on the site, and what does each option cost?
A ratio can also be displayed as a percentage: 2 equals 200%. That change in presentation does not by itself change development rights. Use the governing rule’s definition of counted floor area and its exclusions.
Mumbai shows the mechanism clearly
Mumbai’s Development Control and Promotion Regulations, 2034 (DCPR 2034) separates the zonal or basic FSI from other routes that can change the permitted built-up area. The regulation covers additional FSI on payment of premium, transferable development rights (TDR), and fungible compensatory area under their own eligibility, payment and cap conditions.
That distinction matters. Additional FSI on payment of premium is optional and site-specific; the regulation says it is granted on application, payment and use on the same plot. TDR is a credit created and used under a separate rule set, with generation, receiving-plot and utilisation conditions. Fungible compensatory area has its own conditions and premium treatment.
Why a single number misleads
A quoted FSI can hide different rights, costs and eligibility conditions. A useful estimate separates the base allowance from each additional layer before calculating the result.
- base or zonal FSI;
- premium FSI, including its price and cap;
- TDR, including receiving-plot eligibility and loading limits; and
- fungible or excluded areas, where applicable.
Without that breakdown, two teams can use the same quoted FSI and reach different buildable areas, costs and underwriting results. The single figure was never the full development rule.
Even a correctly separated FAR figure does not settle the footprint question. Buildable area can be constrained before FAR by setbacks, open space and the physical envelope.
The general lesson
Mumbai’s mechanism is not a template to copy into another city. Premium rights, redevelopment incentives, transferable rights, road conditions and exclusions are local regulatory questions. A usable feasibility number therefore names the governing regulation, its retrieval date, included layers and unresolved approvals.
This is not legal or regulatory advice. For a live transaction or filing, confirm the current regulation and site conditions with the relevant sanctioning authority and qualified professionals.
Sources
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