The short answer
From the applicable floor area ratio or FSI, setbacks, ground coverage and height rules for that zone.
Development potential comes from the zoning regulations: permissible floor area ratio or FSI, ground coverage, setbacks, height limits and any reservations affecting the plot. Multiply the plot area by the FAR and deduct non-saleable areas to estimate buildable saleable area.
Get this confirmed in writing by an architect or liaison consultant against the current regulations before you buy. Brochure claims and older approvals are not reliable inputs for a financial model.
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FSIFARfeasibility