Floor Area Ratio (FAR)

Floor area ratio (FAR) is a zoning measure that limits building size relative to the lot it sits on. It's expressed as a ratio of total enclosed floor area to lot area — so a 0.5 FAR on a 5,000-square-foot lot would generally cap total floor area at about 2,500 square feet. By tying allowable size to lot size, FAR controls bulk and density without dictating a building's exact shape, which it leaves to setbacks and height limits.

FAR matters for ADU projects because adding a unit adds floor area, and on a lot that's already near its FAR cap there may be little room left to build. This is one reason ADU-enabling laws often exempt accessory units from FAR limits, or grant extra allowance for them, so that an existing house near its cap can still add a unit. Whether such an exemption applies — and how it's calculated — depends entirely on local rules.

FAR works alongside other zoning controls to define what fits on a parcel: setbacks determine where you can build, height limits cap how tall, and FAR caps how much total floor area you get. Some codes also factor in lot coverage (the share of the lot a building footprint may occupy), which is related but distinct from FAR. Builders use these limits together to test feasibility before investing in design. Because the applicable ratio, what counts toward floor area (garages, basements, and accessory structures are treated differently in different places), and any ADU exemptions all vary by jurisdiction and can change, confirm the current standards for your parcel with the local planning department.