FAR and Lot Coverage Are the Two Primary Tools for Controlling Building Bulk
Zoning codes use multiple mechanisms to control how much building mass can be placed on a site. Two of the most fundamental are floor area ratio (FAR) and lot coverage. FAR controls the total volume of building relative to the site area by limiting the gross floor area that can be built. Lot coverage controls the footprint of buildings and impervious surfaces relative to the site area. Together, these two measures work with setbacks and height limits to define the three-dimensional limits on what can be built. Understanding FAR and lot coverage calculations is a core ARE PA skill - these are among the most commonly tested numerical concepts in the Programming and Analysis division.
Floor Area Ratio (FAR)
FAR is the ratio of total building gross floor area to the site area. FAR = Total GFA / Site Area. A FAR of 2.0 on a 10,000 sf site allows a maximum of 20,000 sf of gross floor area. This could be achieved as a 20,000 sf single-story building, a 10,000 sf two-story building, a 4,000 sf five-story building, or any other combination of floor areas that totals 20,000 sf - as long as all other dimensional standards (setbacks, height limits, lot coverage) are also satisfied. FAR allows density to be concentrated in height (tall, thin buildings on urban sites) or spread across a larger footprint (lower buildings on larger suburban sites), giving designers flexibility in how floor area is distributed as long as the total is within the FAR limit.
What counts toward FAR is a crucial detail that varies by jurisdiction. Many zoning codes exclude parking structures, basement space, mechanical penthouses, and covered but open-sided areas from the FAR calculation. Some codes use different FAR definitions for different building types. The definition used in each jurisdiction must be carefully reviewed and applied - assuming a standard definition without checking the local code leads to calculation errors.
Lot Coverage
Lot coverage is the percentage of the total site area covered by buildings and other impervious surfaces. Maximum lot coverage = (Building footprint + Other impervious area) / Total site area × 100%. If a zoning code allows 60% lot coverage and the site is 15,000 sf, the maximum total impervious area (building footprint plus parking, walkways, and other hardscape) is 9,000 sf. Unlike FAR, lot coverage does not depend on how many stories the building has - a 5,000 sf footprint building has the same lot coverage regardless of whether it is 2 stories or 10 stories.
Lot coverage limits are particularly important for stormwater management: more impervious surface generates more stormwater runoff. Many jurisdictions have tightened lot coverage requirements or introduced stormwater management requirements (detention, retention, rain gardens) specifically because impervious surface is the primary driver of peak stormwater runoff rates.
FAR Calculation Example
A 20,000 sf site in a commercial zone allows a FAR of 3.0 and a maximum lot coverage of 70%. Maximum GFA = 20,000 sf × 3.0 = 60,000 sf. Maximum footprint = 20,000 sf × 70% = 14,000 sf. To achieve the full 60,000 sf GFA with a 14,000 sf footprint: 60,000 / 14,000 = 4.3 stories minimum. The building must be at least 5 stories to accommodate the allowed FAR within the lot coverage limit - assuming the height limit allows 5 stories.
Key Exam Points
- FAR = Total GFA / Site Area; controls total building volume.
- Lot coverage = Building footprint (+ other impervious) / Site area × 100%; controls footprint size.
- FAR × Site area = Maximum allowed GFA.
- What counts toward FAR varies by jurisdiction - parking, basements, mechanical spaces often excluded.
- FAR and lot coverage work together with setbacks and height limits to define the 3D buildable envelope.
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