Floor Area Ratio and Lot Coverage
Summary
Floor area ratio caps the total floor area of a building as a multiple of the lot area, and lot coverage caps the footprint on the ground as a percentage of the lot. They work with height and setbacks to set the largest building a parcel can hold, so find which control binds first before developing a massing.
Cols
Scope
- Covers the two controls most often used to limit building size on a lot, and how they interact with height and setbacks
- Definitions, exclusions, and numeric limits are set by each local zoning ordinance and differ widely. Always verify the municipal code's definitions and the current zoning for the project parcel
- Not the same as building-code area measures such as allowable area or building area; zoning floor area is defined by the ordinance
Terminology
- Floor area ratio (FAR): total floor area divided by lot area; unitless, written as a decimal (0.45) or a percentage (45 percent)
- Gross floor area: floor area measured to the outside of exterior walls, usually including stairs, halls, closets, and mechanical rooms
- Net floor area: floor area with some spaces, such as stairs and closets, deducted; gross is generally preferred for FAR
- Floor area: the portion of gross floor area an ordinance counts toward FAR, after its exclusions
- Lot area: parcel area used as the divisor; ordinances often deduct street right-of-way, drainage easements, and the access strip of a flag lot
- Lot coverage (building coverage): footprint of buildings and covered structures divided by lot area, as a percentage
- Impervious surface ratio: a separate cap on hard surfaces, including paving, as a percentage of the lot; used by some ordinances beside or instead of lot coverage
- Bulk regulations: the group of controls that set maximum building size on a lot: coverage, setbacks, height, impervious surface, FAR, and open yard
- Floor space index (FSI): another name for FAR, expressed as a percentage (1.5 FAR is 150 percent FSI)
- Binding control: the limit (FAR, coverage, height, or setback envelope) that produces the smallest allowable building
How the controls work
Formulas
- Maximum floor area: lot area × FAR
- Maximum footprint: lot area × lot coverage percentage
- Floors at full coverage: maximum floor area ÷ maximum footprint
- FAR ignores how floor area is stacked on the lot; a 2.0 FAR allows two lot-areas of floor space as one tall tower or as low buildings covering most of the site
- Lot coverage ignores height; a high-coverage, low-FAR lot is a single-story building and a low-coverage, high-FAR lot is a tower
Which control binds
- FAR alone can encourage wide, low buildings that use more of the lot; coverage alone can push toward taller, narrower buildings. Ordinances pair them, so the stricter one governs
- Height and the setback envelope can limit a building before either ratio does
- Test the three on a quick massing: full coverage and FAR give a floor count; compare it with the height limit
What counts
Floor area
- Usually counted: conditioned above-grade space, mezzanines and lofts, enclosed garages, detached accessory dwelling units and sheds, habitable attics in some ordinances, roofed porches and breezeways enclosed on three sides
- Often excluded: open balconies and decks, crawl spaces, fully below-grade basements, lightwells, rooftop mechanical space, unfinished attics
- Varies by ordinance: carports, partly above-grade basements, covered but open porches, interior space above a set height (such as 15 ft), and mechanical rooms
- Planning practice has long favored measuring gross area, since the ratio relates a building to land
Lot coverage
- Footprint of every roofed structure on the lot, including garages, sheds, and covered patios
- Some ordinances add roof eaves and overhangs, and give open porches and patios a small extra allowance
- Paving and driveways are handled by an impervious surface cap where the ordinance has one
Typical ranges
| District type | Typical FAR |
|---|---|
| Single-family residential | 0.3 to 0.6 |
| Two-family and townhouse | 0.5 to 1.5 |
| Low-rise multifamily | 1.0 to 3.0 |
| Neighborhood commercial | 0.5 to 2.0 |
| General commercial and mixed use | 2.0 to 5.0 |
| Downtown and dense commercial | 5.0 to 15.0 |
Ranges are approximate. Each ordinance sets its own values for each district.
Worked examples
Single-family lot
8,000 sq ft lot, FAR 0.5, lot coverage 40 percent, height limit 35 ft.
| Step | Calculation | Result |
|---|---|---|
| Maximum floor area | 8,000 × 0.5 | 4,000 sq ft |
| Maximum footprint | 8,000 × 0.40 | 3,200 sq ft |
| One story | Footprint limited by coverage | 3,200 sq ft total; coverage binds |
| Two stories | 4,000 ÷ 2 = 2,000 sq ft per floor | 4,000 sq ft total; FAR binds, coverage 25 percent |
| With a 400 sq ft garage counted in FAR | 4,000 − 400 | 3,600 sq ft of house |
The 400 sq ft garage also counts toward lot coverage in most ordinances.
Urban infill lot
10,000 sq ft lot, FAR 6.0, lot coverage 80 percent, height limit 100 ft.
| Step | Calculation | Result |
|---|---|---|
| Maximum floor area | 10,000 × 6.0 | 60,000 sq ft |
| Maximum footprint | 10,000 × 0.80 | 8,000 sq ft |
| Floors at full coverage | 60,000 ÷ 8,000 | 7.5, or 8 stories with a partial top floor |
| Smaller floor plate | 6,000 sq ft per floor (60 percent coverage) | 10 floors, 120 ft at 12 ft floor-to-floor; exceeds the 100 ft limit |
| Height limit check | 100 ÷ 12 | 8 floors fit, so floor plates must average at least 7,500 sq ft |
Here height and coverage together decide the plate size: 8 floors need 7,500 sq ft each, which is 75 percent coverage.
Coordination
- Survey: confirm lot area as the ordinance defines it, after any dedication or easement deductions
- Zoning official: confirm in writing how garages, basements, mezzanines, porches, and mechanical space count before the area calculation is final
- Area diagrams: keep a counted and excluded floor area plan beside the building area plan, updated with each design phase
- Massing: test the bulk controls together with the setback envelope and height limit, not one at a time
- Site design: coverage and any impervious surface cap shape paving, driveways, patios, and stormwater measures
- Entitlements: FAR and coverage are among the first items a plan reviewer checks, so list the numbers on the cover sheet or zoning summary
Common issues
- Wrong lot area: using the recorded lot area when the ordinance deducts right-of-way, easements, or flag-lot access; recompute from the survey
- Garage and accessory structures left out: detached garages, sheds, and accessory dwelling units often count toward FAR and coverage
- Late design changes: added mezzanines, enclosed porches, and converted basements push the project over FAR; carry a margin in schematic design
- Measuring to the wrong line: gross area is measured to the outside of exterior walls in many ordinances, not to the interior finish
- Assuming exclusions: parking, mechanical, and basement exclusions are not universal; confirm each one
- Overlooking overlays: overlay districts and design review can add or reduce the base limits
Sources
- American Planning Association: Floor Area Ratio, PAS Report 111, https://planning.org/pas/reports/report111.htm
- Wikipedia: Floor area ratio, https://en.wikipedia.org/wiki/Floor_area_ratio
- Buildability: what is floor area ratio, https://buildability.us/blog/what-is-floor-area-ratio
- City of Cupertino: single family FAQs, floor area ratio and lot coverage, https://www.cupertino.gov/Your-City/Departments/Community-Development/Planning/Planning-FAQ/Single-Family-FAQs
- Law Insider: bulk regulations definition, https://www.lawinsider.com/dictionary/bulk-regulations