Last updated: February 16, 2026
Your Footprint Fills the Lot Before You Run Out of FAR
A homeowner sketches an addition on graph paper, adds a detached garage, and assumes the only constraint is the setback line. Then the permit reviewer pulls up the zoning table and says “your lot coverage is 48 %—the cap is 40.” The addition fits inside the setbacks, clears the height limit, and still gets rejected because the combined building footprint exceeds the maximum ground-plane area the code allows. Coverage ratio is the gate most people forget, and it blocks projects at the permit counter more often than FAR or height.
This calculator takes your lot area, the footprint of every structure on it, and the zoning coverage cap, then shows current coverage percentage, remaining buildable footprint, and whether you pass or fail. It does not look up your zoning district—you need to enter the correct maximum from your local code.
Coverage Ratio: Ground Footprint, Not Total Floor Area
Building coverage ratio (also called lot coverage or building coverage) is the percentage of the lot occupied by the ground-floor footprint of all structures. The formula is:
Coverage = (Total Footprint ÷ Lot Area) × 100 %
A two-storey house and a ten-storey tower with the same ground-floor outline produce the same coverage. Coverage cares only about horizontal spread, not vertical bulk. That is the key difference from floor area ratio (FAR), which measures total floor space across every level. A project can have low coverage and high FAR (slender tower, lots of open space) or high coverage and low FAR (wide single-storey warehouse, no open space). Zoning codes use both metrics together to shape building form.
Structures That Count—and the Grey-Zone Items
Every jurisdiction defines “building footprint” slightly differently. Most agree on the obvious items; the disputes happen at the edges.
- Almost always counted: main house, attached garage, detached garage or workshop, pool house with walls and roof.
- Usually counted: covered porches (roof supported by posts), carports, enclosed breezeways between buildings.
- Depends on the code: eave overhangs beyond a threshold (often 2–3 ft), open pergolas without solid roofing, cantilevered upper floors that extend past the ground-level wall.
- Rarely counted as coverage: uncovered decks, patios at grade, driveways, swimming pools (though these may count toward a separate impervious-surface cap).
On a multi-structure lot—say a house, a detached garage, and a covered patio—the calculator adds every included footprint and divides by lot area. Miss one structure and your calculated coverage is lower than reality, which means the permit reviewer’s number will be higher than yours.
8,000 Square Feet, 40 % Cap, House Plus Garage: Step by Step
Lot: 8,000 sq ft, R-1 zone, 40 % maximum coverage. Structures: main house footprint 2,400 sq ft, detached two-car garage 480 sq ft.
- Total footprint: 2,400 + 480 = 2,880 sq ft
- Coverage: 2,880 ÷ 8,000 = 36.0 %
- Max allowed footprint: 8,000 × 0.40 = 3,200 sq ft
- Remaining capacity: 3,200 − 2,880 = 320 sq ft
The lot passes at 36 %, but only 320 sq ft of footprint capacity remains. A 12 ft × 24 ft covered porch (288 sq ft) would still fit. A 400 sq ft pool house would not—you would need a variance or would have to shrink the garage. If the homeowner later wants to add a carport, they have already consumed 36 % and the carport has to stay under 320 sq ft to avoid a code violation.
Notice that coverage says nothing about how tall the house is. A second storey adds zero to coverage but increases FAR. That is why planners check both metrics: coverage controls the footprint, FAR controls the bulk.
Six Mistakes That Push Coverage Over the Cap
- Forgetting the covered porch. A 200 sq ft covered front porch and a 150 sq ft covered back patio add 350 sq ft of footprint. On a tight 6,000 sq ft lot with a 35 % cap, that is almost 17 % of the total allowance consumed by features most people don’t even think of as “building.”
- Counting total floor area instead of footprint. A two-storey house with 1,800 sq ft per floor has 3,600 sq ft of total area but only 1,800 sq ft of footprint. Using 3,600 in the coverage formula doubles the real percentage and makes a compliant project look like it fails.
- Eave overhangs that cross the threshold. Many codes ignore overhangs up to 2 ft. A deep Craftsman eave at 3 ft adds a perimeter strip around the entire roof. On a 40 ft × 50 ft house, that extra foot of overhang adds roughly 180 sq ft to the footprint.
- Carports treated as open structures. A carport has a roof, posts, and no walls, so some homeowners assume it is exempt. Most codes count any roofed area as coverage regardless of whether walls exist. A two-car carport at 400 sq ft can be the item that tips the lot over the cap.
- Pool houses and cabanas added after occupancy. The original house was approved at 38 % on a 40 % cap. Years later the owner adds a 300 sq ft pool cabana without checking coverage. The lot is now at 41.8 % and technically in violation—discovered when the next buyer orders a survey.
- Multi-structure lots with shared-wall confusion. A house with an attached garage shares a wall. The footprint is the outer perimeter of both, not the sum of each measured separately (which would double-count the shared wall). The error is small—usually under 50 sq ft—but on a lot close to the cap it matters.
What the Calculator Leaves Out
The tool divides footprint by lot area and compares the result to the cap you enter. It does not look up your zoning district, interpret overlay provisions, distinguish roofed from unroofed structures, or measure eave overhangs. Impervious-surface limits (which include driveways, patios, and walkways in addition to buildings) are a separate metric not calculated here. Conversions use the standard 43,560 sq ft per acre. For anything that goes on a permit application, verify the coverage definition with your local planning department and have a licensed surveyor confirm footprint measurements.
Need to check floor area limits in addition to coverage? Run the FAR/FSI calculator. Estimating how many lots a larger parcel can yield? Try the plot division planner. Planning a perimeter fence around a newly subdivided lot? Use the fence materials estimator.