JB PARCELLot Coverage Calculator
Enter the lot size, what is built on it, and the limit from the zoning ordinance. See how much of the lot is covered, the largest footprint the district allows, and how much room is left for an addition, a garage, or an accessory dwelling.
How this calculator works
Lot coverage is the building footprint divided by the lot area. A 1,800 square foot footprint on a 10,890 square foot lot, a quarter acre, covers 16.5 percent of it. The footprint is what sits on the ground, seen from above, so a second story adds floor area without adding any coverage.
The limit comes from the zoning ordinance for the district the lot sits in. Multiply the lot area by that percentage and you have the largest footprint the district allows. Subtract what is already built and you have the room left for an addition, a detached garage, or an accessory dwelling unit.
Impervious surface is a separate and usually larger number. It adds driveways, parking, patios, walks, and pool decks to the buildings, because all of them shed stormwater instead of soaking it in. Many ordinances cap it on its own, and on a lot with a long driveway it is often the limit you hit first.
Before you underwrite an addition or an ADU, check the ordinance definitions. Jurisdictions disagree about whether decks, covered porches, eaves, and pools count, and a few square feet of definition can decide whether the project fits.
Worked example: room for an addition on a quarter-acre lot
A 10,890 sf lot with a 2,400 sf house and garage footprint, a 900 sf driveway, and a 400 sf patio, in a district that caps building coverage at 30 percent.
| Building coverage | 22.04% |
|---|---|
| Largest footprint allowed | 3,267 sf |
| Footprint left to build | 867 sf |
| Impervious surface | 33.98% |
A 1,000 sf single-story addition would push coverage to 31.2 percent and fail. The same floor area split over two stories needs only a 500 sf footprint and fits with room to spare. That is the whole reason the second story exists on so many infill projects.
Where the numbers come from
Coverage limits and what counts toward them come from each jurisdiction's zoning ordinance. Nothing on this page is the legal limit for a specific lot. Read the ordinance for the district the parcel is zoned and confirm with the planning department.
Questions investors ask
What is lot coverage?
The share of a lot covered by buildings, measured by footprint. A 2,400 square foot footprint on a 10,890 square foot lot is 22 percent coverage. Zoning ordinances cap it to keep open space, light, and drainage on each lot.
What counts toward lot coverage?
Buildings with roofs almost always count, attached or detached. Beyond that the ordinances differ: some count decks, covered porches, pools, or roof overhangs past a set depth, and some do not. The definition section of your zoning ordinance settles it.
What is the difference between lot coverage and impervious surface?
Lot coverage counts buildings. Impervious surface counts everything that sheds water, so it adds driveways, parking, walks, patios, and pool decks. Where an ordinance limits both, the impervious limit is usually the higher percentage and often the harder one to meet.
How do I find the coverage limit for a lot?
Find the zoning district first. It is on the parcel in the JB PARCEL map for counties that publish zoning, and on the local zoning map everywhere. Then read that district in the zoning ordinance. Confirm with the planning department before you buy on the strength of it.
Does a second story count toward lot coverage?
No. Coverage is footprint, so building up is how you add space on a lot that is at its limit. Floor area ratio is the rule that counts every story, which you can check in the floor area ratio calculator.
Related calculators
Screening a real property? Pull its appraised value, assessed value, acreage, land use, and last recorded sale off the JB PARCEL parcel map and drop the real numbers into this calculator instead of guessing. Coverage runs across every county we carry.