What Is Lot Coverage?

Key takeaways

• Lot coverage is the share of your lot that sits under a roof, seen from above.
• Your city sets the cap. It can differ from one zoning district to the next.
• A second story adds floor area but no new coverage.
• Turning an existing garage into an ADU adds no new footprint.
• In California, a coverage cap cannot block an ADU of up to 800 sq ft with 4-foot side and rear setbacks ¹.
• Above 800 sq ft in California, and in most other states, the local cap applies in full.

Why lot coverage matters to you

Coverage is often the first number that limits a backyard ADU. Your house and garage already use part of the cap. The ADU gets only what is left.

If you guess wrong, the plan checker (the city staff member who reviews your drawings) will send the plans back. That costs a redesign and more weeks.

Knowing the number early tells you which ADU shape makes sense. You may choose a smaller unit, a two-story unit, or a garage conversion.

What lot coverage measures

Picture the lot from directly above. Every roofed structure throws a shadow on the ground: the house, a detached garage, a shed above a certain size, a covered patio. Add those footprints together, divide by the area of the lot, and the result is lot coverage. The formula below writes it out.

Cities use it to control how much of a parcel is built on, which protects light, drainage, trees and the open feel of a street.

It is one of three numbers that together set a lot’s building envelope, alongside the setback lines that fix where a building may stand and the height limit that fixes how tall it may be.

Lot coverage is not the same as floor area ratio. Coverage counts ground footprint only, so a second story adds nothing to it. FAR counts every floor, so a second story adds a lot. A two-story ADU can pass a coverage cap that a one-story ADU of the same floor area would break.

What usually counts toward coverage

Worked example: a hypothetical 6,000 sq ft lot

All inputs below are hypothetical, chosen to show the arithmetic. The lot is 50 feet wide and 120 feet deep, and the zoning district caps lot coverage at 40%.

Under the local rule alone, this owner could convert the garage (no new footprint), build up, or build an ADU of about 300 sq ft. In California the answer changes: state law says the coverage cap cannot stop an ADU of at least 800 sq ft that meets 4-foot side and rear setbacks, so the cap gives way for a unit up to that size, as explained below.

California’s rule: coverage cannot block a modest ADU

In California, Government Code section 66321(b)(3) bars a city or county from applying limits on lot coverage, floor area ratio, open space, front setbacks and minimum lot size in a way that does not permit an ADU of at least 800 square feet of interior livable space with four-foot side and rear setbacks, built in compliance with all other local development standards ¹.

The section was last amended by SB 543 (Stats. 2025, ch. 520), effective 1 January 2026; this rule was formerly in Gov. Code 65852.2(c) before SB 477 moved the ADU law into Chapter 13 in 2024.

Two limits on that protection matter. First, it covers 800 square feet, not any size: above 800, the local coverage cap applies again. Second, the ADU must still meet the city’s other objective standards, such as height, which section 66314 allows a local ordinance to impose ².

“Interior livable space” is the habitable floor area inside the unit. The Chapter 13 definition of living area excludes a garage or any accessory structure ³.

This is California’s rule only. Other states set their own ADU protections, and many have none; see the ADU laws by state tracker.

Sources: [1] [2]

How to check lot coverage on your own lot

1. Find the lot area on the county assessor record or a survey.
2. Look up the zoning district and its coverage cap in the city’s zoning code, and read how that code defines coverage.
3. Measure the footprint of every roofed structure, including covered patios.
4. Divide the total footprint by the lot area to get current coverage.
5. Subtract from the cap to see the footprint left; in California, compare that with the 800 sq ft protection.
6. Show the numbers on the permit set’s site plan, where plan check will verify them.

Common misreadings

Myth A garage conversion adds lot coverage.

Reality Converting an existing roofed structure does not add footprint, which is one reason conversions are easier to fit.

Myth Lot coverage and FAR are the same limit.

Reality Coverage counts footprint; FAR counts total floor area on all floors. A lot can pass one and fail the other.

Myth In California, lot coverage no longer applies to ADUs.

Reality It still applies above the 800 sq ft protection and to every other structure on the lot.

Where coverage sits in the zoning chain

1. Zoning district → lot size → coverage cap → footprint left → ADU size and shape

Second example: a small lot and a garage conversion

Again, every input is hypothetical. The lot is 40 feet wide and 100 feet deep. The district caps coverage at 45%.

Older homes are often over today’s cap because the rules changed after they were built. Converting the existing garage adds no footprint. In California, state law also says no setback can be required for an existing structure converted to an ADU ².

Where you will see lot coverage

• The zoning code table for your district, often next to setbacks and height.
• The site plan in your permit set. It lists each building’s footprint and the total coverage.
• A plan check comment letter, if the numbers do not add up.
• A designer’s or builder’s feasibility report, before any drawings start.
• A survey, which gives the exact lot area the math depends on.

California’s “must approve” path for a detached ADU

California has a second rule that helps small lots. Government Code section 66323 lists ADUs a city must approve, even if its other rules say no ⁴.

One is a new detached ADU with side and rear setbacks of no more than 4 feet on a lot with a single-family home. The city may limit that unit to 800 sq ft of livable space and to the state height floors. It cannot use lot coverage to stop it.

This is California law only. It applies in residential and mixed-use zones.

Sources: [4]

Rules in other states

Outside California, lot coverage is almost always a purely local number. Some states have passed ADU laws, and a few limit how cities can apply size rules. Many have not.

Rules vary, so check the state-by-state ADU law tracker and then your city’s code.

Lot coverage compared with related limits

What coverage does to cost

Coverage does not have a price of its own. It shapes cost by shaping the design.

A tight cap can push you toward a second story. That adds stairs, structure and often more engineering. It can also push you toward a garage conversion, which reuses a slab and walls you already have.

Removing a shed or patio cover to free footprint is another cost to weigh. For real numbers, see the ADU cost guide and the cost calculator.

Benefits and limits of a coverage cap

What to ask the planning department

• What is the coverage cap for my zoning district?
• Do covered patios, eaves or sheds count, and above what size?
• Is there a separate limit on paving or impervious surface?
• Does the state 800 sq ft protection apply to my lot, and how do you apply it?
• If my lot is already over the cap, can I still convert my garage?
• Do you want coverage shown as a table on the site plan?

Checklist for your project

1. Get the exact lot area from a survey or the assessor.
2. Write down your district’s cap and its definition of coverage.
3. Measure every roofed structure you will keep.
4. Work out the footprint left for the ADU.
5. Compare one-story, two-story and conversion options.
6. In California, check if your plan fits the 800 sq ft and 4-foot rule.
7. Put the coverage math on the site plan before you submit. In California, the city then has 15 business days to tell you if the application is complete ⁵.

Check your lot

• ADU size envelope calculatorFootprint, setbacks and height together
• ADU feasibility checkerWhether your lot can take an ADU

Related guides and research

• ADU regulations explained
• How big ADUs actually are
• California ADU ordinance tracker
• ADU rules by location

Related terms

Floor Area Ratio

Floor area ratio (FAR) is the total floor area of all buildings on a lot, on every story, divided by the lot’s area, so a FAR of 0.5 allows 3,000 square feet of building on a 6,000 square foot lot.

Setback

A setback is the minimum distance a zoning code requires between a building and a property line, measured horizontally from the line to the nearest wall or other regulated part of the structure.

Height Limit

A height limit is the maximum vertical height a zoning code allows for a building, measured from a reference point on the ground to a reference point on the roof that the local code defines.

Minimum Lot Size

A minimum lot size is the smallest parcel area, and sometimes width or depth, on which a zoning code allows a certain building or use.

Buildable Area

Buildable area is the part of a lot where a building may legally and physically go, found by subtracting setbacks, easements and other no-build areas from the whole parcel.

ADU Zoning

The local zoning rules that say where an ADU may go on a lot and how big and tall it may be. In California, state law requires cities to allow ADUs on lots zoned for residential use and limits how far local zoning can restrict them.

ADU Permit

An ADU permit is the building permit a city or county issues to allow construction of an accessory dwelling unit, after checking the plans against local zoning standards and the building code.

Also referenced from: ADU Size, ADU Site Plan, Zoning District, Floor Area Ratio, Setback, Height Limit, Minimum Lot Size, Buildable Area