Quick answer
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.
It controls the bulk of building on a lot rather than its footprint. An ADU adds to the total, so a lot already near its FAR has little room left.
Quick facts
- Formula
- Gross floor area of all buildings ÷ lot area
- Written as
- A decimal (0.45) or ratio (0.45:1); occasionally a percentage
- Who sets it
- The local zoning code
- Counts
- Every story, not just the ground floor
- Often excluded
- Basements below grade, some garages, attic space under a set height (varies by code)
Key takeaways
- FAR is total floor area on every story divided by lot area.
- It limits how much building a lot holds, not where it stands.
- Each city writes its own FAR and its own list of what counts.
- An ADU usually adds to the total, so a full lot has little room left.
- In California, FAR cannot block an ADU of at least 800 sq ft with 4-foot side and rear setbacks 1.
- Outside California, rules vary by state and city.
Why FAR matters to you
FAR is the number that decides how big your ADU can be once your house is counted. If your house is large for its lot, the FAR may already be used up.
It also shapes later plans. Floor area you give the ADU is floor area you cannot add to the main house.
Plan check counts it closely. A missed room, a tall ceiling or a garage can change the total and send drawings back for changes.
How to work out FAR for your lot
- Find your exact lot area on a survey or the county assessor record.
- Look up your zoning district and its FAR in the city zoning code.
- Read how that code defines floor area. Note what it leaves out.
- Add up the floor area of every building on the lot, on every floor.
- Divide by the lot area. That is your current FAR.
- Multiply the lot area by the allowed FAR and subtract what exists. The rest is room for the ADU.
What FAR controls
FAR is a single number for how much building a lot may hold. The formula below shows it both ways. Multiply the lot area by the FAR and you get the maximum total floor area, which the owner can spread over one story or stack over several.
Where lot coverage limits how much ground a building covers, FAR limits its total size. A city that wants smaller houses on large lots, or less bulk on small ones, usually reaches for FAR. Many suburban codes use both, plus setbacks and a height limit.
The “floor area” in the formula is defined by each code. Some count a garage, some exempt the first few hundred square feet of it; some count stair openings twice, once per floor; some exempt basements that sit mostly below grade.
The definition section of your local zoning code decides, and plan check applies it line by line.
FAR = F ÷ L so Maximum F = L × FAR
F- Total floor area of every building on the lot, all stories, in square feet, as your local code defines floor area
L- Lot area in square feet, from a survey or the assessor record
FAR- The ratio your zoning district sets, written as a decimal (0.5) or a ratio (0.5:1)
Room left for an ADU = (L × FAR) − floor area already built. In California, state law can still allow a modest ADU where this number is zero; see the California section.
Worked example: FAR on a hypothetical 6,000 sq ft lot
Hypothetical inputs, for the arithmetic only: a 50 × 120 foot lot in a district with a FAR of 0.45, and a two-story house of 2,450 sq ft with a 400 sq ft detached garage that the code counts.
| Step | Math | Result |
|---|---|---|
| Lot area | 50 ft × 120 ft | 6,000 sq ft |
| Maximum floor area | 6,000 × 0.45 | 2,700 sq ft |
| Existing floor area | 2,450 + 400 | 2,850 sq ft (FAR 0.475, already over) |
| Floor area left for a new ADU | 2,700 − 2,850 | None under the local rule |
| FAR with a 750 sq ft ADU added | (2,850 + 750) ÷ 6,000 | 0.60 |
Many older houses exceed a FAR adopted after they were built, which makes them legal nonconforming. Under a strict local reading this lot could take no new ADU at all.
In California it can: state law stops FAR from blocking an ADU of at least 800 sq ft with 4-foot side and rear setbacks, so the 750 sq ft unit is protected, provided it meets the city’s other objective standards.
California’s rule for ADUs and FAR
California Government Code section 66321(b)(3) says a city or county may not impose limits on floor area ratio, lot coverage, open space, front setbacks or minimum lot size that do 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; before SB 477 moved the ADU law into Chapter 13 in 2024, this rule sat in Gov. Code 65852.2(c).
The same paragraph also bars a minimum or maximum ADU size set as a percentage of the primary dwelling where that would prevent the 800 sq ft unit. Above 800 square feet, the local FAR applies again, along with the city’s other objective standards 3.
Chapter 13 defines living area as interior habitable area including basements and attics but not a garage or accessory structure 2. That is the state definition for ADU purposes; your city’s FAR definition may count space differently for the rest of the lot.
Outside California, whether FAR can be waived for an ADU depends on state and local law. See ADU laws by state.
FAR vs lot coverage vs setbacks
| Limit | Measures | A second story… | Typical ADU effect |
|---|---|---|---|
| Floor area ratio | Total floor area on all stories | Adds to it | Caps total ADU size |
| Lot coverage | Footprint on the ground | Adds nothing | Caps a one-story ADU’s footprint |
| Setback | Distance from property lines | Adds nothing (some codes add step-backs) | Fixes where the ADU may stand |
| Height limit | Vertical height | Must fit under it | Decides if two stories are possible |
Where FAR shows up in an ADU project
- On the site plan in the permit set, as a floor-area table that lists every building and the resulting ratio.
- In plan-check corrections, when a reviewer counts a space the designer left out.
- In design choices: a loft, a high ceiling counted as two floors, or a garage conversion can change the total.
- In future additions: floor area used by an ADU is not available for enlarging the main house later.
Misreadings that cost time
- Myth A FAR of 0.5 means half the lot can be built on.
- Reality That describes lot coverage. FAR 0.5 means total floor area equal to half the lot area, on any number of stories.
- Myth If my house is over the FAR, I cannot add an ADU in California.
- Reality State law protects an ADU of at least 800 sq ft with 4-foot side and rear setbacks from FAR limits, as long as it meets other standards.
Second example: one story or two on a 5,000 sq ft lot
Hypothetical inputs only. The lot is 50 by 100 feet. The district allows a FAR of 0.5 and caps coverage at 40%. The one-story house has 1,600 sq ft of floor area and a 1,600 sq ft footprint.
| Step | Math | Result |
|---|---|---|
| Lot area | 50 ft × 100 ft | 5,000 sq ft |
| Floor area allowed | 5,000 × 0.5 | 2,500 sq ft |
| Floor area left | 2,500 − 1,600 | 900 sq ft |
| Footprint allowed by coverage | 5,000 × 40% | 2,000 sq ft |
| Footprint left | 2,000 − 1,600 | 400 sq ft |
| A 600 sq ft ADU, one story | 600 sq ft footprint | Fits FAR, fails coverage |
| A 600 sq ft ADU, two stories of 300 | 300 sq ft footprint | Fits both, if the height limit allows |
This shows why FAR and coverage pull in different directions. Going up saves footprint but not floor area. In California, both limits give way for a unit up to 800 sq ft with 4-foot side and rear setbacks, so the one-story option would also be protected there 1.
How codes write and count FAR
| Variation | What it means | What to check |
|---|---|---|
| Decimal (0.45) | Floor area may equal 45% of the lot area | The most common form |
| Ratio (0.45:1) | Same number, written as a ratio | Read it the same way |
| Sliding FAR | The ratio drops as lots get bigger | Find the row for your lot size |
| Gross floor area | Measured to the outside of the walls | Most codes use this |
| Exempt spaces | Basements, some garage area, low attics | Your code’s definition list |
California’s size limits beside FAR
FAR is a local tool. California also sets its own ADU size rules. Under Government Code section 66314, a detached ADU may not exceed 1,200 sq ft of floor area 3.
Section 66323 lists ADUs a city must approve. One is a new detached ADU with 4-foot side and rear setbacks on a single-family lot. The city may cap it at 800 sq ft of livable space 4.
So in California, 800 sq ft is the size FAR cannot block. A unit larger than that must fit within the local FAR as well.
FAR rules in other states
Outside California, FAR for an ADU is usually set by the city or county. A few states have ADU laws that limit local size rules. Most leave FAR alone.
Rules vary. Start with the state ADU law tracker, then read your local code.
How FAR affects cost
FAR has no fee of its own. It changes cost by changing what you can build.
A low FAR pushes you toward a smaller unit. A small unit costs less in total but often more per square foot, because the kitchen and bathroom cost the same in any size.
Converting space that already counts, such as a garage the code includes, may add no new floor area. See the ADU cost guide for cost drivers.
Benefits and limits of a FAR cap
What it does well
- Keeps building bulk in line with lot size.
- Gives the owner freedom over shape: one story or two.
- Simple to check with one division.
Where it falls short
- Large existing homes may leave no room for an ADU.
- Each city counts floor area differently.
- Can push owners to small units that cost more per square foot.
What to ask the planning department
- What FAR applies to my district and lot size?
- Does my garage count? Does a basement or attic?
- Is a tall ceiling counted twice?
- How do you apply the state 800 sq ft protection to FAR?
- Does the ADU’s floor area reduce what I can add to my house later?
Checklist for your project
- Confirm lot area and district.
- Copy the FAR rule and the floor-area definition.
- Total the floor area of everything you will keep.
- Compare the room left with the ADU size you want.
- In California, check if the plan fits the 800 sq ft protection.
- Show a floor-area table on the site plan.
- Keep the numbers the same on every sheet. The city must decide completeness within 15 business days in California 5.
Run the numbers
- ADU size envelope calculatorFloor area, footprint and height limits together
- ADU type selectorDetached, attached or conversion
Related guides and research
Questions about floor area ratio
How do you calculate floor area ratio?
Divide the total floor area of all buildings on the lot, counting every story, by the lot area. A 2,700 sq ft total on a 6,000 sq ft lot is a FAR of 0.45. Your zoning code defines which spaces count.
What does a FAR of 1.0 mean?
Total floor area may equal the lot area. On a 5,000 sq ft lot that is 5,000 sq ft of building, for example two stories of 2,500 sq ft each.
Does an ADU count toward FAR?
Usually yes under local codes, because it is floor area on the lot. In California, though, FAR cannot be used to block an ADU of at least 800 sq ft with 4-foot side and rear setbacks (Gov. Code 66321(b)(3)).
Is FAR the same as lot coverage?
No. FAR counts total floor area on every story; lot coverage counts only ground footprint. A two-story building raises FAR without changing coverage.
What is a good floor area ratio for a house?
There is no single good number. Each city picks a FAR for each district to match the look it wants. A lower FAR means smaller buildings for the lot size. The only number that matters for your ADU is the one in your local zoning code.
Does a garage count in floor area ratio?
It depends on the local code. Some cities count all garage space, some exempt the first few hundred square feet, and some exempt it fully. In California, the state definition of ADU living area leaves out garages, but your city’s FAR definition may still count them for the rest of the lot.
Do basements count toward FAR?
Often not, if they sit mostly below ground. Many codes exempt basement space below a set height above grade. Check your code’s floor-area definition before you plan a basement ADU or count one out.
How big can a detached ADU be in California?
Government Code 66314 caps a detached ADU at 1,200 sq ft of floor area. Cities can set lower limits, but they cannot use FAR or similar rules to stop a unit of at least 800 sq ft with 4-foot side and rear setbacks.
What does a FAR of 0.5 mean on a 6,000 sq ft lot?
It means the total floor area of all buildings can be up to 3,000 sq ft. You could build that on one floor, if coverage allows, or spread it across two floors. The ADU counts toward that total under most local codes.
Sources
- California Legislature, Government Code §66321 (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
- California Legislature, Government Code §66313, definitions (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
- California Legislature, Government Code §66314, objective standards (as amended by AB 1170, Stats. 2025, ch. 67), retrieved .
- California Legislature, Government Code §66323, ADUs a local agency must approve ministerially (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
- California Legislature, Government Code §66317, ministerial review and timelines (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The California rule was read in the current text of Government Code §§66313, 66314 and 66321 on leginfo.legislature.ca.gov on 7 October 2026. Cities may impose objective standards under §66314 [g66314].
The worked example uses stated hypothetical figures only. Government Code §§66314 (1,200 sq ft detached cap), 66317 and 66323 were also read on 7 October 2026. The second worked example uses stated hypothetical inputs only.
Suggest a correction. We fix errors and say what changed.