Quick answer
ADU cost per square foot is the cost of an accessory dwelling unit divided by its floor area, a shorthand for comparing units of different sizes.
It is only as good as its two inputs: which costs went into the top number (construction only, or design, permits and site work too) and which area went into the bottom (interior livable space or gross footprint). Two honest quotes for the same unit can differ by a third on this alone.
Quick facts
- Formula
- Total cost ÷ square feet
- One California benchmark
- $250/sq ft median, ADUs permitted or built 2018–19 (statewide owner survey, 2021) 1
- Smaller units
- Cost more per square foot, because fixed costs are spread over fewer feet
- Not the same as
- A city’s valuation rate for permit fees (Sacramento: $170.80/sq ft in 2026) 3
- Check before comparing
- What is included above the line, and which area is below it
Key takeaways
- Cost per square foot = a cost ÷ an area. Both parts must match before two numbers can be compared.
- Small ADUs cost more per foot. Kitchens, baths and hookups cost about the same at any size.
- One dated California benchmark: a $250 per square foot median for ADUs permitted or built in 2018–19 1.
- A city’s permit valuation rate is a fee input, not a build price 3.
- Builder per-foot quotes usually leave out design, fees and site work.
- Use per-foot figures to compare, never to set your budget.
What it means, in plain English
Say an ADU cost $200,000 and has 500 square feet of floor. Divide one by the other. You get $400 per square foot.
That single number lets you compare a small unit with a big one. It is handy. It is also easy to misuse.
The trouble is the words “cost” and “square foot”. Which costs? Just the builder’s work, or design and fees too? Which square feet? The inside living space, or the outside footprint? Change either answer and the number changes.
Why it matters to you
You will hear per-foot numbers everywhere. Builders quote them. Neighbors share them. Websites list them.
If you plan a budget from someone else’s per-foot figure, you may be far off. Their number may leave out site work. Their unit may be twice the size of yours.
Used well, the number helps you spot a bid that is out of line. Used badly, it hides the costs that sink budgets. The rest of this page shows how to use it well.
How the number is built
The arithmetic is simple. The definitions are not. A builder quoting “$X per foot” usually means its own construction scope divided by the unit’s conditioned (heated and cooled) floor area. A homeowner asking “what did your ADU cost per foot?” usually means everything they paid divided by the size on the listing.
Before comparing two figures, ask three things. Does the top line include soft costs such as design and fees? Does it include site work and utility runs? Those are hard costs that bids often exclude. And is the area interior livable space or the exterior footprint?
California’s ADU law measures size in interior livable space. It defines livable space as space intended for living, sleeping, eating, cooking or sanitation 4. A garage, a porch or wall thickness does not count toward that.
The formula below shows both parts. Two per-foot numbers compare only when the top line and the bottom line were built the same way.
Cost per sq ft = C ÷ A
C- The cost being counted, in dollars: builder scope only, or all-in (hard costs + soft costs + site work)
A- The area, in square feet: interior livable space (California’s ADU measure) or the outside footprint
State which C and which A you used next to every per-foot number you write down.
Source: [4]
Sources: [4]
Worked example: why a smaller ADU costs more per foot
A hypothetical, for arithmetic only. Assume every unit carries the same fixed costs (a kitchen, a bathroom, a utility trench, a permit) of a hypothetical $90,000, plus variable costs of a hypothetical $200 for each square foot of floor, walls and roof.
| Unit size | Fixed + variable | Total | Per sq ft |
|---|---|---|---|
| 400 sq ft | $90,000 + (400 × $200) | $170,000 | $425 |
| 600 sq ft | $90,000 + (600 × $200) | $210,000 | $350 |
| 800 sq ft | $90,000 + (800 × $200) | $250,000 | $313 |
Doubling the size from 400 to 800 sq ft raises the total by about 47% but lowers the per-foot figure by about 26%. That is why a per-square-foot price from a large unit understates a small one. The inputs are invented; the pattern is the point.
Worked example: same cost, different area
A hypothetical $240,000 all-in project, for arithmetic only. The only thing that changes is which area goes below the line.
| Area used | Square feet (hypothetical) | Math | Per sq ft |
|---|---|---|---|
| Interior livable space | 560 | $240,000 ÷ 560 | $429 |
| Gross footprint (outside of walls) | 620 | $240,000 ÷ 620 | $387 |
| Footprint plus covered porch | 700 | $240,000 ÷ 700 | $343 |
One unit, one cost, three per-foot numbers 25% apart. Always ask which area a figure uses.
Types of per-square-foot number
| Number | Who produces it | What it is for |
|---|---|---|
| Builder’s quote per sq ft | A contractor or prefab maker | Pricing its own scope; often excludes site work, design and fees |
| All-in cost per sq ft | You, after the project | Comparing your real outlay with another project |
| Survey median per sq ft | A researcher, from owner reports | A dated benchmark; the 2021 California survey gives $250 1 |
| Permit valuation per sq ft | The city’s fee table | Calculating permit fees; Sacramento uses $170.80 for new ADU floor area on its 2026 estimate 3 |
Valuation rate vs build price
A city’s permit valuation per square foot sets the fee. It is not what a builder will charge. Sacramento’s 2026 rate of $170.80 for new ADU floor area is a fee input only 3.
What published data says, and its limits
The statewide California owner survey from UC Berkeley’s Center for Community Innovation was published by the Terner Center in April 2021.
It found a median of $150,000, or $250 per square foot, for ADUs permitted or completed in 2018 and 2019.
It also found 37% of units cost less than $100,000 and 71% less than $200,000.
A Terner Center financing paper from July 2022 adds that 13% of ADUs in that data were estimated to cost more than $300,000 2. So the spread is wide. The figures below put that spread side by side.
Read all of this as a dated, California-only benchmark from owners who finished. It is not a current price. Region, unit type and size all move it. Our California cost study breaks the published figures down.
ADU cost statistics lists every figure with its source. Outside California we know of no comparable statewide survey; treat national per-foot numbers you see online as unverified.
- $250Median cost per square footMedian total $150,000 1
- 37%Cost under $100,0001
- 71%Cost under $200,0001
- 13%Estimated over $300,0002
California only, owner-reported, 2018–19 projects. A dated benchmark, not a current price. Source: [1] [2]
Where you will see a per-foot number
- A builder’s sales sheet or first call. Ask what scope it covers.
- A prefab price list. Usually the factory unit only, before site work and delivery.
- A city fee sheet. A valuation rate used to set permit fees 3.
- An appraisal. A value per foot, which is about price, not cost.
- Research and news stories. Check the year, place and what was counted.
What pushes the per-foot figure up or down
- Size: fixed costs spread over fewer feet raise it (see the first example).
- Type: a garage conversion reuses a slab and walls; a detached new build pays for all of them. Compare with the conversion vs new build tool.
- Kitchens and bathrooms: the most expensive rooms per foot; a second bathroom moves the figure more than extra living space does.
- Site: slope, access and long utility runs add cost without adding a single square foot.
- Fees: in California, crossing 750 sq ft of interior livable space can switch on impact fees. See ADU permit fees.
- Region: labor costs differ sharply between metro areas.
The rules: how California measures ADU size
No law sets a price per square foot. But size rules decide which square feet count. In California, ADU size limits and fee thresholds use interior livable space, as defined in Government Code § 66313 4.
Other states and cities measure differently. Some use gross floor area. Some count lofts or exclude them. Check the local code, and see ADU laws by state. Use the same area for every number you compare.
Sources: [4]
Benefits and limits of per-foot pricing
What it does well
- Lets you compare units of different sizes quickly.
- Flags a bid that is far above or below others on the same scope.
- Easy to explain to a lender or partner.
Where it falls short
- Hides fixed costs, so small units look cheap when priced from big ones.
- Means different things to builders, owners and cities.
- Ignores site conditions, which add cost but no floor area.
- Goes stale fast; survey medians are years old.
Cost per square foot compared with related terms
| Term | What it is | Use it for |
|---|---|---|
| Cost per square foot | A cost divided by an area | Comparing like with like |
| Cost estimate | An itemized prediction of total cost | Budgeting and contracts |
| Hard costs | Labor, materials, site work | What most per-foot quotes cover |
| ADU ROI | Return measured against total cost | Judging if the unit pays off |
| Permit valuation | A city fee input | Fee math only |
Common mistakes and misreadings
- Myth A per-foot quote times my size is my budget.
- Reality It leaves out fixed costs, site work and soft costs. Use a full estimate.
- Myth A bigger ADU always costs more per foot.
- Reality Usually the reverse. Fixed costs spread over more feet lower it.
- Myth The city valuation is what it costs to build.
- Reality It is a fee input. Sacramento’s 2026 rate is $170.80, which is not a build price 3.
- Myth The 2021 survey median is today’s price.
- Reality It covers 2018–19 California units 1. Costs have changed since.
What to ask when someone quotes a per-foot price
- What costs are inside this number? Design? Permits? Site work? Utility trench?
- Which square feet did you divide by: interior livable space, or the outside footprint?
- What size unit was it? Is mine similar?
- What year, and what city or region?
- What finish level and how many bathrooms?
- Can you give me a full itemized bid instead?
Checklist: work out your own cost per square foot
- Get an itemized bid on your drawings.
- Add any excluded site work and utility runs at real quotes.
- Add design, engineering and permit fees from your city’s fee sheet.
- Add loan costs if you borrow.
- Measure interior livable space from your plans.
- Divide the all-in total by that area.
- Compare only with figures built the same way.
- Test sizes in the size envelope calculator and the cost calculator.
From quote to comparable figure
- Builder quote
- + site work and utility runs
- + design, engineering and permit fees
- = all-in total
- ÷ interior livable sq ft
- = comparable cost per sq ft
Related guides, tools and research
- Bid comparison toolPut two quotes on the same scope
- ADU size distributionHow big California ADUs actually are
- ADU size statisticsSize figures with sources
- ADU cost guideEvery cost line in one place
Questions about adu cost per square foot
How much does an ADU cost per square foot?
The statewide California owner survey found a median of $250 per square foot for units permitted or built in 2018–19 1. That is a dated benchmark. Your figure depends on size, type, site and region, and on whether design, fees and site work are counted.
Why does a small ADU cost more per square foot?
A kitchen, a bathroom, utility connections and permits cost roughly the same whatever the unit’s size. Spread over fewer square feet, those fixed costs push the per-foot figure up.
Is a garage conversion cheaper per square foot?
It often is, because the slab, walls and roof already exist. But the old structure may need upgrades to meet code. Compare both options with your own numbers in the conversion vs new build tool.
Is the permit valuation the cost per square foot?
No. A city’s valuation rate is used to calculate fees. Sacramento’s 2026 estimate uses $170.80 per square foot 3; it is a fee input, not a construction price.
How do you calculate cost per square foot for an ADU?
Add every cost you want to count, such as the bid, site work, design and fees. Then divide by the floor area, ideally interior livable space. Say which costs and which area you used, so others can compare.
Does cost per square foot include permits and design?
Not when a builder quotes it, usually. Builder quotes tend to cover construction only. An all-in figure you calculate yourself should include design, permits and fees.
What square footage counts for an ADU in California?
State ADU law uses interior livable space: space for living, sleeping, eating, cooking or sanitation 4. Ask your city how it measures, because that number also sets fee thresholds.
Is it cheaper per square foot to build a bigger ADU?
Usually yes per foot, because fixed costs are spread wider. But the total goes up, and in California a unit over 750 sq ft of interior livable space can owe impact fees. Check both numbers.
Are online ADU cost per square foot numbers reliable?
Treat them with care. Many give no year, place or scope. Prefer figures with a named primary source and date, and confirm with bids on your own drawings.
Sources
- Terner Center for Housing Innovation, UC Berkeley, “First Ever Statewide ADU Owner Survey Shows Growth, Room for Improvement” (22 April 2021), reporting the Center for Community Innovation survey, retrieved .
- Terner Center for Housing Innovation and USC Lusk Center for Real Estate, “ADU Construction Financing: Opportunities to Expand Access for Homeowners” (July 2022), retrieved .
- City of Sacramento Community Development, CDD-0419 ADU Fee Estimate, New Construction (fee schedule worksheet, revised 07-12-2026), retrieved .
- California Legislature, Government Code § 66313 (ADU definitions, including “livable space”), 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: Survey figures from the Terner Center publication of the 2021 CCI survey and its July 2022 financing paper; Sacramento rate from the city’s published PDF; definitions from Gov. Code § 66313 on leginfo, all read 7 October 2026. Worked examples use hypothetical inputs for arithmetic only.
Suggest a correction. We fix errors and say what changed.