ADU costs

How much does an ADU cost in California?

Thinking about a backyard cottage, a garage conversion, or a junior ADU and wondering what it costs? Here's what published research actually says about California ADU costs, what goes into the total, and where the numbers run out. Every figure below names its source and the year it describes.

Last reviewed October 19, 2026

1 · The short answer

Short answer

There is no current, statewide price list for ADUs in California. The most-cited research is a statewide survey of ADU owners by UC Berkeley's Center for Community Innovation (CCI), published in April 2021. It found a median total cost of $150,000, or $250 per square foot, for ADUs permitted or completed in 2018 or 2019. That total includes design, labor, materials, and permits. In that survey, 37% of ADUs cost less than $100,000 and 71% cost less than $200,000.

Those are 2018–2019 projects. Construction costs have moved since then (see Why older numbers are a starting point only below), and we don't know of a newer statewide survey of what California homeowners paid. So treat these numbers as history, not a price for today.

Sources: UC Berkeley Center for Community Innovation, Implementing the Backyard Revolution (April 2021, PDF) · Terner Center, First Ever Statewide ADU Owner Survey (April 22, 2021)

2 · The survey

What California ADUs cost: survey results by region and type

The 2021 CCI survey is the only statewide survey we found that reports what homeowners actually paid. Median total cost and median cost per square foot, for ADUs permitted or completed in 2018 or 2019:

CCI survey, 2021. Per-square-foot figures are rounded to the dollar from the survey's table.

Group (CCI survey, 2021)	Median total cost	Median cost per sq ft
All of California	$150,000	$250
Los Angeles County	$100,000	$197
Orange and San Diego Counties	$130,000	$200
Central Coast	$140,000	$223
San Francisco Bay Area	$177,500	$329
Detached ADU	$180,000	$300
Addition to the main house (attached)	$150,000	$250
Garage ADU (conversion, or space added to an existing garage)	$90,000	$189
Conversion of space inside the main house	$100,000	$173
Studio	$100,000	$241
1 bedroom	$150,000	$267
2 bedrooms	$212,500	$250

"Cost" here is the total the homeowner spent, including design, labor, materials, and permits.

What the survey can't tell you:

Inland Empire, Kern, Imperial, and other inland areas aren't broken out. The survey reports four regions with enough responses (Bay Area, Los Angeles County, Central Coast, Orange and San Diego Counties). Everything else was grouped as "other." We have no sourced median for Riverside or San Bernardino County.
It's a survey of people who finished or permitted an ADU. About 4.8% of the households contacted responded. Respondents earned more than typical Californians, and the authors note the Bay Area is over-represented and Los Angeles County is under-represented.
It describes 2018–2019 projects, before the 2020 state ADU law changes took effect.

Sources: UC Berkeley Center for Community Innovation, Implementing the Backyard Revolution (April 2021, PDF)

3 · Other figures

Other published cost figures (and how they differ)

You'll see other numbers online. Here are the ones from sources we consider credible, with what each one is:

Terner Center and CCI policy brief (August 2020): an analysis of ADU construction data found an average (not median) ADU cost in California of $167,000; $148,000 in Los Angeles and $237,000 in the Bay Area. The brief says the gap is driven largely by labor costs. This is a different statistic from the survey median above, from a different data set.
Casita Coalition (statewide ADU nonprofit), 2022: says most projects are $250–$500 per square foot, and that costs typically range from $30,000 for a simple interior conversion to $500,000+ for a large unit with high-end finishes on a hillside lot. The page doesn't say how these figures were developed. It was published in February 2022 and last updated in September 2022.
City of Sierra Madre ADU page: says that "as of 2022," the average cost to build an ADU ranged from $270 to $600 per square foot, typically $120,000 to $500,000, and that city fees averaged $8,000 to $18,000 of that. The city doesn't say where the figures come from or what area they cover.

None of these is a survey of 2025 or 2026 projects. If you see a precise "2026 cost per square foot" online, check what it's based on.

Sources: Terner Center and CCI, Reaching California's ADU Potential (August 2020, PDF) · Casita Coalition, How to build an ADU (published February 2022, updated September 2022) · City of Sierra Madre, Accessory Dwelling Units page

4 · Older numbers

Why older numbers are a starting point only

Construction costs change. One public yardstick: the State Allocation Board adjusts the statewide cap on school developer fees every two years using a construction cost index (the Class B index from RS Means / Gordian). That cap went from $4.08 per square foot in January 2020 to $5.38 in January 2026, about 32% higher. (That percentage is our arithmetic from the Board's published figures.)

That index measures commercial-style building costs, not ADUs, so we don't apply it to the survey numbers above. It's a reminder that a 2018–2019 median is a starting point for your own research, not a price for today.

Sources: DGS/OPSC, Annual Adjustment to SFP Grants and Developer Fee History · State Allocation Board item, Index Adjustment on the Assessment for Development (January 28, 2026, PDF) · Cal. Gov. Code §65995 (school facility fee caps)

5 · What's included

What goes into the total cost

An ADU budget is more than construction. Items that published sources list as part of ADU costs:

Design and planning: architectural designs, property surveys, soil tests, and energy reports. CalHFA's former ADU grant listed these as "pre-development costs," along with site prep, permits, and impact fees.
Permits and fees: city building permit and plan check fees, plus any impact fees, utility connection fees, and school fees that apply. State law limits some of these. See our page on ADU permit and impact fees in California.
Construction: labor and materials. In the CCI survey, "construction cost" included design, labor, materials, and permits.
Site and utility work: demolition, water, sewer, or septic connections, utility improvements, and site leveling. Long Beach's ADU loan program lists these as normal ADU project costs.

Sources: CalHFA ADU Grant Program page · UC Berkeley Center for Community Innovation, Implementing the Backyard Revolution (April 2021, PDF) · City of Long Beach, Backyard Builders Program · Cal. Gov. Code §66311.5 (ADU and JADU fees)

6 · By type

Costs by ADU type

Garage conversion. In the 2021 CCI survey, garage ADUs (conversions, or space added to an existing garage) had a median total cost of $90,000 ($189 per sq ft), the lowest of the ADU types in the survey. Conversions of space inside the main house came in at a median of $100,000 ($173 per sq ft). These are 2018–2019 projects.

Detached ADU. Detached units had the highest median in the survey: $180,000 ($300 per sq ft).

Attached ADU (addition). Additions to the main house had a median of $150,000 ($250 per sq ft).

Junior ADU (JADU). A JADU is up to 500 sq ft of interior livable space inside a single-family home. The CCI survey doesn't report JADUs separately, and we didn't find another credible source with JADU cost figures.

Prefab or modular ADU. We didn't find a credible, independent source with California prefab ADU cost figures. Prices on manufacturer websites usually cover only part of the total (for example, the unit but not site work, foundation, utilities, or fees), so ask any company exactly what its price includes.

Sources: UC Berkeley Center for Community Innovation, Implementing the Backyard Revolution (April 2021, PDF) · Cal. Gov. Code §66313 (ADU and JADU definitions)

7 · After you build

Ongoing costs after you build

Property tax. Under California property tax law, an addition to land or improvements is "new construction." When it's finished, the county assessor determines its market value and sets a base year value for the newly built part. If the work is only partly done on January 1, the assessor values the work in progress on that date. The California State Board of Equalization (BOE) describes how this works. How much your tax bill changes depends on your county assessor's valuation. Ask your county assessor's office or a tax professional.

Insurance. An ADU can change your homeowner's insurance needs. See our page on ADU insurance.

Sources: California State Board of Equalization, New Construction

How to get a real number for your project

A practical order of operations, from the city pages and guides cited below:

Find your city's or county's ADU page and fee schedule, and ask the permit counter which fees apply to your ADU's size and type.
Check whether your city offers free or reduced-fee standard ADU plans. Some do. For example, Chula Vista says using its standard plans saves about $1,000 in plan review fees.
Get at least three written bids from licensed contractors, based on the same drawings. Casita Coalition suggests being specific about what each bid covers and doesn't cover. For the license check, the contract, and the down-payment limit, see how to get ADU contractor bids.
Add design, survey, and engineering costs, utility work, and fees to the construction bids.
Leave room for surprises. Long Beach's ADU loan program, for example, requires the project budget to include a construction contingency.

Sources: City of Chula Vista, Accessory Dwelling Units page · Casita Coalition, How to build an ADU (published February 2022, updated September 2022) · City of Long Beach, Backyard Builders Program

Quick answers

How much does an ADU cost in California?

There's no current statewide figure. The 2021 UC Berkeley CCI survey of ADU owners found a median total of $150,000, or $250 per square foot, for ADUs permitted or completed in 2018 or 2019, including design, labor, materials, and permits. Costs have changed since then, and they vary a lot by region, size, and type.

What does an ADU cost per square foot?

In the 2021 CCI survey, the statewide median was $250 per square foot (2018–2019 projects), from $197 in Los Angeles County to $329 in the Bay Area. Casita Coalition said in 2022 that most projects run $250–$500 per square foot, without saying how that range was developed.

How much does an ADU cost in Southern California, Los Angeles, or San Diego?

In the 2021 CCI survey, the median was $100,000 in Los Angeles County and $130,000 in Orange and San Diego Counties, for 2018–2019 projects. A 2020 Terner Center brief reported an average of $148,000 in Los Angeles. We have no sourced figure for San Bernardino or Riverside County.

How much does a garage conversion ADU cost in California?

In the 2021 CCI survey, garage ADUs had a median total cost of $90,000, or $189 per square foot, for 2018–2019 projects.

Can I build an ADU for $100,000? For $50,000?

In the 2021 CCI survey, 37% of ADUs cost less than $100,000 (2018–2019 projects). Casita Coalition said in 2022 that a simple interior conversion can start around $30,000. Whether your project lands in that range depends on your lot, the type of ADU, your city's fees, and today's prices. We don't have a sourced figure for how many ADUs cost under $50,000.

Is there an ADU cost calculator for California?

We don't offer one, and we didn't find a calculator from a government or university source. Online calculators use their own assumptions. Ask what they're based on.

Does an ADU raise my property taxes?

Generally, new construction is assessed when it's finished, and the county assessor sets a value for the newly built part. See the BOE's New Construction page and ask your county assessor.

Is it worth it to build an ADU?

That depends on your goals and your numbers, and we can't answer it for you. For context, the 2021 CCI survey found about half of new ADUs were rented out, and the Terner Center reported a median rent of $2,000 a month for new ADUs in the survey. A lender, accountant, or financial advisor can help you look at your own situation.

Sources

The survey, research briefs, statutes, and city pages this page is based on.

Check ADU rules for your address