The short answer: The only independent statewide data on what California homeowners actually paid is a 2021 UC Berkeley survey: a median of $150,000, or about $250 per square foot. Those units were mostly built before recent cost increases. A June 2026 guide from a California builder puts a typical all-in ADU at $200,000 to $400,000, with junior ADUs and garage conversions at the low end and detached backyard units at the high end. That second set of numbers is a vendor’s estimate, not a study, so use it to frame a budget and confirm it with written local bids.
Last verified: October 5, 2026 ยท Written by the ADU Scope editorial team
Where the numbers come from: Almost every “ADU cost” page is written by a company that builds ADUs. This guide separates the two kinds of evidence. The 2021 and 2020 figures come from the Terner Center at UC Berkeley, a university research center. The 2026 ranges come from one California builder, ADU Scale, and are labeled as vendor estimates wherever they appear. We did not find a neutral 2023 to 2026 study that measures new ADU construction costs.
What Homeowners Actually Paid: The 2021 Survey
In April 2021, UC Berkeley’s Center for Community Innovation and the Terner Center published the first statewide survey of California ADU owners. It is based on 823 completed surveys from property owners, a 4.8% response rate after filtering, and the costs are self-reported. It remains the best neutral benchmark we found, but it is more than five years old.
| Measure (2021 owner survey) | Result |
|---|---|
| Median statewide construction cost | $150,000, or about $250 per sq ft |
| Built for less than $100,000 | 37% of ADUs |
| Built for less than $200,000 | 71% of ADUs |
| Median rent of a new ADU | $2,000 per month |
| How owners paid | 53% with cash, 43% with bank loans, 10% with money from friends or relatives (the shares add up to more than 100%, so owners evidently reported more than one source) |
The report also gives regional medians. They show how much location moves the answer:
| Region | Median cost | Per sq ft |
|---|---|---|
| San Francisco Bay Area | $177,500 | $329 |
| Central Coast | $140,000 | $223 |
| Orange and San Diego counties | $130,000 | $200 |
| Los Angeles County | $100,000 | $197 |
The Bay Area figure per square foot is about two-thirds higher than Los Angeles County’s. A 2020 Terner Center report on earlier data points the same way: an average estimate of $167,000 statewide, $148,000 in Los Angeles and $237,000 in the Bay Area, a gap the report attributes largely to labor costs. The same report notes that Bay Area costs “can exceed $800 per square foot, equaling $400,000 for a 500 square foot ADU.” That is a high-end case, not a typical price, and it is an estimate, so do not compare it directly with the survey medians.
Key fact: The survey measured what owners spent, mostly on projects built before the recent rise in construction costs. Do not apply the 2021 median to a 2026 budget without adjusting it. Treat it as the floor of what is plausible for a comparable project, not as a current price.
2026 Builder Ranges by ADU Type
ADU Scale, a California builder, publishes the following ranges in a guide it last updated in June 2026. We include them because they separate cost by unit type, which the survey does not. They are one vendor’s view.
| ADU type | Per sq ft | Total project |
|---|---|---|
| Junior ADU (JADU) | $200 to $350 | $80,000 to $160,000 |
| Garage conversion | $250 to $450 | $120,000 to $220,000 |
| Attached ADU | $300 to $500 | $200,000 to $320,000 |
| Backyard detached | $300 to $550 | $240,000 to $380,000 |
| Prefab or modular (delivered) | $200 to $400 | $180,000 to $320,000 |
The ordering is the reliable part, and it follows the construction work involved:
- JADUs and conversions reuse existing walls, roof and foundation, which avoids the most expensive parts of construction.
- Attached ADUs share some structure but need new foundations, framing and a roof tie-in.
- Detached ADUs need everything new, plus utility trenches to a separate building.
- Prefab units move part of the work into a factory, but foundations, site work and utility connections still cost money.
For the same square footage, a conversion should cost less than an addition, and an addition less than a new detached building. The same builder lists hillside or historic-overlay locations at $500 to $700 per sq ft for new detached work.
Tip: When you compare a prefab with a site-built ADU, compare the total installed project cost, not the price of the unit. The builder’s own all-in prefab range ($180,000 to $320,000) overlaps the lower half of its detached range once foundation and utility work are counted.
Why a Per-Square-Foot Price Misleads
“Price per square foot” is the number everyone asks for and the one that hides the most. A small unit costs more per square foot than a large one because the kitchen, bathroom, utility hookups, permits and design cost roughly the same at 400 or 800 sq ft. The same builder’s ranges for a new detached unit show the pattern:
| Detached ADU size | Per sq ft | All-in |
|---|---|---|
| 400 sq ft | $350 to $600 | $140,000 to $240,000 |
| 600 sq ft | $300 to $550 | $180,000 to $330,000 |
| 800 sq ft | $280 to $500 | $225,000 to $400,000 |
Going from 600 to 800 sq ft adds about $57,500 at the midpoint of these ranges ($255,000 to $312,500), roughly $287 for each extra square foot. That is below the headline rate. The practical lesson is to ask bidders for prices at two sizes and compare the cost of the extra space, instead of multiplying a per-square-foot figure by the size you want.
Location
ADU Scale gives all-in ranges for a 600 sq ft detached unit by region: Los Angeles $200,000 to $340,000, San Diego $180,000 to $300,000, Orange County $230,000 to $370,000, and San Francisco and the Bay Area $300,000 to $450,000. Both sources put the Bay Area highest, but they disagree on the order of the other regions (the survey has Los Angeles County lowest, the vendor has San Diego lowest), another reason to treat regional figures as orientation. Note that these totals are not simply 600 sq ft times the same vendor’s per-square-foot range for each region (in Los Angeles that product would be $180,000 to $330,000), so the all-in figures presumably include items the per-foot rate leaves out. Compare totals with totals and do not mix the two.
Site conditions
Two lots with the same unit can differ by tens of thousands of dollars. The builder lists items that are often missing from a first quote:
| Item | Vendor range |
|---|---|
| Sewer lateral upgrade | $15,000 to $30,000 |
| Hillside soils and foundation work | $20,000 to $60,000 |
| Structural retrofit of a pre-1970 garage | $5,000 to $25,000 |
| Water meter upsize | $10,000 to $25,000 |
| Electric panel upgrade | $5,000 to $15,000 |
| Soft costs (design, engineering, permits and related) | 12% to 18% of total project value |
A single lot will have some of these items and not others. On a $300,000 project, soft costs alone at 12% to 18% would be $36,000 to $54,000. Our guide to hidden costs and utility hookups covers the utility lines in detail.
Fees You May Not Owe
State law removes one cost category for smaller units. Under Government Code section 66311.5, a city or special district may not charge impact fees on an ADU of 750 sq ft or less of interior livable space or on a JADU of 500 sq ft or less. Larger ADUs pay impact fees in proportion to their size compared with the main house. Permit, plan-check and utility charges still apply. Our articles on impact fees and permit cost by city explain what each city publishes.
How to Build a Realistic Budget
- Pick the unit type first. Conversion, attached or detached drives cost more than finishes do.
- Choose a size and ask for pricing at two sizes.
- Get three written bids with line items, not lump sums.
- Ask each bidder what is excluded: utilities, permits, design, site work, landscaping.
- Hold back a contingency. A reserve of 10% to 15% is a reasonable planning allowance, though no legal or industry source sets the figure.
- Subtract fees you may not owe, using the size thresholds above.
Our ADU budget breakdown works through three sample budgets with the arithmetic shown.
Frequently Asked Questions
What is the average cost of an ADU in California?
The 2021 Terner Center and UC Berkeley survey found a median of $150,000, about $250 per square foot. Current builder guides put typical all-in costs at $200,000 to $400,000, but those are vendor estimates.
What is the cheapest type of ADU?
A junior ADU or a garage conversion, because they reuse existing structure. See our guide to the cheapest way to build an ADU.
Does a bigger ADU cost less per square foot?
Usually yes, because fixed costs are spread over more space. The total still rises.
What is a good price per square foot for an ADU?
There is no single good price. One California builder lists about $200 per sq ft for a JADU up to $550 or more for a detached unit, and $500 to $700 on hillsides. Compare all-in totals with the same inclusions instead.
Is the 2021 survey figure still valid?
As a benchmark, yes. As a budget, no, because construction costs have risen since the units in the survey were built.
Next Steps
- ADU Budget Breakdown: Three Sample Budgets and a Line-by-Line Framework
- Hidden ADU Costs and Utility Hookups
- ADU Design Cost: Architect vs. Pre-Approved Plans
- ADU Impact Fees: When You Pay Them and How to Avoid Them
- California ADU Laws 2026
General information, not financial or legal advice. Costs vary by site and change over time; get written bids.
Sources: Terner Center and CCI: First statewide ADU owner survey (April 2021) ยท Terner Center and CCI: Reaching California’s ADU Potential (August 2020) ยท ADU Scale: California ADU cost guide (vendor, updated June 2026) ยท HCD ADU Handbook, March 2026