ADU construction cost by city
| City | Permit type | Usual filed value | Middle half of firms' usual job | Permits | Usual days, issue to final | City page |
|---|---|---|---|---|---|---|
| Anaheim, CA | ADU | $100,000 | $60,000–$150,000 | 330 | 307 | Cost page |
| Palo Alto, CA | ADU | $108,045 | $66,111–$133,896 | 206 | — | Cost page |
| Raleigh, NC | Backyard Unit (ADU) | $112,256 | — | 139 | — | Cost page |
| Nashville, TN | Backyard Unit (ADU) | $114,788 | $103,120–$127,315 | 407 | — | Cost page |
| San Diego, CA | ADU (Companion Unit) | $127,372 | $105,805–$174,174 | 1,278 | 442 | Cost page |
| San Diego County, CA (unincorporated) | ADU | $128,106 | $110,493–$160,022 | 334 | 351 | Compare firms |
| Sacramento County, CA (unincorporated) | ADU | $133,039 | $99,402–$173,840 | 207 | 254 | Cost page |
| Denver, CO | ADU | $137,144 | $102,992–$175,000 | 262 | — | Cost page |
| Bellingham, WA | ADU | $140,755 | $118,076–$152,242 | 110 | 329 | Cost page |
| Honolulu, HI | ADU & Ohana Unit | $150,000 | $120,000–$215,000 | 297 | 533 | Cost page |
| San Francisco, CA | ADU | $169,365 | $28,364–$224,488 | 333 | 567 | Cost page |
| Marin County, CA (unincorporated) | ADU | $205,000 | — | 111 | 555 | Cost page |
Austin files ADU permits without a value and is left out. An ADU type can mix garage conversions with new detached units, which is part of why the middle-half ranges are wide.
What drives the price
- New build or conversion. Converting a garage reuses the slab, walls and roof; a detached unit builds all of them. In San Francisco the middle half of firms' usual jobs runs from $28,364 to $224,488, the widest band here.
- Size. San Diego allows "attached or detached" ADUs with a gross floor area from 150 to 1,200 square feet (City of San Diego IB 400). A 1,200-square-foot unit is a small house; a 400-square-foot studio is not.
- Fees. In California an agency "shall not impose any impact fee upon the development of an accessory dwelling unit" of "750 square feet of interior livable space or less", and fees on larger units are charged "proportionately in relation to the square footage of the primary dwelling unit" (Gov. Code § 66311.5). Ask your city for its fee estimate at both sizes. San Diego notes its ADUs are still "subject to Development Impact Fee requirements" under its municipal code, so read your own city's rules.
- The middle half elsewhere. In San Diego, 93 builders' usual jobs sit mostly between $105,805 and $174,174; in Honolulu, 31 builders sit between $120,000 and $215,000.
Filed value is not the final price
The filed value is the builder's figure on the permit application, set before construction and sometimes before the finishes are chosen. Nobody audits it, it often excludes design, permit fees, utility connections and the site work to reach the back of a lot, and change orders over a year-long build are rarely refiled. Use it to check whether a construction quote is in the normal band for your city, not as the total you will spend.
Permit rules in brief
Every ADU needs a building permit in the departments whose rules we read, and usually several: Denver lists zoning, building and sewer use and drainage permits, plus trade permits. Zoning decides whether you can build one at all. The city-by-city list is in do I need a permit for an ADU, and what is an ADU covers the basics. For one county in depth, see ADU cost in San Diego County.
How long the permit stays open
ADUs are slow. Sacramento County closes them in a usual 254 days and Anaheim in 307; Honolulu (533), Marin County (555) and San Francisco (567) take a year and a half. Only 52.5% of San Francisco's ADU permits a year or more old reached a final, against 94.0% in Anaheim. Tie the last payment to the final inspection and certificate of occupancy.
Check an ADU quote in five minutes
- Find your city in the table and open its cost page.
- Put the construction quote next to the middle-half range.
- Ask what is excluded: design, permit and impact fees, utility connections, site work.
- On your city's compare page, read the builder's ADU permits and finished share, and ask about any open past the city's usual days.
See every San Diego ADU builder by finished share →
What these numbers cannot tell you
Permits record no floor area, bedroom count or construction type, so there is no per-square-foot figure here. Conversions and new units are mixed in the same figure. Units built without a permit are not in the record; can an unpermitted ADU be legalized covers those.
How we know this
ADU permits are those each city files under an ADU, companion unit or backyard unit work type over its three-year window ending September 2026 (a few cities end earlier; each city page names its window). The usual filed value is the median of permits that carry one; the middle-half range uses firms with three or more ADU permits and is blank where fewer than eight such firms exist; days is the median from issue to final, blank where a city records no final date. Size and fee rules are quoted from the pages under Sources, read 23 October 2026.
Numbers come from City of San Diego Development Services approvals; San Diego County Planning & Development Services building permits on homes in the unincorporated county (the County's open-data permit, contractor and status-history files); Sacramento County building permits on single-family, duplex and townhouse homes in the unincorporated county (the County's Accela permit layer); City and County of Denver residential construction and demolition permits; City & County of Honolulu building permits (DPP); San Francisco DBI building permits on one- and two-family homes, with the contractor from the city's permit contacts file; City of Anaheim Accela building permits on single-family homes and ADUs; City of Bellingham permits on single-family homes, duplexes and townhomes, data through October 17, 2026. Nobody paid to be on this page and nobody can.
Questions
- What is the average ADU construction cost?
Going by permits, the usual ADU was filed at $100,000 in Anaheim, $114,788 in Nashville, $127,372 in San Diego, $137,144 in Denver and $169,365 in San Francisco. That is the middle filed value, so a few very large units do not pull it up.
- How much does ADU construction cost per square foot?
The permit record cannot say, because cities record a job value, not the unit's floor area. San Diego allows ADUs from 150 to 1,200 square feet, so its usual $127,372 could be a small studio or a two-bedroom unit. Ask each builder for the floor area and a total and compare the totals, not a per-foot rate.
- Is there an ADU construction cost estimator for my city?
Your city's ADU cost page, linked in the table, is the closest thing: it shows the 10th to 90th percentile of filed values and, where home values are available, the usual job by home value. San Diego's is built on 1,278 permits, the most of any city here.
- How much does an ADU cost in California?
In our California cities the usual ADU permit was filed at $100,000 in Anaheim, $128,106 in unincorporated San Diego County, $133,039 in Sacramento County and $205,000 in unincorporated Marin County. A California ADU of 750 square feet of interior living space or less owes no impact fees under Government Code § 66311.5.
- How long does it take to build an ADU?
Longer than most jobs we track. The usual ADU permit went from issue to final in 254 days in Sacramento County, 442 in San Diego, 533 in Honolulu and 567 in San Francisco. That clock starts after the permit is issued, so design and plan review come on top.
Build your own list
Every firm with that kind of permit on record. Pick what you value. The list moves.
Related guides
- ADU vs home addition: what the permit file shows
- What is an ADU? ADU vs DADU vs JADU, California's rules, and what ADU permits show
- Can an unpermitted ADU be legalized? California's AB 2533 rule, and what it costs and takes
- Do I need a permit for an ADU? Yes; here is what 9 cities require, and how long it takes
- Building an ADU or ohana unit in Honolulu: the pre-check form, the recorded covenant, converting an old structure, and what permits show
- Sacramento County Shelf Ready ADU plans: five free plan sets, what you still pay, and what ADU permits show
For one city
- Information Bulletin 400: ADU/JADU (August 2025) — City of San Diego Development Services (accessed 2026-10-23)
- California Government Code § 66311.5 (ADU impact fees; formerly § 66324) (accessed 2026-10-23)