The types of ADU
| Type | What it is | California rule (HCD ADU Handbook, March 2026) |
|---|---|---|
| Detached ADU (DADU, backyard cottage) | A separate building on the lot | Must be allowed at 800 sq ft with 4-ft side and rear setbacks; height 16 ft, or 18 ft in some cases |
| Attached ADU | An addition joined to the house | Height up to 25 ft or the main house's limit, whichever is lower |
| Conversion ADU | Inside existing space: a garage, basement or accessory building | Converted accessory structures may expand by 150 sq ft; lost garage parking need not be replaced |
| JADU (junior ADU) | A unit inside the house, up to 500 sq ft | Owner must live on site if it shares a bathroom with the house; one per single-family lot |
On a single-family lot, California must allow at least one ADU in existing space, one JADU and one new detached ADU. The terms differ by state: Seattle uses "DADU" for the detached kind, capped them at 1,000 square feet and allowed two ADUs per lot under its 2019 rules (City of Seattle).
California's main rules at a glance
| Question | Rule | Gov. Code |
|---|---|---|
| Minimum size a city must allow | 800 sq ft with 4-ft side and rear setbacks | § 66321(b)(3) |
| Smallest unit | 190 sq ft efficiency unit | § 66313(a)(1) |
| Parking | No more than 1 space per unit or bedroom; none within half a mile walking distance of public transit | § 66322(a) |
| Owner-occupancy | Not allowed for ADUs; required for a JADU with shared sanitation | §§ 66315, 66333(b) |
| Permit review | Approve or deny a complete application within 60 days | § 66317(a)(3) |
| Impact fees | Exempt at 750 sq ft or less | § 66311.5(c) |
| Renting | Terms longer than 30 days | §§ 66323(e), 66333(g) |
| Minimum lot size | A city may not use lot size or coverage rules to block an 800 sq ft ADU | § 66321(b)(3) |
What ADU permits are filed at, and how long they stay open
| City | ADU permits (3 years) | Usual filed value | Usual days, issue to final | Reached final |
|---|---|---|---|---|
| San Diego | 1,278 | $127,372 | 442 | 75.1% |
| San Diego County (unincorporated) | 334 | $128,106 | 351 | 85.1% |
| Sacramento County (unincorporated) | 207 | $133,039 | 254 | 92.8% |
| Anaheim | 330 | $100,000 | 307 | 94.0% |
| San Francisco | 333 | $169,365 | 567 | 52.5% |
| Honolulu (ADU & ohana unit) | 297 | $150,000 | 533 | 79.6% |
The 60-day rule covers plan review, not construction. Once issued, the usual ADU permit stayed open 307 days in Anaheim and 567 in San Francisco before its final inspection, and only 52.5% of San Francisco's ADU permits a year or more old had reached a final. Across the cities in the table, the usual permit took between 254 and 567 days to close, so plan on most of a year at least. The filed value is the builder's own figure for the construction, usually above $100,000; our ADU vs addition guide compares it with additions, and San Diego County's ADU cost guide breaks it down by size.
Before you call an ADU builder
- Check your lot against the rules above: a detached unit needs room for 4-foot side and rear setbacks; a garage conversion needs none.
- Look up your city's ADU page for local design rules and pre-approved plans; the state rules above are the floor.
- Open your city's ADU compare page (for example San Diego) and look at each builder's permit count and finished share; a builder with permits that reached final has carried an ADU through inspection.
- If a unit already exists without a permit, read whether an unpermitted ADU can be legalized.
See every San Diego ADU builder's permits →
What this does not tell you
Permits do not say whether an ADU is detached, attached or a conversion, so the filed values mix all three. The rules above are California's; other states set their own, and Seattle's figures are from its 2019 rules. Days to final measure the permit, not when the unit was first lived in.
How we know this
ADU permits are those each city files under an ADU, companion-unit or ohana work type in its three-year window ending September 2026 (Honolulu's ends June 2025). The usual filed value is the median of permits that carry one; days is the median from issue to final; "reached final" is the share of permits a year or more old that the city closed with a final. California rules are quoted from HCD's ADU Handbook (March 2026) and Seattle's from the City's 2019 submission guide, read 22 October 2026.
Numbers come from City of San Diego Development Services approvals; Sacramento County building permits on single-family, duplex and townhouse homes in the unincorporated county (the County's Accela permit layer); San Francisco DBI building permits on one- and two-family homes, with the contractor from the city's permit contacts file; City & County of Honolulu building permits (DPP); City of Anaheim Accela building permits on single-family homes and ADUs; 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), data through October 17, 2026. Nobody paid to be on this page and nobody can.
Questions
- What is the difference between an ADU and a DADU?
A DADU is one kind of ADU: a detached accessory dwelling unit, a separate small house on the lot, often called a backyard cottage. An ADU can also be attached to the main house or built inside it. Seattle uses the DADU term and capped DADUs at 1,000 square feet under its 2019 rules.
- When to choose a detached ADU or an attached ADU?
Choose detached when the lot has room behind the house and you want a unit with its own walls and privacy; California must allow a detached ADU of at least 800 square feet with 4-foot side and rear setbacks. Choose attached or a conversion of existing space (a garage, a basement) when the lot is tight, since converted space can grow by 150 square feet and needs no new footprint.
- What is the permitting process for ADUs?
Submit plans to the city or county building department as a building permit. In California the agency must approve or deny a complete ADU application within 60 days (Gov. Code § 66317). The permit then stays open through inspections: the usual ADU permit took 442 days from issue to final in San Diego and 254 in Sacramento County.
- What are the ADU size limits in Sacramento?
Sacramento follows California's ADU law: a JADU may be up to 500 square feet, a local agency must allow at least an 800-square-foot ADU with 4-foot side and rear setbacks, and ADUs of 750 square feet or less are exempt from impact fees (HCD ADU Handbook, March 2026). The usual Sacramento County ADU permit was filed at $133,039.
- Can I convert my garage into an ADU in Sacramento?
Yes, under state law: California's handbook lists attached garages and accessory structures as space an ADU can be built in, lets a converted structure expand by 150 square feet, and bars the city or county from requiring the lost garage parking to be replaced.
- Is owner-occupancy required for ADUs in Sacramento?
Not for an ADU; HCD says a local agency cannot impose an owner-occupancy requirement on an ADU. A JADU that shares a bathroom with the main house does require the owner to live on the property.
Build your own list
Every firm with that kind of permit on record. Pick what you value. The list moves.
Related guides
- Can an unpermitted ADU be legalized? California's AB 2533 rule, and what it costs and takes
- ADU vs home addition: what the permit file shows
- How to check a contractor's license in California
- What's a good completion rate for a contractor? The benchmarks from 12 trades and cities
For one city
- Accessory Dwelling Unit Handbook — California Department of Housing and Community Development (2026-03; accessed 2026-10-22)
- Pre-approved Plans for Accessory Dwelling Units: Submission Guide — City of Seattle, Office of Planning and Community Development (2019; accessed 2026-10-22)