Brentwood permitted 153 ADUs from 2014 to 2025 — among the most active markets in California. Here's what it costs, what's allowed, and who builds them.
All-in design-build estimates (architecture + permits + construction + soft costs). Real quotes vary by lot conditions, finishes and contractor — get at least three before signing.
| ADU type | All-in cost, low to high |
|---|---|
| Detached ADU~750 sqft typical | $128,250–$342,000typical $213,750$171–$456 per sqft |
| Attached ADU~700 sqft typical | $99,400–$252,700typical $159,600$142–$361 per sqft |
| Garage Conversion~400 sqft typical | $34,000–$91,200typical $56,800$85–$228 per sqft |
| Junior ADU (JADU)~400 sqft typical | $34,000–$91,200typical $56,800$85–$228 per sqft |
| Prefab ADU~600 sqft typical | $76,800–$205,200typical $127,800$128–$342 per sqft |
| Basement Conversion~600 sqft typical | $59,400–$159,600typical $99,600$99–$266 per sqft |
Per-square-foot ranges reflect 2026 pricing in the default market. Permit fees ($5–25k), site work ($10–40k) and high-end finishes can push totals 20–40% higher. See cost methodology for the full regression model and the 8 California builder-pricing sources it's calibrated against.
California state law sets a floor that every city must meet. Brentwood may allow more than the minimums shown — confirm with the Brentwood planning department before finalising plans. See how we verify city-specific ordinance data.
| Rule | Standard |
|---|---|
| ADUs allowed on single-family lot | Yes — required by state law |
| Max units per lot | Single-family lot: 1 ADU converted from existing space, 1 JADU and 1 detached new ADU. From Jan 1, 2027 a second detached ADU, though a city need not then also approve a JADU. Lot with an existing multifamily building: up to 8 detached ADUs, not more than its existing units, plus conversions of non-livable space |
| Owner-occupancy required | Not allowed to require for ADUs (state preempts) |
| Side setback (detached) | 4 feet |
| Rear setback (detached) | 4 feet |
| Max height | 16 ft; 18 ft within one-half mile walking distance of a major transit stop or high-quality transit corridor |
| Off-street parking | None may be required within one-half mile walking distance of public transit, or in the other cases listed in Government Code section 66322; otherwise at most one space per ADU or per bedroom, whichever is less |
| Max ADU size | 850 sqft for studio/1BR; 1,000 sqft for 2+BR (city may allow more) |
| Min lot size | A city may not impose a minimum lot size requirement for ADUs |
| Permit timeline (ministerial) | 60 days from complete application (state law) |
State law, effective statewide. What the 2027 ADU laws change, and what they don't →
Annual ADU permits issued, 2014–2025. Sources: Big West Coast ADU Dataset for 2013–2017 and the California HCD Annual Progress Report (October 2026 release) for 2018–2025.
Among Brentwood ADUs with a completion in the state's records, half were finished within 230 days (about 7.6 months) of the building permit being issued — 116 days faster than the statewide median of 346 days.
Measured from California HCD Annual Progress Report records (October 2026 release): building-permit date to certificate of occupancy or final inspection, for ADUs permitted 2018–2023. The clock starts when the permit is issued, so design and plan check come on top. 64% of Brentwood ADUs permitted in those years have a completion on record; projects that were abandoned, are still under way, or were never reported as complete are not in the figure. Method, limits and every city →
Active CA-licensed B-class general contractors with ADU-specialist branding, sourced from the CSLB Public Data Portal snapshot from April 28, 2026. Click any license number to verify current status at the CSLB Instant License Check — status can change at any time. See builder-directory methodology.
We don't yet list ADU specialists with offices in Brentwood. Some builders from neighbouring cities may serve this area: you can ask for an introduction below, check our full builder directory, or contact your local planning department for referrals.
Talk to a builder
Run the lot checker for your address. You get the rules that apply to your lot, a cost range and Brentwood's measured build-time figures first, and at the end you can ask to be introduced by email to one builder or builder-matching service that serves your area. Builders may pay us a referral fee for an introduction; it never changes anything on this page, and no builder can pay to appear in our directory.
Most Brentwood ADU projects fall in a $100,000–$400,000+ range, all-in (design + permits + construction). The big drivers are size, type (detached costs more than a garage conversion), and finish level. See the per-type cost table above for typical ranges in this market.
Yes — every ADU in California requires a building permit. Brentwood processes ADU applications under the state's 60-day ministerial review rule (no public hearings, no discretionary review for code-compliant designs). Permit fees vary but typically run $5,000–$25,000 depending on size and impact fees.
It depends on the city. Under the general ADU ordinance rules, a city cannot set a maximum ADU size below 850 sqft, or 1,000 sqft for more than one bedroom, but the separate state-mandated detached ADU path may be limited to 800 sqft. Many California cities allow larger detached ADUs — check the Brentwood planning department or the ordinance link in our rules table above for the local maximum.
No, not for an ADU. AB-976, in effect since January 1, 2024, made the state's ban on owner-occupancy requirements for ADUs permanent: California cities may not require an owner to live on the property as a condition of permitting an ADU. JADUs are different. A JADU that shares sanitation facilities with the existing structure requires the owner to live in either the house or the JADU; owner-occupancy is not required if the JADU has separate sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization (Government Code section 66333).
A JADU (Junior ADU) is a unit ≤500 sqft created within an existing single-family home, sharing a wall with the main residence. JADUs are cheaper and faster to build. A JADU that shares sanitation facilities with the existing structure requires the owner to live in either the house or the JADU; owner-occupancy is not required if the JADU has separate sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization. Most homeowners build a separate ADU for rental flexibility.
According to California HCD permit records, Brentwood permitted 38 ADUs in 2023, its peak year on record (2013–2025). California-wide ADU permits have grown roughly 30× since 2016 — homeowner demand is outpacing every other housing-supply tool the state has tried.
Among Brentwood ADUs with a completion in the state's records, half were finished within 230 days (about 7.6 months) of the building permit being issued; the middle half took between 160 and 295 days, and 82% were finished within a year. That is measured over 56 completed ADU projects permitted 2018–2023, from building permit to certificate of occupancy. Design and plan check happen before the permit is issued, so allow for those on top. The statewide median is 346 days.