California ADU rules · Contra Costa County
Can I Build an ADU in Brentwood?
Yes — you can build an ADU in Brentwood. California's statewide ADU law requires every city, including Brentwood, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
CA state law1 ADU + 1 JADU
At least one ADU and one JADU per single-family lot (a conversion ADU may also be possible).
Units on a multifamily lot
Local ruleconversions (>=1 or 25% of units) + up to 2 detached (proposed multifamily) or up to 8 detached (not to exceed number of existing units, existing multifamily)
Detached ADUs on multifamily lots: up to 2 if proposed multifamily, up to 8 if existing multifamily, but not to exceed number of existing units.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule1,000 sq ft
Local cap: 850 sq ft (studio/1-bed), 1,000 sq ft (2+ bed); state-exempt ADUs must be allowed at least 800 sq ft.
Max attached ADU size
Local rule850 (studio/1-bed), 1,000 (2+ bed), and no more than 50% of primary dwelling
Local cap: 850/1,000 sq ft and 50% of primary dwelling; state-exempt ADUs must be allowed at least 800 sq ft.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
CA state lawNo front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone
Separation from main house
Local rule4 ft
Detached ADUs must be set back at least 4 ft from the primary dwelling.
Parking
Parking required
Local ruleUp to 1 space
Max 1 space per ADU; JADUs require no parking.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required when a garage/carport is converted to an ADU.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
Local rule30 days
30 days for preapproved plans; 60 days for standard applications.
Owner-occupancy
Local ruleOnly for a JADU
Owner occupancy required for JADUs only; not required for ADUs.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
CA state lawBuilding permit
Only a ministerial building permit — no conditional use permit or discretionary design review for a qualifying ADU.
Brentwood-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Fire sprinklers
Fire sprinklers required in ADUs only if required for the primary dwelling.
Historic district parking exemption
No parking required for ADUs in historic districts.
Brentwood caps detached and attached ADUs at 850/1,000 sq ft (studio/1-bed/2+ bed) and requires a 4 ft separation from the primary dwelling. Up to 8 detached ADUs may be allowed on existing multifamily lots, but not exceeding the number of existing units.
Frequently asked questions
Can I build an ADU in Brentwood?
Yes. California's statewide ADU law requires Brentwood to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Brentwood?
A detached ADU of at least 1,000 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Brentwood?
Side and rear setbacks are limited to 4 ft. No front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone.
Is parking required for an ADU in Brentwood?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Brentwood?
A complete application must be acted on within 30 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Brentwood?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Brentwood Municipal Code Chapter 17.715 (Ord. 1083, effective 2/25/2025)
- BMC § 17.715.050(A)-(B)
- BMC § 17.715.050(F)(2)
- BMC § 17.715.050(B)(2)(a)
- BMC § 17.715.050(M)
- BMC § 17.715.050(M)(4)
- BMC § 17.715.050(B)(2)
- BMC § 17.715.050(L)
- BMC § 17.715.050(K)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of Brentwood's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Brentwood Planning before relying on it.
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