Buying
Can You Build an ADU in Brentwood? The 2026 Rules
Brentwood must approve a complete ADU application within 60 days, an 800-square-foot detached unit skips most design rules, and a rebate of up to $15,000 expires September 30, 2026.
I get this question more than almost any other right now. Someone wants a smaller footprint without leaving the street they have lived on for twenty years. Someone needs a self-contained unit out back for a caregiver. Someone wants rental income against a tax bill that keeps climbing.
An accessory dwelling unit answers all three. Brentwood's rules changed in February 2025, the state pushed back on part of them that October, and a rebate program expires this month.
What counts as an ADU in Brentwood, and how is a JADU different?
Brentwood's rules live in Chapter 17.715 of the Municipal Code, adopted as Ordinance No. 1083. An ADU is a full second dwelling, with living, sleeping, eating, cooking, and sanitation, on a lot with an existing or proposed home. It can be detached, attached, or carved out of an existing garage or outbuilding.
A JADU, or junior accessory dwelling unit, is smaller: no more than 500 square feet, contained entirely within the existing house, with its own exterior entrance and at least an efficiency kitchen. It can share a bathroom with the main house.
The difference that matters most is occupancy. A JADU requires you to live in either the main house or the JADU. An ADU carries no such rule, which changes the plan for anyone thinking about renting the main house and moving into the back unit.
The city has 60 days to answer
Under section 17.715.040, an ADU application is handled through the building permit process, ministerially, without a hearing. The city has 60 days from a completed application to approve or deny. Miss that window and the application is deemed approved. On preapproved plans, the clock is 30 days.
That word "completed" is doing a lot of work. The clock does not start on a partial packet. The difference between a 60-day approval and a nine-month slog is almost entirely whether the drawings and site plan were right the first time.
The city publishes a running permit log on its Accessory Dwelling Units page. As of September 8, 2026, it showed 14 applications under review and 11 units under construction.
City of Brentwood permit log, September 8, 2026
Fourteen ADU applications under review and eleven units under construction.
The smallest was a 300-square-foot conversion, the largest a 1,180-square-foot detached unit. Source: City of Brentwood Accessory Dwelling Units page.
14
applications under review
How big can it be, and why the printed code is not the whole story
Almost nobody knows this part, and I go looking for it before a client spends money on plans.
Brentwood's ordinance, at section 17.715.050(F)(2), caps ADUs at 850 square feet for a studio or one-bedroom and 1,000 square feet for two or more bedrooms, and limits an attached ADU to 50 percent of the floor area of the existing house.
On October 15, 2025, the California Department of Housing and Community Development sent the city written findings that this provision violates state law. Government Code section 66321(b)(3) prohibits a size limit based on a percentage of the primary dwelling where it prevents an 800-square-foot ADU with four-foot setbacks. HCD flagged a second problem too. The ordinance requires replacement parking when a garage becomes a JADU, which state law does not allow.
So the printed code and the enforceable code may not match on those two points. If a plan check cites the 50 percent rule or demands replacement parking for a garage conversion, that is a conversation to have with the Community Development Department, not a reason to shrink the project. Call the city at (925) 516-5400 and ask where the ordinance stands today.
Separately, the ordinance already carves out a "state exempt ADU": one detached new-construction unit of 800 square feet or less, with four-foot side and rear setbacks, permitted ministerially and exempt from the district's development, design, and parking standards. For a lot of Brentwood lots, that 800-square-foot box is the path of least resistance.
The rebate runs out on September 30, 2026
The state-funded ADU Accelerator Program put rebate money behind ADU construction in 15 East Bay cities, and Brentwood is one of them. The city received a one-time allocation of $121,194.85 to run it.
ADU Accelerator rebate tiers
- Not deed-restricted: $7,500 for units under 500 sq ft · $5,000 for 501–750 sq ft · $2,500 for 751–1,000 sq ft
- Deed-restricted to low-income households for at least 20 years: $15,000 under 500 sq ft · $10,000 for 501–750 sq ft · $5,000 for 751–1,000 sq ft
- Nothing over 1,000 sq ft, and one rebate per parcel
- Program period ends September 30, 2026. Final inspection sign-off and notice to staff both have to happen by that date.
If your ADU is finishing up right now, stop reading and email the Development Services Division today. If you are still sketching, budget as though the rebate is not there and watch for an extension.
What does an ADU do to your property taxes?
Adding an ADU is new construction, so the county will assess it. What it does not do is reset the assessed value of everything else you own.
Contra Costa's reassessment process works on the difference between the new value and the value already on the roll. That difference becomes a supplemental assessment, and you will get one or more supplemental tax bills on top of the annual one. Your existing Proposition 13 base year value on the house and land is not disturbed. The Assessor's office is at (925) 313-7400 for a read on your parcel. The dates and the lender trap are in The Supplemental Tax Bill Every New Brentwood Owner Gets.
If you are in a Mello-Roos district, that assessment is separate from all of this. I broke it down in Why Is My Brentwood Tax Bill So High?
City impact fees are their own category. None are charged for a JADU or for an ADU under 750 square feet; at 750 and above they are charged proportionally to the size of the main house.
Can an HOA like Trilogy say no?
Not flatly. California Civil Code section 4751 makes any governing-document provision that "effectively prohibits or unreasonably restricts" an ADU on a single-family lot in a planned development void and unenforceable.
What an association can do is impose reasonable restrictions on design, materials, roof pitch, and color, as long as they do not unreasonably raise the cost or block the unit. In a community like Trilogy at the Vineyards, architectural review is real and the lots are compact, so setbacks and building separation are usually the binding constraint long before the CC&Rs are.
Settle two things in writing before you spend a dollar on plans: what the architectural committee requires, and how the community's age qualification applies to a second unit. Get both from the association, not a neighbor.
What I check from the escrow side
If you are weighing an ADU against a move, send me the address and one sentence about what the space needs to do. I'll pull the parcel, tell you whether the 800-square-foot box fits, and run that number against what a single-story home on the market would cost instead. I would rather talk it through than watch you spend six figures solving a problem a different house would have solved.
(925) 200-0717 — I answer my own phone, and this is a conversation to have before you pay for drawings.
This post is for general informational purposes only and is not legal, tax, or construction advice. Building codes, city ordinances, and rebate programs change. Confirm current requirements with the City of Brentwood Community Development Department, the Contra Costa County Assessor, and your own attorney or tax professional before making decisions about your property.
Margie Lupo, Broker Associate, SRES® · eXp Realty of Northern California, Inc. · DRE #01193496 · Brokerage DRE #02188495 · Equal Housing Opportunity
Sources
- City of Brentwood Municipal Code, Chapter 17.715, Accessory Dwelling Units and Junior Accessory Dwelling Units (Ord. 1083) — accessed September 8, 2026
- City of Brentwood, Accessory Dwelling Units — permit log accessed September 8, 2026
- City of Brentwood, ADU Accelerator Program — accessed September 8, 2026
- California Department of Housing and Community Development, Review of City of Brentwood's ADU Ordinance — October 15, 2025
- California Civil Code section 4751 — accessed September 8, 2026
- Contra Costa County, Reassessment — accessed September 8, 2026
Frequently asked questions
Do I need to add a parking space for an ADU?
Brentwood cannot require more than one space per ADU, and cannot require any if the unit sits within a half mile walking distance of public transit or is a conversion of the existing house or an accessory building. No parking may be required for a JADU at all.
Will I have to sprinkler the whole house?
No. Under section 17.715.050(K), fire sprinklers are required in an ADU only where they are already required in the primary dwelling, and building an ADU does not trigger a sprinkler requirement in the existing house.
Is an ADU cheaper than moving to a single-story home?
Sometimes, sometimes not. It depends on your lot, your equity, and what the space needs to do. If the goal is a home that works for the long haul, run the ADU math against buying differently first. The features that matter are in Aging in Place in Brentwood.
Does an ADU add resale value?
It can, but appraisers treat a permitted, separately metered unit very differently from a converted garage with no paperwork. Permits turn square footage into value.
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