Brentwood allows an ADU of up to 1,000 square feet on a single-family lot, and the city must act on a complete application within 60 days. With preapproved plans that drops to 30. Those two numbers settle most of what people want to know before they start pricing a build.
Here are the rules that actually govern what you can put in the back yard, taken from the city's own ordinance.
How big can an ADU be in Brentwood?
It depends on the type and the bedroom count.
A detached or attached ADU is capped at 850 square feet for a studio or one bedroom, and 1,000 square feet for two bedrooms or more. A junior ADU, which sits inside the existing house, is capped at 500 square feet.
There is also a state-exempt category: a detached ADU of 800 square feet or smaller that a city cannot block on most local standards. If your plan fits under 800 feet, you are on much firmer ground.
How close to the property line can it sit, and how tall?
Standard ADUs need a minimum four-foot side and rear setback, and a detached unit must sit at least four feet from the primary dwelling. State-exempt units only need setbacks sufficient for fire and safety.
Height is where most designs get decided. A detached ADU may not exceed sixteen feet. That rises to eighteen feet near transit, or twenty feet if the roof pitch aligns with the primary dwelling. An attached ADU may go to twenty-five feet or the underlying zone's limit, whichever is lower, and no more than two stories.
Do I have to add parking?
At most one space per ADU, and often none.
Parking is waived if the property sits within half a mile of transit, lies in a historic district, involves converting part of the primary dwelling, or has car-share available nearby. A junior ADU never requires parking.
This matters more than it sounds. Losing a parking space to a code requirement is what kills a lot of otherwise workable back yard plans.
How long does approval take?
The city must act within sixty days of receiving a completed application, or thirty days if you use preapproved plans.
Those are decision clocks, not construction schedules, and they start when the application is complete. Incomplete submissions are the usual reason a project that should take two months takes six.
If you're buying or selling in Brentwood
If you're buying: confirm any existing ADU was permitted, because an unpermitted unit is a liability rather than a feature and lenders and appraisers treat the two very differently. If you are buying with a build in mind, check the setbacks and the half-mile transit question for that specific parcel before you write. An HOA can also restrict what the city would allow.
If you're selling: gather the permit and final inspection records for any ADU before you list. A documented unit and an undocumented one are not the same asset, and the difference shows up in the appraisal rather than in the offer. If yours was never permitted, talk to the city about your options before a buyer's inspector raises it.
Frequently asked questions about ADUs in Brentwood
What is the difference between an ADU and a JADU?
A junior ADU is created within the walls of the existing single-family home and is capped at 500 square feet. A standard ADU can be detached or attached and runs up to 850 or 1,000 square feet depending on bedroom count. JADUs also carry an owner-occupancy requirement that standard ADUs do not.
Do I have to live on the property to build an ADU?
For a standard ADU, no. Brentwood's owner-occupancy requirement applies to junior ADUs, where the owner must live in either the JADU or the remaining portion of the house.
Is there financial help for building an ADU in Brentwood?
The city has run an ADU Accelerator Program offering rebates of up to $15,000 for units deed-restricted to low-income occupancy for at least twenty years, or up to $7,500 without that restriction, with no rebate for units over 1,000 square feet and one rebate per parcel. Brentwood's allocation was a one-time payment of $121,194.85 and the stated program period ran through September 30, 2026. Confirm current availability with the city before counting on it.
Will building an ADU increase my property taxes?
The new construction is assessed and typically generates a supplemental tax bill, but the existing home is not reassessed because of it. That supplemental bill arrives separately from the annual one and is often not covered by a mortgage impound account.
Can an ADU be sold separately from the main house?
Do not assume so. California has narrow circumstances in which separate conveyance is possible and it depends on whether the city has adopted the enabling provisions. Ask the City of Brentwood directly about your parcel rather than relying on general articles.
The Bottom Line on ADUs in Brentwood
The governing numbers are 850 or 1,000 square feet depending on bedrooms, four-foot side and rear setbacks, sixteen feet of height for a detached unit, at most one parking space and often none, and a sixty-day decision clock.
The two questions that decide most projects are whether your plan fits under the 800-square-foot state-exempt threshold and whether your parcel is within half a mile of transit. Answer those before you spend money on design.
Thinking about an ADU on a Brentwood property?
Whether a specific lot works is a parcel-level question, not a general one. Send me an address and I will tell you what I can see before you commission plans.
See homes for sale in Brentwood
Across 460+ East Bay closings over twenty years, an unpermitted ADU has cost sellers more at the negotiating table than the unit ever added in value.
Selling a home that already has one? Start with a valuation, or call or text 925-250-9611.
Sources
City of Brentwood Municipal Code, Accessory Dwelling Units and Junior Accessory Dwelling Units for size limits, setbacks, height, parking, owner-occupancy and the sixty and thirty day decision timelines. City of Brentwood, ADU Accelerator Program for rebate amounts, the size cap on rebates, the per-parcel limit, the city's allocation and the program period. City of Brentwood, Accessory Dwelling Units for permitted zoning districts and the application process.
ADU law changes frequently at both state and city level. Confirm current requirements with the City of Brentwood before relying on any of this.
Jaz Chand, Broker Associate | Homes With Accent
The Jaz Team at Merrill Signature Properties | DRE #01751823
Phone/Text: 925-250-9611 | Jaz@HomesWithAccent.com
General information only, not legal, tax or financial advice, and not a representation about what may be built on any specific parcel. Zoning, building and ADU requirements change. Confirm all details with the City of Brentwood and your own professionals.