Do I need a permit for an Accessory Dwelling Unit (ADU) in
Brentwood, California?

Brentwood ADU permits — the key details

California Government Code § 65852.2 is your legal backbone. Brentwood must approve a proposed ADU on a single-family lot if: (1) the lot is zoned for single-family use; (2) the ADU is no more than 50 percent of the existing dwelling's square footage (or 1,200 sq ft, whichever is less) for an attached unit, or 800 sq ft for a detached junior ADU; (3) you provide a separate utility meter or sub-meter; and (4) the ADU does not exceed 55 feet in height and respects state-mandated setbacks (zero-lot-line front and side setbacks are allowed for attached ADUs; detached ADUs get 4 feet rear, 5 feet side, unless the local code says less — Brentwood allows these). There is no owner-occupancy requirement, no ADU-per-lot cap (you can add two ADUs per SB 9 on certain lots), and no parking requirement for ADUs within half a mile of transit (Brentwood is served by County Connection bus; call the transit authority to verify your lot's distance). The city's building permit application must include site plans showing lot dimensions, setbacks, utility routes, and parking layout; floor plans and sections; and structural design for detached units. If your ADU includes an electric vehicle charging station, NEC 690.12 (240V outlet) and local utility interconnection rules apply — Brentwood's power provider is either Pacific Gas & Electric (PG&E, East County) or East Bay Municipal Utility District power partner. Garage conversions, second-story additions above existing structures, and detached new-build ADUs all require permits. The only category that moves faster is a junior ADU (JADU) — a unit carved out of an existing primary dwelling with no new exterior wall — which can sometimes pass plan review in 4–6 weeks if the interior layout, egress, and kitchen are straightforward.

IRC R310 egress is non-negotiable. Every sleeping room (including the primary bedroom in a JADU) must have an emergency escape and rescue opening (EERO) — a window or door leading to grade, a deck, or a fire-escape landing. Minimum sill height is 44 inches above floor; minimum dimensions 5 sq ft opening, 3 feet wide, 4 feet 6 inches tall (or 3 sq ft for units above the first story of a building). In Brentwood's foothill areas (climate zone 6B), where winter frost can reach 18–30 inches, basement egress windows in a detached ADU must sit above grade or be protected by an egress well (IRC R310.2); the building inspector will verify on-site. Kitchens require a 3-foot clear floor space and a vent hood exhausting to outdoors per IRC M1503 (not into the attic or garage). Bathrooms must have exhaust fans vented to outdoors; a bathroom window is not a substitute. If your detached ADU has only one exit stairway, IRC R311 requires that hallway to be a dead-end corridor no more than 20 feet long, with no bedrooms off the main path. Single-exit layouts are allowed but trigger this strict geometry rule — the building inspector checks this on framing inspection.

Utility connections are where many Brentwood ADU applicants falter. California law requires a separate utility meter or sub-metering for water, sewer, gas, and electric. Your application must include a letter from the water purveyor (City of Brentwood Public Works or Brentwood Westside Improvement District) confirming that a separate water meter is available at the ADU address and that sewer capacity exists. PG&E or the local power provider must confirm a second electric panel is feasible. Septic systems are not used in Brentwood proper (municipal sewer only), but if your ADU is in unincorporated Contra Costa County just outside Brentwood, septic design becomes critical — leach fields must be 50 feet from property lines and 100 feet from wells (Title 24, Chapter 15.5). Brentwood's utility districts sometimes charge application fees ($200–$500) and require a new meter set ($500–$1,000), plus monthly billing under two accounts. Some applicants mistakenly ask if a sub-meter (one meter, divided billing through a utility provider's software) is cheaper — it is, but not all utilities accept sub-metering for ADUs; confirm before designing.

Parking is usually waived for ADUs in Brentwood, thanks to California Government Code § 65852.2(e), which exempts ADUs within half a mile of transit from local parking minimums. Brentwood is served by County Connection (bus routes 609, 610, 611, 614). If your lot is more than half a mile from a stop, parking rules may apply — check the zoning code section on parking minimums. Many ADU applicants deduce they can eliminate parking by proposing a lot with street parking nearby; the planning department may accept this if you submit a letter from the city's public works confirming on-street parking is unrestricted. Never assume zero parking. The building inspector will drive the neighborhood and may flag a site lacking parking if the zoning code requires it.

Brentwood's timeline is locked by state law: 60 days from a complete application to approval or denial under AB 671 (Government Code § 65852.2(b)). However, 'complete' is the key word. If your first submittal is missing utility letters, structural calcs, or a clear floor plan, the city issues a 'Request for Information' (RFI) that pauses the clock. Expect a first RFI if you don't pre-coordinate with the water and power districts. Plan for 90–120 days total to be safe. Once approved, building inspections follow: foundation (if detached), framing, roof sheathing, rough trades (plumbing, electrical, HVAC in-wall), insulation, drywall, and final walkthrough. Each inspection must pass before the next; failed inspections add 1–2 weeks. Utility companies also inspect meters and service connections before you energize. Full permit processing from application to certificate of occupancy typically takes 5–7 months for a detached ADU, 3–4 months for a garage conversion or JADU.

Three Brentwood accessory dwelling unit (adu) scenarios

State law preempts Brentwood's local zoning — why this matters for your ADU

Before 2016, Brentwood (like most California cities) could block ADUs with restrictive zoning, lot-size minimums, and owner-occupancy rules. AB 68 (2021) and earlier AB 68 companion bills changed the game. California Government Code § 65852.2(e) explicitly states that local agencies (including Brentwood) cannot impose parking requirements on ADUs, cannot require owner-occupancy, cannot limit ADUs to one per lot, and cannot impose more stringent setbacks than the statewide minimums. Brentwood's local code is effectively overridden on these points. This means: (1) you do not need to live in the primary dwelling to rent out your ADU (common misconception); (2) you do not need to demonstrate financial hardship or have a family member move in; (3) you can propose a detached ADU on a 0.25-acre lot even if the city's old code said 'one ADU per lot, 0.5 acres minimum' — the state law wins. However, Brentwood still enforces IRC codes (egress, kitchen egress hoods, foundation), utility standards (separate meters), fire code (egress stairway design), and any local amendments that do not contradict state ADU law. This creates a gray zone: Brentwood's planning department might initially deny your ADU application citing a local parking requirement or owner-occupancy rule. You escalate to the building director or city manager, cite Government Code § 65852.2(e), and they reverse the denial. Avoid this delay by proactively referencing the state law in your application cover letter: 'Per CA Gov. Code § 65852.2(e), this ADU project is not subject to parking minimum, owner-occupancy, or lot-size restrictions.' The city will still review plan compliance with state codes, but they will not impose local zoning barriers.

The 60-day shot clock in AB 671 is also state-mandated. Brentwood must approve or deny your ADU permit application within 60 calendar days of a complete submission. If the application is incomplete, the city sends a detailed RFI (Request for Information) and the clock pauses while you respond. Once you resubmit, the clock restarts. Many applicants misunderstand this: they think the 60 days covers construction. It does not — 60 days is plan review and permit issuance only. Construction and inspections happen after you receive the permit. If Brentwood denies your application after 60 days, you can appeal to the city council or seek a writ of mandate in superior court claiming the denial violates state law. This rarely happens because Brentwood's planning department is aware of the statewide mandate, but it's a legal cudgel if the city tries to slow-walk your ADU.

Brentwood's building department doesn't have a published local ADU ordinance on its website (unlike Oakland or San Francisco, which have detailed ADU design guidelines). This means Brentwood reviews your ADU application against the California Building Code, the IRC, and any city-specific amendments on egress, utilities, and parking. Call the building permit office at the start and ask: 'Do you have a sample ADU site plan or a local ADU checklist?' If they don't, you're working from the IRC and state law alone. This is actually cleaner — no surprise local rule — but it means your architect or engineer must be thorough. Missing an egress window, for example, is a plan-review defect in any city; in Brentwood, it will trigger an RFI because the inspector has no local template to point you to.

Utility metering, septic (if applicable), and climate-zone foundation rules in Brentwood's mixed terrain

Brentwood spans two climate zones: 3B (coastal foothills, frost depth ~8 inches) and 5B–6B (inland and mountain areas, frost depth 12–30 inches). Your ADU's foundation depth and frost protection depend on where on the map you sit. If you're in the western part of Brentwood near the bay interface (climate zone 3B), a slab-on-grade with 6 inches of sand or gravel below meets IRC R403.3. If you're in the eastern foothills (climate zone 6B, frost depth 18–24 inches), the IRC R403.1 requires footings below the frost line — typically 24 inches deep in the foothills, sometimes 30 inches in the highest elevations. The building inspector will verify your site with a soil boring or at a minimum a visual inspection of nearby homes' foundation depth. Choosing the wrong depth triggers a costly rework: if you pour a shallow slab and the inspector flags frost-heave risk, you may need to underpin or abandon the slab. Always call the building department with your address and ask, 'What is the required footing depth for my location?' Brentwood's permit system ties address to a climate zone, so the answer is definitive.

Water, sewer, and electric metering is non-negotiable and often the longest utility-coordination phase. Brentwood's water is supplied by the City of Brentwood Public Works or Brentwood Westside Improvement District, depending on your address. Before you start design, contact the utility and request a 'letter of availability' confirming (1) a water meter can be installed at the ADU service address, (2) the district has capacity for the ADU's demand (typically 2–3 residential units = 1.5–2.5 acre-feet per year), and (3) sewer service is available (Brentwood is 100% municipal sewer — septic is not used in city limits, though it may apply in unincorporated county areas). The letter usually takes 2–3 weeks and costs $200–$400. PG&E and the local electric utility require a similar 'capacity and feasibility' letter before you design the ADU's electrical service. If your main service is 100 amps and you're adding an 800 sq ft ADU, you'll likely need a 200-amp service upgrade (estimated $3,000–$5,000). Sub-metering is an alternative if allowed by the utility: one main meter feeds two sub-panels with individual breaker feeds billed separately. This saves money but requires written approval from the utility and the city building department. Never assume sub-metering is cheaper without verifying the utility accepts it for ADUs.

Septic systems are not used in Brentwood proper. If your ADU lot is in unincorporated Contra Costa County just outside Brentwood city limits, septic design becomes critical. Septic tank size must be 1,500 gallons minimum for a two-bedroom ADU per Title 24, Chapter 15.5. The leach field (or dispersal field) must be at least 50 feet from property lines, 100 feet from water wells, and 25 feet from streams. Soil boring and percolation tests are required to size the field; cost is $1,500–$3,000. If soil percolation is poor (clay-heavy, which is common in Brentwood's inland areas), a costly engineered system (aerobic treatment unit, sand filter, or mound system) may be required — adding $8,000–$20,000. Always get a septic feasibility letter from Contra Costa County Environmental Health before you commit to a detached ADU on an unincorporated parcel. Brentwood's building department reviews septic design, but the county health department has final say.

Drainage and grading are often overlooked. If your ADU is on a slope (common in Brentwood's foothills), the site plan must show how stormwater is diverted away from the ADU foundation. A 2–5 foot swale or perimeter drain downslope from the structure is typical. If the ADU is in a flood zone (Brentwood has FEMA flood zones along creeks and low-lying areas), the foundation elevation must be above the 100-year flood elevation plus freeboard (usually 1–2 feet). Check FEMA's Flood Map Service Center for your address. If you're in a flood zone, your plan must show flood-proofed utilities (meter pedestals above flood elevation, sump pump with battery backup, etc.). Brentwood's planning or public works department will flag this on plan review if applicable. Missing flood protection is a common deficiency in foothill ADU applications.

Common questions