ADU in Brentwood, CA
ADU design and permitting in Brentwood usually runs $18,000-$35,000 for the full permit set, and the work that trips up most projects here is the HOA response letter, not the drawings themselves. Brentwood’s late-1990s through 2010s stucco-and-tile tract homes sit on lots large enough for a compliant detached unit, but a surprising number carry CC&Rs that claim to forbid rentals or extra units outright. We quote a written price before any work starts, per Haven Standard Clause 1, and we include the HOA response in the permit set when it’s needed. Call (971) 497-2402 to start with a site assessment.
Why Brentwood Homes Are Different
Brentwood’s housing stock is overwhelmingly late-1990s through 2010s stucco-and-tile single-family tract homes on lots from 6,000 to over 10,000 square feet. That’s a younger, more uniform building stock than most Bay Area cities, and it changes the ADU math in two directions at once. The attached 2-3 car garages and modern 200-amp electrical panels mean a garage conversion or detached build rarely requires the kind of infrastructure replacement you’d expect in a 1940s bungalow. But the cookie-cutter subdivision layouts are tightly mirrored: your setback line, your neighbor’s fence, and the utility easement all line up the same way on every lot, which means a siting error on one plan is a siting error on forty similar plans. We read the recorded plat map, not just the assessor’s parcel, before we draw anything.
The other Brentwood reality is the heat. Summer temperatures routinely hit 100-105°F, and the Delta breeze doesn’t reach every subdivision equally. A ductless mini-split and Title 24-compliant insulation are not upgrades in this climate; they’re the floor. And because winter nights can dip near freezing while summer afternoons bake the south wall, any plumbing or HVAC that crosses an unconditioned garage conversion needs to be insulated as if it’s exterior work. We treat it that way from the first drawing.
Finally, a significant share of Brentwood homes sit inside HOA-governed master-planned communities, including the large Trilogy 55+ development. Homeowners in these neighborhoods frequently receive written warnings claiming their CC&Rs prohibit ADUs. California’s AB 670, passed in 2019, voids any HOA rule that effectively bans a state-compliant ADU. We cite that statute by name in the response letter we send to the HOA, and that single document has converted more than a few “blocked” homeowners into permitted projects.
Our Main Brentwood Services
Two services account for most of the ADU calls we take from Brentwood homeowners: garage conversions, which leverage the large attached garages common to the city’s tract housing, and new detached ADUs, which sit better on the larger lots in the city’s newer subdivisions. Here’s how each one works in Brentwood specifically.
Garage Conversion ADU in Brentwood
Brentwood’s tract homes were built with attached 2-3 car garages that often exceed 400 square feet, and the slab under them is typically 4 inches of post-1990 concrete, well within range for a conversion without a new foundation. The work turns an underused garage into a permitted living unit with its own entrance, kitchen, and bath, while keeping the roofline and setbacks untouched, which tends to make HOA review smoother too. The main Brentwood-specific condition we check first is the attic insulation above the garage ceiling: many of these homes were built with minimal insulation there, and a conversion locks in whatever’s already above the drywall. Full scope and local pricing live on the dedicated page for Garage Conversion ADU in Brentwood.
New Detached ADU in Brentwood
The larger Brentwood lots, many over 8,000 square feet, leave genuine room for a detached unit without crowding the main house or the neighbor’s fence line. A new detached ADU is a clean-sheet build, which means we can orient it for shade on the west wall, size the mini-split for a 105°F summer day, and place windows where the Delta breeze actually reaches. On Brentwood’s mirrored subdivision lots, the siting exercise is more precise than on a custom-home lot, because the same setback and easement geometry repeats across every parcel on the street. Detailed scope, siting logic, and Brentwood-specific pricing are on the New Detached ADU in Brentwood page.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
ADU Design & Permitting in Brentwood
ADU design and permitting in Brentwood carries an extra document that most Bay Area cities don’t require: an HOA response letter for the high percentage of homes inside master-planned communities like Trilogy, where CC&Rs often claim to ban rentals or additional units. We write that letter ourselves, cite AB 670 by name, and submit it alongside the permit set, because the permit counter isn’t the obstacle here; the HOA’s legal counsel is. The other Brentwood condition that shapes this service is the heat: every plan set we produce for this city includes a Title 24 compliance path sized for 100°F-plus summer afternoons, not the coastal baseline most energy software defaults to. Here’s what the work actually involves, including the line-item costs. For the full scope across every project type, see ADU Design & Permitting.
Site Assessment
Signs you need it
You’ve been told by an HOA or a contractor that your lot can’t take an ADU, or you don’t know whether your setback, easement, or panel capacity allows one at all.
What it costs in Brentwood
A site assessment for an ADU project in Brentwood typically runs $400-$900, depending on lot complexity and whether an HOA response is required.
How we do it
We measure the lot against the recorded plat map, check the panel rating and service drop, walk the setback lines, and confirm whether the CC&Rs on file actually prohibit anything, then deliver a written go/no-go with the reasons named.
HOA & Zoning Compliance
Signs you need it
You live in an HOA-governed Brentwood subdivision and have received, or expect, a written notice claiming your CC&Rs prohibit an ADU or a rental unit.
What it costs in Brentwood
HOA and zoning compliance work in Brentwood, including the AB 670 response letter, typically runs $1,200-$2,500 as part of the permit set.
How we do it
We read the recorded CC&Rs, match them against current state ADU law, draft the response letter to the HOA’s management, and submit it with the permit application so the city and the HOA see the same legal position.
What ADU Design & Permitting Costs in Brentwood
| Item | Typical range in Brentwood |
|---|---|
| Site assessment | $400 - $900 |
| Full permit set (architectural, structural, Title 24) | $18,000 - $35,000 |
| HOA & zoning compliance response letter | $1,200 - $2,500 |
How long does ADU permitting take in Brentwood?
Brentwood planning review typically takes 60 to 120 days for a complete ADU application, plus HOA response time when a master-planned community is involved. A complete, code-cited permit set shortens the review, and we handle every resubmission. Call (971) 497-2402 to get started.
Does AB 670 really override my Brentwood HOA?
Yes. California AB 670, effective 2019, voids any HOA rule that effectively bans a state-compliant ADU, and this is settled law, not a legal gray area. We cite it by name in every Brentwood HOA response letter. Call (971) 497-2402 for a written second opinion if you’ve already been told no.
Junior ADU & Interior Conversion in Brentwood
Junior ADUs and interior conversions in Brentwood are the fastest permitted path to a rentable unit, and the city’s tract floor plans actually reward them: the mirrored layouts mean a standard bedroom-over-garage conversion repeats cleanly across dozens of homes on the same street. The Brentwood catch is insulation, because a JADU built out of a conditioned bedroom doesn’t face the same heat problem as a garage conversion, but an interior conversion that touches an exterior wall on the south or west side of the house has to be insulated and glazed for 105°F reality, not for a coastal Title 24 baseline. The full service scope is on Junior ADU & Interior Conversion.
Interior JADU
Signs you need it
You have a primary bedroom with an attached bath, or a large bedroom wing that can be walled off with its own entrance, and you want a permitted rental unit without touching the exterior.
What it costs in Brentwood
An interior JADU in Brentwood typically runs $80,000-$130,000 all-in, including the kitchenette, separate entrance, and the required owner-occupancy filing.
How we do it
We draw the separation wall and entrance, file the JADU owner-occupancy covenant with the county, and pull the permit as a single scope with a written price before any work starts.
Room-to-ADU Conversion
Signs you need it
Your home has a finished bonus room, den, or formal dining space that sits underused, with a window pattern and access point that already work for a separate unit.
What it costs in Brentwood
A room-to-ADU conversion in Brentwood typically runs $90,000-$150,000, with the spread driven by whether the existing walls and electrical meet current code or need opening up.
How we do it
We inspect the existing framing and wiring, confirm what stays and what gets replaced, and draw the conversion to preserve as much of the finished space as the code allows, with a written price before the first wall is touched.
What Junior ADU & Interior Conversion Costs in Brentwood
| Item | Typical range in Brentwood |
|---|---|
| Interior JADU (full scope) | $80,000 - $130,000 |
| Room-to-ADU conversion | $90,000 - $150,000 |
| Above-garage ADU | $250,000 - $400,000 |
Does a Brentwood JADU require owner-occupancy?
Yes. A California junior ADU requires the owner to live in either the main house or the JADU, and that covenant is recorded with the county at permitting. We file it for you as part of the permit set. Call (971) 497-2402 for a written price.
Can I convert an above-garage room in Brentwood?
Usually yes, if the garage ceiling structure and the access stair meet code, though many Brentwood above-garage bonus rooms need structural reinforcement and insulation upgrades for a permitted conversion. We’ll tell you in the site assessment. Call (971) 497-2402 to schedule one.
ADU Site Prep & Utilities in Brentwood
Site prep and utilities in Brentwood are defined by two local constants: the 200-amp panels that came standard on the city’s late-1990s and newer tract homes, and the hard, dry clay soil that makes trenching straightforward in summer but slow in winter when the ground holds water. Most Brentwood ADUs can tap the existing panel without an upgrade, which removes a $4,000-$8,000 line item that older Bay Area cities routinely hit. The work we watch closest is the plumbing lateral, because crossing a 10,000-square-foot lot to reach the sewer line is a different dig than a 6,000-square-foot lot, and the difference shows up in the trenching line item, not the fixture count. Scope detail is on ADU Site Prep & Utilities.
Electrical Panel Upgrade
Signs you need it
Your panel is 100 amps, or your 200-amp panel has no open breaker slots and no room for a subpanel feed to the ADU.
What it costs in Brentwood
A 200-amp panel upgrade in Brentwood typically runs $4,000-$8,000, and many Brentwood homes already have the 200-amp service that lets them skip this line item entirely.
How we do it
We run a load calculation against the ADU’s Title 24 design, confirm whether the existing panel carries it, and upgrade only when the math requires it, never as a default upcharge.
Foundation & Grading
Signs you need it
Your lot slopes toward the house or the street, or the ADU sits on a corner of the parcel where the existing grade won’t shed water away from the new slab.
What it costs in Brentwood
Foundation and grading for a detached ADU in Brentwood typically runs $18,000-$35,000 depending on lot slope, soil bearing, and the foundation type the plan calls for.
How we do it
We take soil borings before we finalize the foundation detail, then grade, form, and pour to the permitted plan, with a documented photo record of every stage before the slab is covered.
What ADU Site Prep & Utilities Costs in Brentwood
| Item | Typical range in Brentwood |
|---|---|
| Electrical panel upgrade (200-amp) | $4,000 - $8,000 |
| Foundation & grading (detached ADU) | $18,000 - $35,000 |
| Plumbing lateral & gas line extension | $5,000 - $12,000 |
Do I need a panel upgrade for a Brentwood ADU?
Not automatically. Most Brentwood homes built after the late 1990s have 200-amp panels, which typically support a studio or one-bedroom ADU without an upgrade. We run the load calculation before assuming. Call (971) 497-2402 for a site assessment.
How long does trenching take in Brentwood’s clay soil?
Utility trenching for a typical Brentwood ADU takes two to five working days in dry summer soil and longer in winter when the clay holds water and slows the dig. We schedule trenching for the driest window on the permit calendar. Call (971) 497-2402 to plan yours.
Brands We Service in Brentwood
We specify and coordinate prefab and modular ADU systems from Cover, Dvele, Mighty Buildings, and Boxabl within the same single-contract design-build scope, including Brentwood site prep and foundation coordination. Parts and panel lead times vary by brand, but our our brands page tracks the current turnaround for each. We never ask you to call the factory; that’s our job.
Pricing for ADU in Brentwood, CA
| Service | Typical range in Brentwood |
|---|---|
| Site assessment | $400 - $900 |
| Garage conversion ADU (full scope) | $120,000 - $200,000 |
| New detached ADU (full scope) | $250,000 - $400,000 |
| Full permit set (architectural, structural, Title 24) | $18,000 - $35,000 |
| HOA & zoning compliance response letter | $1,200 - $2,500 |
| Interior JADU (full scope) | $80,000 - $130,000 |
| Room-to-ADU conversion | $90,000 - $150,000 |
| Electrical panel upgrade (200-amp) | $4,000 - $8,000 |
| Foundation & grading (detached ADU) | $18,000 - $35,000 |
| Plumbing lateral & gas line extension | $5,000 - $12,000 |
What pushes a Brentwood ADU toward the top of these ranges is usually one of three things: a lot that slopes enough to require extra grading, an HOA response that goes to legal counsel, or an existing structure that needs structural opening-up rather than finish work. A newer 200-amp panel and a flat tract lot pull the number down. Every project we quote carries a written price before any work begins, per Haven Standard Clause 1, and that price includes the drawings, the permit set, and the construction as one contract, no designer-versus-builder gap.
Neighborhoods We Serve in Brentwood
We work throughout Brentwood, from the older subdivisions around downtown to the newer master-planned communities on the city’s edges. Every neighborhood in the city gets the same written-price-before-work rule and the same documented photo record on every visit. Typical response time for a Brentwood site assessment is within five to seven business days, depending on the season.
- Trilogy at the Vineyards
- Shadow Lakes
- Garaventa Ranch
- Apple Hill Estates
- Deer Ridge
Service Area - Cities Near Brentwood
Beyond Brentwood, our crews cover the surrounding East Contra Costa corridor, including Oakley, CA to the west, Antioch, CA on the river side, and Pittsburg, CA up toward the 4. We also take ADU design and permitting work in Danville, CA when the scope suits a single-contract design-build. Scheduling for Brentwood is coordinated through our San Francisco Bay Area office.
Serving Brentwood, CA - Our Local Coverage Area
We’re based in the Brentwood area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs About ADU in Brentwood
We typically schedule Brentwood site assessments within five to seven business days, with a written price and a go/no-go delivered after the walk. The assessment covers lot geometry, panel capacity, HOA status, and soil conditions, and it’s the starting line for any design or pricing work. Call (971) 497-2402 to book one.
California no longer imposes a statewide owner-occupancy requirement on standard ADUs, though a Junior ADU does require the owner to live on the property. Some Brentwood HOAs still claim an occupancy restriction in their CC&Rs, which AB 670 overrides. We’ll check your recorded documents in the site assessment.
Yes. We write and send the AB 670 response letter to the HOA or its management company, and we submit a copy with the permit application so the city and the HOA are looking at the same legal argument. Homeowners who’ve already been told no by an HOA should call (971) 497-2402 for a free second opinion on their written denial.
What happens when you call
- 1A real person answersNo phone trees - talk to a local adu pro.
- 2You get an upfront price rangeHonest numbers before anyone is dispatched. No surprises.
- 3We come out and measureLicensed & insured, at a time that suits you - the visit and the quote are both free.
- 4You get the price in writingLine-itemed, yours to keep, and nothing starts until you say go.
What Brentwood Customers Say
"They walked us through setbacks and utility capacity before we paid anything, and told us one of our two options was not going to get approved."
- Mireille T."Every allowance was itemised at the start, so the finish selections did not turn into a surprise at the end."
- Benedict H."Handled the whole planning process. We went to one hearing and that was it."
- Rowena C.