ADU in Brentwood, CA
Junior ADU & Interior Conversion in Brentwood typically runs $85,000-$175,000 all-in, drawings through final inspection, while ADU Site Prep & Utilities adds $15,000-$65,000 depending on what’s already in place. We’re Ellery ADU Studio, and we handle both under one contract at one written price. Call (213) 698-7591 to schedule your free estimate.
Brentwood’s 1990s-2000s building boom left a housing stock almost purpose-built for ADU work: large lots, attached 3-car garages, and newer infrastructure that cuts the surprise costs common in older cities. But those same tract builders tied garage ceilings into main-house trusses in ways that complicate height compliance, and summer temperatures hitting 110°F push mechanical cooling from optional to essential. We design and build around both realities.
Why Brentwood Homes Are Different
Brentwood sits in an inland valley where summer highs routinely reach 100-110°F, making it the hottest jurisdiction in greater Contra Costa County. Afternoon Delta breezes off the Sacramento-San Joaquin Delta help, but they’re inconsistent, so every ADU we spec here is sized for full peak cooling loads with mini-splits and heavy envelope insulation. Coastal Contra Costa standards don’t cut it.
The housing stock matters too. Virtually everything residential here was built between 1990 and 2015 in planned subdivisions: two-story and single-story tract homes on 6,000-10,000+ sq ft lots with attached multi-car garages, wide side yards, and electrical panels that usually don’t need immediate replacement. That profile gives most properties ample coverage and setback room for detached ADUs or straightforward garage conversions.
But there’s a catch. Several master-planned communities, including 55+ developments like Trilogy at the Vineyards, carry HOA CC&Rs recorded before California’s full ADU preemption statutes took effect. City permits are obtainable, yet HOA enforcement threats can still stall projects. We review recorded restrictions before any drawings begin.
The garage framing quirk is the one that catches most owners. Brentwood’s tract builders commonly tied 3-car garages, 600-700 sq ft, structurally into the main home’s truss system. That complicates ceiling height compliance with California’s 7-foot minimum in ways we don’t see in cities with older, separately built garage stock. We assess this on every garage conversion walkthrough.
Our Main Brentwood Services
Most Brentwood homeowners start with one of these three services. Each has its own dedicated Brentwood page with deeper detail.
ADU Design & Permitting in Brentwood
We treat the permit set as the core product, not an afterthought. In Brentwood, that means navigating Contra Costa County’s specific fire-safety and energy-code triggers while documenting the existing structure to head off plan-check revisions. Our permit sets include structural calculations, Title 24 energy compliance, and photodocumentation of existing conditions. ADU Design & Permitting in Brentwood covers our full process.
Garage Conversion ADU in Brentwood
Brentwood’s large attached garages are technically ideal for conversion, but the truss-tied ceiling issue requires early structural assessment. We evaluate ceiling height, slab condition, and utility routing before any pricing, then deliver a written scope that accounts for foundation, framing modification, and mechanical routing. Garage Conversion ADU in Brentwood has the full breakdown.
New Detached ADU in Brentwood
With 6,000-10,000+ sq ft lots common across Brentwood, setback and coverage compliance is usually straightforward. We coordinate prefab systems from brands like Abodu and Plant Prefab within the same single contract, or build stick-frame where site access or customization demands it. New Detached ADU in Brentwood explains sizing and options.
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.
Junior ADU & Interior Conversion in Brentwood
Junior ADU & Interior Conversion in Brentwood solves the spatial puzzle created by the city’s large-lot, large-home stock: owners want rental income or multigenerational housing without the cost or footprint of a detached unit. Junior ADU & Interior Conversion covers our full service, but Brentwood’s specific conditions - newer electrical panels, truss-tied garages, and extreme cooling loads - shape every project we design here. Interior work avoids setback and coverage math entirely, and often moves faster through permit.
Room-to-ADU Conversion
Signs you need it
You have a ground-floor bedroom suite or den with exterior access, a window that meets egress, and you want rental income without adding square footage.
What it costs in Brentwood
Room-to-ADU conversion in Brentwood typically runs $85,000-$140,000 all-in, including the permit set, kitchen and bath rough-in, and final inspection.
How we do it
We verify egress and fire separation, design the kitchen and bath layout to meet JADU code, produce the permit set, and build through final with a documented photo record at each visit.
Interior JADU
Signs you need it
You want the smallest legal rental unit inside your existing home, sharing systems with the main house, with no exterior construction required.
What it costs in Brentwood
Interior JADU work in Brentwood typically runs $75,000-$125,000, including the permit set, utility tie-ins, and code-compliant kitchen and bath.
How we do it
We identify the smallest qualifying space, design for shared mechanical and electrical, produce the JADU permit set, and build to the 500 sq ft maximum with a written price locked at signing.
What Junior ADU & Interior Conversion Costs in Brentwood
| Item | Typical range in Brentwood |
|---|---|
| Room-to-ADU conversion | $85,000-$140,000 |
| Interior JADU | $75,000-$125,000 |
| Above-garage ADU (if applicable) | $120,000-$175,000 |
Can a JADU have its own kitchen and bathroom?
Yes, a JADU must have efficiency kitchen facilities and may have a separate bathroom or share one with the main dwelling; we design either layout to code.
Does interior work need the same permits as a detached ADU?
Yes, JADUs and interior conversions still require building permits, fire separation review, and utility inspections, though they skip setback and coverage review.
ADU Site Prep & Utilities in Brentwood
ADU Site Prep & Utilities in Brentwood addresses the gap between a permitted design and a buildable site: foundation work, electrical capacity, water and sewer laterals, and the hardscape restoration that follows. ADU Site Prep & Utilities is our full service hub. Brentwood’s newer infrastructure helps, but 1990s-era electrical panels and Delta-adjacent soil conditions still create surprises we catch in pre-construction assessment.
Electrical Panel Upgrade
Signs you need it
Your existing panel is rated under 200 amps, has no open breaker slots, or shows corrosion from inland valley temperature swings.
What it costs in Brentwood
Electrical panel upgrade in Brentwood typically runs $4,500-$8,500, including PG&E coordination, permit, and inspection.
How we do it
We assess load calculations for the ADU plus main house, specify the new panel and service route, pull the permit, and coordinate PG&E meter work with our own crew.
Foundation & Grading
Signs you need it
Your planned ADU site has slope over 2%, expansive clay soils, or an existing garage slab under 4 inches thick.
What it costs in Brentwood
Foundation and grading work in Brentwood typically runs $12,000-$35,000, including soils assessment, engineered fill if needed, and slab or stem wall.
How we do it
We verify slab thickness and soil conditions, design for Brentwood’s expansive clay where present, excavate or grade, pour to engineered spec, and document with photos before backfill.
What ADU Site Prep & Utilities Costs in Brentwood
| Item | Typical range in Brentwood |
|---|---|
| Electrical panel upgrade | $4,500-$8,500 |
| Foundation & grading | $12,000-$35,000 |
| Plumbing lateral extension | $3,500-$12,000 |
Will PG&E require a service upgrade for my ADU?
PG&E requires a service upgrade when the combined load of main house plus ADU exceeds your existing meter capacity; we calculate this during design and include it in scope if needed.
Does site prep include landscaping restoration?
Hardscape restoration is quoted separately; we restore disturbed areas to grade and compacted soil, with paving or planting by others unless specified in your scope.
Brands We Service in Brentwood
We specify and coordinate prefab and modular systems from Plant Prefab, Tuff Shed, and James Hardie within our single-contract model, with parts and submittals stocked for Brentwood-area delivery timelines. Turnaround on brand-specific submittals typically runs 2-3 weeks once the permit set is approved. See our brands for full compatibility.
Pricing for ADU in Brentwood, CA
| Service | Typical range in Brentwood |
|---|---|
| Room-to-ADU conversion | $85,000-$140,000 |
| Interior JADU | $75,000-$125,000 |
| Above-garage ADU | $120,000-$175,000 |
| Electrical panel upgrade | $4,500-$8,500 |
| Foundation & grading | $12,000-$35,000 |
| Plumbing lateral extension | $3,500-$12,000 |
| Gas line extension | $2,500-$7,500 |
| Landscaping & hardscape | $5,000-$18,000 |
What pushes price up in Brentwood: truss modification for garage conversions, 200-amp service upgrades when the main panel is maxed, and enhanced insulation and mini-split sizing for 110°F peak loads. What doesn’t: the newer infrastructure common here. Under Haven Standard Clause 1, your written price is locked before any work starts.
Neighborhoods We Serve in Brentwood
We work across Brentwood’s full residential footprint, from the established subdivisions near downtown to the newer master-planned edges. Crews dispatch from our San Francisco Bay Area coordination office with scheduling tied to Brentwood permit inspection windows.
- Downtown Brentwood
- Shadow Lakes
- Trilogy at the Vineyards
- Apple Hill
- Summers Ranch
Service Area - Cities Near Brentwood
Our crews serve the full Contra Costa and central Bay Area corridor. Homeowners in Sherman Oaks, CA, Studio City, CA, West Hollywood, CA, and Beverly Hills, CA can access the same single-contract design-build model and written-price guarantee.
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
Contra Costa County plan review for ADUs typically runs 8-14 weeks depending on backlog, with faster track for JADUs and interior conversions that skip setback review. We submit a complete permit set with structural calculations and Title 24 compliance to minimize revision rounds.
California law limits HOA restrictions on ADUs, but Brentwood’s older CC&Rs in communities like Trilogy at the Vineyards create enforcement grey zones; we review recorded restrictions during design and document the legal basis before permit submission.
We pull all permits as part of our single-contract scope; the permit set is our core product, and we manage plan check, revisions, and inspection scheduling through final.
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
"The rental income has covered the payment since the second month. They were realistic about the numbers rather than optimistic."
- Odette N."The permit set and the build were one contract with one number. Our neighbours used two firms and spent a year watching them blame each other."
- Helena V."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.