New Detached ADU in Brentwood - Plans, Permits & Build, One Fixed Price

Fast, Reliable New Detached ADU Across Brentwood

A new detached ADU in Brentwood, CA typically runs $280,000-$420,000 all-in for a 600-1,000 sq ft unit, from permit set through final inspection. That range covers site work, foundation, construction, and the documentation you’ll need for every visit. If an HOA has already told you no, don’t stop reading. California’s AB 670 voids any CC&R language that effectively bans a state-compliant ADU, and we handle the HOA response directly. Call (971) 497-2402 and we’ll tell you what’s buildable on your lot before you spend another dollar.

Call (971) 497-2402

Ellery ADU Studio has designed, permitted, and built New Detached ADU projects throughout Brentwood’s master-planned communities, and we know which lots inside Trilogy, Shadow Lakes, and the Garin Ranch subdivisions actually clear setback rules on paper. Our crews serve Brentwood directly, coordinated from our San Francisco Bay Area office. No franchise. No designer pointing at a builder. One contract, one written price, one team accountable.

Why Ellery ADU Studio Is Brentwood’s Preferred New Detached ADU Company

Most Brentwood homeowners we sit down with have already been told their lot can’t support a detached unit. Sometimes that’s true. More often, the person who told them never checked the rear-yard setback against Brentwood’s specific lot dimensions, or never applied California’s ADU mandates, or simply doesn’t want the paperwork. We read the title report. We pull the zoning worksheet from the city. We give you an answer in writing.

Nadia Ellery founded this studio in 2015, the year California’s ADU permitting landscape shifted, and built the practice around a simple idea: the permit set is the product. The drawings, the structural engineering, the Title 24 energy compliance, and the construction are one contract at one written price. No change orders at the door. No designer-versus-builder blame gap. Under Haven Standard Clause 1, you get a written price before any work begins, and under our 365-Day Done Right Promise, if the finished ADU isn’t built to the permitted drawings, we make it right, in writing, before the project starts.

We maintain a documented photo record on every visit to your Brentwood site. You’ll receive evidence of what was found and what was done, not a summary of how it felt.

Our New Detached ADU Services in Brentwood

Custom Built ADU

A custom detached ADU gives you full control over layout, roofline, and interior finish, and on Brentwood’s 6,000-10,000 sq ft lots, it’s often the better financial move than trying to force a conversion inside an existing footprint. We design the unit to clear your specific rear-yard setback, orient it for the Delta breeze, and detail the exterior to handle Brentwood’s summer heat without making the interior a glass box that cooks by 2 p.m. You get a permit set that a plan checker in Brentwood will actually approve, not a generic drawing that bounces twice.

Prefab ADU Installation

For Brentwood homeowners who want a shorter site timeline, we specify and coordinate prefab systems from Abodu, Cover, Dvele, and Mighty Buildings within the same single contract. Prefab doesn’t skip engineering; it front-loads it off-site. We still run the same foundation review, same utility tie-in, same Title 24 calcs. The difference is that fewer weeks of framing happen in your backyard. On Brentwood lots with tight access or HOA construction-hour restrictions, that matters.

Modular ADU

Modular construction is a subset of prefab: the unit arrives in modules, set by crane or truck, and we finish the cladding, porch, and site work on-site. Brentwood’s wide streets and newer subdivision layouts make module delivery straightforward compared to older East Bay cities with single-lane access. We coordinate module set, utility connections, and the final site grading under one written scope.

Studio ADU

A studio ADU in Brentwood, typically 400-600 sq ft, runs $190,000-$290,000 all-in, drawings through final. That’s where we put the real number on the page. For a unit that size, a ductless mini-split with a Title 24-compliant insulation package is not an upgrade here; it’s the minimum that keeps the space livable through July and August, when Brentwood regularly sits at 100-105°F with no marine layer relief. We spec the insulation and the HVAC as standard on every studio, not as a line-item upsell.

Trusted Brands We Service in Brentwood

We specify and coordinate prefab and modular systems from Abodu, Cover, and Dvele for Brentwood projects. That means your detached ADU goes through the same design review, same written pricing, and same documentation standard as a fully custom build. When a prefab module arrives on your Brentwood lot, we’ve already confirmed the site pad, the utility trench, and the foundation line against the manufacturer’s drawings. You aren’t coordinating three vendors. You have one point of accountability and one written price, per Haven Standard Clause 1.

Common New Detached ADU Problems We See in Brentwood Homes

• HOA violation letters that misstate the law. Brentwood’s master-planned communities, particularly Trilogy, send written warnings claiming CC&Rs forbid detached ADUs. AB 670 (2019) voids any HOA rule that effectively bans a state-compliant ADU. We respond to the HOA in writing, cite the statute, and move the permit forward.
• Interior-grade window frames and unsealed paver joints. Brentwood’s Delta air carries enough salt and humidity to start white rust on standard zinc-plated hardware by the second winter. We spec stainless or hot-dip galvanized fasteners, marine-grade window frames, and polymer-based joint sand on every exterior detail.
• Slabs poured without a vapor barrier. A monolithic slab without an underslab vapor barrier, combined with a mini-split lineset run through an uninsulated chase, will condense in Brentwood’s hot-cold swings. That condensation becomes mold inside the wall cavity. We detail the barrier and insulate every chase, not just the conditioned space.
• Fence posts and deck framing rotted at grade. Pressure-treated posts set without a bituminous coating or concrete collar rot at the soil line within five years in Brentwood’s clay-heavy soils. We spec pressure-treated posts with a bituminous coating, set in concrete, with stainless post-to-beam connectors.

What Salt Air Near Brentwood Decides About Your Materials

Brentwood sits far enough inland that people assume salt air isn’t a factor. It is. The Delta carries airborne salt and humidity east through the Carquinez Strait, and Brentwood’s flat, open lots get the full summer UV load with almost no tree canopy. The failure we see most often isn’t structural; it’s fastener corrosion. Standard zinc-plated screws, brackets, and post anchors show white rust within 18 to 24 months on exposed exterior connections. On a detached ADU with a wooden deck, pergola, or covered porch, that means every post-to-beam connection, every railing bracket, and every joist hanger is a future replacement point if the spec is cheap.

We replaced a corroded lightweight aluminum patio cover with a hot-dip galvanized steel frame and stainless steel fasteners on a New Detached ADU in the Trilogy community. The original structural brackets showed white rust at every post-to-beam connection within two years of installation. Our crew also sealed all paver joints with polymer-based sand and swapped the fence posts for pressure-treated posts set in concrete with a bituminous coating. The previous owner’s standard wood posts had rotted at grade in under five years. That’s the Brentwood pattern: the cheap spec fails fast, and the repair costs more than the coastal-spec upgrade ever would have.

The price gap between a coastal-spec and an inland-spec version of the same detached ADU is roughly $8,000-$14,000 on a full project: stainless fasteners instead of zinc, marine-grade window frames, sealed paver joints, polymer-based sand, a vapor barrier under the slab, and insulated chases. Within a decade, the inland-spec version will need a re-roof fastener replacement, window frame repair, and paver joint rehab, and that invoice lands around $18,000-$25,000. We don’t offer the cheap spec. We write the coastal-spec price, explain why, and let you take the line-item comparison to any other builder in Brentwood.

How Brentwood’s HOA Reality Changes a Detached ADU Project

Brentwood’s concentrated HOA-governed master-planned communities, including the large Trilogy 55+ development, mean a New Detached ADU job often starts with a written CC&R violation notice on the kitchen counter. The homeowner calls us already frustrated, sometimes already resigned. California’s AB 670 changes that conversation. The law voids any HOA rule that effectively bans a state-compliant ADU, which means the HOA’s own document is legally unenforceable on this point. We cite AB 670 by name in our written response to the HOA. We send it on our letterhead. We copy the homeowner on everything. The permit still goes through the City of Brentwood like any other project.

Generalist contractors walk away from these jobs because they don’t know the state law overrides the HOA document. We don’t. If your HOA has already told you no, call (971) 497-2402 and we’ll tell you whether you actually have a path forward, at no cost.

Pricing for New Detached ADU in Brentwood, CA

We Also Serve Cities Near Brentwood

Our Brentwood service area includes the surrounding East Contra Costa communities of Oakley, Antioch, Pittsburg, and Danville. The same written-price standard, the same documentation, the same one-contract design-build model applies everywhere we work. If you’re just over the Brentwood line, we’ll still assess your lot and give you a written number.

FAQs - New Detached ADU in Brentwood