Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
Before
After
Before
After
Before
AfterFast, Reliable Home Additions & ADU Across Brentwood
A detached ADU or room addition in Brentwood typically runs $45,000-$130,000 for attached work and $90,000-$220,000 for a standalone unit, with most projects taking 4-8 months from permit to final inspection. We coordinate dispatch and scheduling from our San Francisco Bay Area office, and our crews serve Brentwood directly - no subcontractor handoffs, no out-of-area crews learning the local code on your dime. Call (213) 652-7670 to schedule your free estimate and written price.
We’ve worked across Brentwood’s 2000s-era subdivisions long enough to know the specific failure patterns of these builds: the undersized roof overhangs, the standard-grade windows that buckle under Delta heat, and the expansive clay subsoils that heave flatwork within a few seasons. That local knowledge changes what we spec and what your project costs. A contractor who treats Brentwood like any inland California city misses the soil and climate variables that determine whether your addition lasts 10 years or 50.
Why Rowan Construction Group Los Angeles Is Brentwood’s Preferred Home Additions & ADU Company
Our reputation in Brentwood was built project by project, not by marketing spend. Homeowners here research contractors the way they research appliances - they read the three-star reviews, they screenshot prices, they want documentation before they commit. We fit that workflow. Every Rowan project since 2007 has started with a flat written price and written scope, and every crew visit produces a documented photo record the customer keeps. No asterisks, no “call for details.”
Response time to Brentwood is same-week for estimates, with a live person answering calls 24/7 to schedule. We know the local permit landscape: Brentwood falls under Contra Costa County’s unincorporated jurisdiction for some parcels and the city’s own planning department for others, with different setback and utility-connection rules for each. Our permitting team files in both offices regularly. We also understand the neighborhood-specific soil conditions - from the stable ground near downtown Brentwood to the expansive clay fields east of Marsh Creek Road - and we adjust foundation specs accordingly.
Grant Rowan, our founder, established the Haven Standard in 2007 with Clause 1 as the non-negotiable: the price is written before anyone starts. That discipline is why our projects finish within a few percent of the agreed number across 4,000-plus jobs. Brentwood homeowners who’ve been burned by open-ended construction contracts tend to notice the difference.
Our Home Additions & ADU Services in Brentwood
ADU Design & Permitting
ADU permitting in Brentwood requires navigating either Contra Costa County or city planning depending on your parcel, plus utility coordination with East Bay Municipal Utility District for water and sewer tie-ins. Our design-build team produces permit-ready plans that account for Brentwood’s specific requirements: fire separation distances for detached units, parking replacement rules for garage conversions, and the utility-capacity letters EBMUD demands before approving new meters. We handle the submittal, the corrections, and the resubmittal - the administrative work that stalls most homeowner-driven ADU projects for months.
Garage Conversion ADU
Garage conversions in Brentwood run $30,000-$80,000, and they’re the fastest path to rental income for homeowners on standard 6,000-square-foot lots where setbacks prevent detached construction. The catch: most Brentwood garages from the 2000s tract builds sit on slabs poured over expansive clay without engineered fill, and the conversion to habitable space requires a structural evaluation. We’ve converted garages in the Shadow Lakes neighborhood and off Balfour Road where the existing slab had to be cut, re-poured with over-excavated fill, and tied to the new ADU foundation. We quote that work upfront, not as a surprise change order.
New Detached ADU
A new detached ADU in Brentwood is our most specified project type here, and it’s where the local soil and climate variables matter most. East Brentwood’s subdivisions built on former Marsh Creek farmland sit on expansive clay that moves seasonally, so any ADU slab here needs post-tension or over-excavated engineered fill - a step most inland California cities with stable soils never require. On a detached ADU off Marsh Creek Road, our crew pulled up the existing patio flatwork and found the clay subsoil had heaved nearly two inches since the 2004 pour, so we re-engineered the slab with over-excavated fill and hot-dip galvanized anchor bolts before framing. We spec’d stainless fasteners, aluminum-clad window frames, and sealed paver joints on the new hardscape because the Delta’s summer dust and 105°F swings eat standard-grade hardware fast. The punch list passed inspection sign-off on the first walk, and the ADU’s draw schedule stayed on track despite the slab correction.
Detached ADU construction in Brentwood runs $90,000-$220,000 depending on size, finish level, and utility-extension complexity. We quote that as a flat price with a written scope, not a per-square-foot estimate that balloons.
Junior ADU & Interior Conversion
Junior ADUs - converted interior space with a separate entrance, no new foundation required - work well in Brentwood’s larger tract homes where a bedroom wing or attached garage can be reconfigured. The 2000s-era floor plans often have generous master suites or bonus rooms that convert efficiently. We assess the existing HVAC capacity, the electrical panel’s spare amperage, and the sewer line’s slope before quoting. In Brentwood’s summer heat, we also evaluate whether the existing insulation and window specs can handle the thermal load of a now-occupied space, or whether the addition needs its own mini-split and upgraded glazing.
ADU Site Prep & Utilities
Site prep for Brentwood ADUs means addressing the clay subsoil first. We over-excavate and recompact with engineered fill on any east Brentwood parcel where geotechnical reports or neighboring flatwork failure indicates expansive soil. Utility work means coordinating with EBMUD for water and sewer, PG&E for gas and electric service upgrades, and sometimes Comcast or AT&T for communication drops. We handle the applications, the trenching, and the inspections - and we document each phase with the photo record that’s standard on every Rowan visit, not an upgrade.
Room Additions & Second Story Additions
Second-story additions in Brentwood face a specific challenge: the 2000s tract homes were built with standard-grade materials and minimal roof overhangs, so the thermal performance is already marginal. Adding conditioned space above without addressing the original envelope means the new room fights the same heat load the builder never solved. We typically spec upgraded attic insulation, radiant barrier sheathing, and aluminum-clad windows as part of the structural package - not as optional upsells, but as necessary for the addition to perform. The clay soil question also applies: a second story adds load to a foundation that may already be moving seasonally. We evaluate the existing slab and footings before finalizing structural plans.
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.
Before
AfterTrusted Brands We Service in Brentwood
Our sourcing depth across fifty-plus vetted brands means the written quote contains honest numbers rather than inflated ones. For Brentwood ADU and addition projects, we regularly specify KraftMaid and Wellborn cabinetry, Cambria and Caesarstone countertops, and Kohler and Toto fixtures - brands with established distribution in the East Bay, so replacement parts don’t require cross-country shipping if a warranty issue arises. We also work with prefabricated ADU manufacturers including Abodu and Cover for homeowners who want a faster path to permit and construction, and Dvele for net-zero-ready units that address Brentwood’s extreme summer cooling loads. Local inventory and established vendor relationships keep material lead times predictable, which keeps your draw schedule on track.
Common Home Additions & ADU Problems We See in Brentwood Homes
- Slab and flatwork cracking from expansive clay movement. Additions built on standard-grade concrete without over-excavated engineered fill heave within a few seasons in east Brentwood subdivisions. We see this across entire neighborhoods near Marsh Creek, not as isolated bad luck.
- Fastener and hardware corrosion from Delta heat cycling and dust. Standard galvanized or electroplated fasteners on roofing, railings, and fence posts corrode and fail early here. We spec stainless or hot-dip galvanized hardware as standard - the coastal-spec version of the job that costs more upfront and saves thousands in replacement.
- Thermal overload from undersized roof overhangs and standard-grade windows. The 2000s tract homes trap heat, so additions end up fighting the same thermal load the original build never solved. We upgrade the envelope as part of the structural package, not as an optional extra.
- Utility capacity limits on older mains. EBMUD and PG&E infrastructure in the fastest-built 2000s neighborhoods sometimes requires service upgrades for ADU water and electric meters. We identify this during pre-construction so it doesn’t stall permit approval.
Pricing for Home Additions & ADU in Brentwood, CA
Home additions and ADU work in Brentwood carries a specific cost profile shaped by local conditions: the clay-soil foundation engineering, the coastal-spec materials that survive Delta heat, and the permit complexity of Contra Costa County’s dual jurisdiction. Here’s what we see in the current market:
| Project Type | Price Range in Brentwood |
|---|---|
| Home Addition (attached) | $45,000-$130,000 |
| ADU Construction (detached) | $90,000-$220,000 |
| Garage Conversion ADU | $30,000-$80,000 |
| Paver Patio Installation (ADU hardscape) | $3,500-$11,000 |
These ranges reflect flat written prices with written scopes - the Haven Standard’s Clause 1. What moves a project within the range: size, finish level, foundation complexity (clay soil remediation adds $8,000-$25,000 depending on depth and area), and utility-extension distance. We deliver the exact number before work starts, and no change proceeds without a signed change order. Bring a competitor’s written estimate and we’ll review it line by line at no charge - our Free Second Opinion.
We Also Serve Cities Near Brentwood
Our Home Additions & ADU crews also work in Brentwood‘s neighboring communities: Tarzana, Encino, Van Nuys, and Sherman Oaks. Each city’s soil conditions, permit offices, and housing stock create different project profiles, and we adjust our specs accordingly rather than applying a one-size template.
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 - Home Additions & ADU in Brentwood
The clay subsoil east of Marsh Creek Road requires over-excavated engineered fill or post-tension slabs that stable-soil cities don’t need, and the Delta’s 105°F summer swings demand stainless fasteners, aluminum-clad windows, and sealed paver joints rather than standard-grade materials. The cheap bid skips these specs and costs more in repairs within a decade. Call (213) 652-7670 for an exact quote - estimates are free.
Yes, every garage conversion ADU in Brentwood requires a building permit, and most require electrical, plumbing, and mechanical permits as well. Contra Costa County or the City of Brentwood planning department handles approval depending on your parcel’s jurisdiction, with EBMUD utility coordination required for new meters. We file all permits as part of our standard service. Call (213) 652-7670 for an exact quote - estimates are free.
The expansive clay subsoil adds seasonal movement to the foundation load calculation, so we evaluate the existing slab and footings before finalizing structural plans. In east Brentwood near Marsh Creek, we often specify deeper footings or grade-beam reinforcement that stable-soil neighborhoods don’t require. This engineering step is quoted upfront, not discovered mid-project. Call (213) 652-7670 for an exact quote - estimates are free.
The gap covers stainless or hot-dip galvanized fasteners instead of electroplated, aluminum-clad window frames instead of vinyl, sealed paver joints instead of dry-laid, and upgraded attic insulation with radiant barrier sheathing. In Brentwood’s Delta heat, the inland-spec version corrodes, warps, or fails within 5-10 years; the coastal-spec version lasts 25-plus. We quote only the coastal-spec version because we’re responsible for the 365-Day Done Right Promise. Call (213) 652-7670 for an exact quote - estimates are free.
The draw schedule is tied to inspection milestones - foundation, framing, mechanical/electrical/plumbing, insulation, and final - with each release documented in your written contract. No work proceeds on a change until the change order is signed, which is the administrative discipline Haven acquired Rowan for and the reason our projects finish within a few percent of the agreed price. The documented photo record at each visit shows progress and any conditions that might affect the schedule. Call (213) 652-7670 for an exact quote - estimates are free.
Written by Grant Rowan, Owner at Rowan Construction Group, serving Brentwood since 2007.