Home Additions & ADU in Brentwood - Design, Permits and Build Under One Contract

Fast, Reliable Home Additions & ADU Across Brentwood

Home additions and ADU construction in Brentwood typically run $45,000-$130,000 for room additions and $90,000-$220,000 for detached ADUs, with most projects requiring engineered footings due to the city’s expansive clay soils. We also offer Bathroom Remodeling - Brentwood services for homeowners looking to upgrade within their existing footprint. We’re familiar with the 2000s-era tract homes from Deer Ridge to Shadow Lakes, and we coordinate dispatch from our San Francisco Bay Area office so crews can reach Brentwood properties quickly for site evaluations and ongoing work. Call (510) 686-6370 to schedule your free estimate - we’ll bring a written price, not a pitch.

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Why Rowan Construction Group Walnut Creek Is Brentwood’s Preferred Home Additions & ADU Company

We’ve worked the subdivisions off Balfour Road, Marsh Creek Road, and throughout the Sand Creek corridor long enough to recognize the patterns. The diagonal cracking at interior door corners. The garage slabs that heaved after the third wet winter. These aren’t mysteries to us anymore - they’re the starting point for how we scope and price work in Brentwood.

Our Home Additions & ADU team has delivered 4,000-plus projects since 2007, every one with a written price before work started and a documented photo record when the crew left. Under Clause 1 of The Haven Standard, that written price is non-negotiable - it’s the foundation everything else builds on.

Brentwood customers get the same discipline: a flat written quote with line-item scope, a signed change order before any adjustment proceeds, and our 365-Day Done Right Promise printed on every agreement. If it’s not done right, we make it right. The guarantee lives on a terms page, not an asterisk.

Calls are answered by a live person 24/7. Our crews are background-checked and uniformed. And we offer a Free Second Opinion on any written estimate - bring a competitor’s quote and we’ll review it line by line at no charge.

Our Home Additions & ADU Services in Brentwood

ADU Design & Permitting

ADU design and permitting in Brentwood starts with understanding what the city will actually approve on your specific lot. The 2000s-era tracts have varying setback requirements, HOA restrictions in newer subdivisions, and utility capacity that wasn’t planned for secondary dwellings. We handle the site survey, soil report coordination, and permit package submission to Contra Costa County. A typical garage conversion ADU in Brentwood runs $30,000-$80,000; new detached ADU construction runs $90,000-$220,000. Under The Haven Standard, the permit costs and timeline are written into your quote before we file anything.

Garage Conversion ADU

Garage conversions are popular in Brentwood’s larger-format tracts where the original 2.5- or 3-car garage exceeds what the household actually uses. But there’s a catch: many of these garages sit on the same unengineered slab that heaved with the clay soil cycle, and the builder-grade framing and OSB sheathing are already aging out. We assess the slab condition, specify engineered repairs where needed, and design the conversion with proper insulation for Brentwood’s 105-112°F summer heat. The result is a permitted, separately metered living unit that doesn’t repeat the original construction’s shortcuts.

New Detached ADU

New detached ADUs in Brentwood demand the most rigorous site prep of any addition type. The Altamont clay beneath the old orchard ground shrinks and swells dramatically with the seasonal wet-dry cycle. We recently took on a room addition and detached ADU for a family off Balfour Road in the Deer Ridge area, where the original 2003 slab had already heaved enough to crack the garage floor and rack the door frames. We pulled a soil report, engineered deeper footings with a moisture barrier, and tied the new structure to the existing home with a properly flashed, load-bearing connection. The owner now has a fully permitted ADU with its own utility rough-in and a punch list we closed out in a single visit. This is how we build every detached ADU in Brentwood - not as an upgrade, but as baseline practice.

Junior ADU & Interior Conversion

Junior ADUs work well in Brentwood’s larger 3,500-square-foot tracts where a portion of the existing footprint can be sectioned off with a separate entrance. These projects stay under the $45,000-$130,000 room-addition range and avoid the soil engineering complexity of ground-up construction. We focus on fire separation, independent HVAC zoning for summer heat loads, and matching the existing stucco and trim profiles so the conversion doesn’t read as an afterthought.

Room Additions

Single-story room additions in Brentwood - family rooms, primary suite extensions, expanded kitchens - tie into the existing structure at the roofline and foundation. The critical detail is the load transfer: the addition’s footer must be engineered for the clay soil profile, and the connection must be properly flashed to handle the thermal expansion that Brentwood’s temperature swings produce. We specify James Hardie fiber-cement siding rather than repeating the original builder-grade exterior trim, which gives the addition a longer service life and better fire resistance.

Second Story Additions

Second-story additions are less common in Brentwood’s already-large tracts but make sense where the footprint is constrained by setbacks or HOA rules. These projects require structural analysis of the existing first-floor walls and foundation - walls that were built for single-story loads and may need reinforcement. We engineer for the additional load, specify matching roof pitches and eave details, and coordinate crane access for material delivery in the tighter cul-de-sac layouts common in 2000s subdivisions.

ADU Site Prep & Utilities

Site prep for Brentwood ADUs includes soil testing, utility rough-in coordination with the city, and often trenching for separate electrical and water service. The clay soils make trenching conditions unpredictable - dry summer clay is concrete-hard, winter-saturated clay is unstable. We schedule excavation in the viable shoulder seasons and specify proper bedding and backfill to prevent utility settlement.

Sunrooms & Enclosed Patios

Enclosed patios and sunrooms in Brentwood need to handle the summer heat without becoming unusable. We specify low-E glazing, insulated roof panels, and dedicated mini-split HVAC rather than tapping the main house system, which was already marginal in many 2000s builds. The foundation gets the same engineered footing treatment as any addition - the patio slab was rarely built to support enclosed, conditioned space.

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Trusted Brands We Service in Brentwood

We source across fifty-plus vetted brands, and we specify them in our written quotes so the numbers are honest, not inflated. For Brentwood’s ADU and addition projects, we regularly use KraftMaid and Wellborn cabinet lines for kitchenette and full-kitchen ADU builds, Cambria quartz surfaces for countertops that handle the thermal cycling without seam failure, and James Hardie fiber-cement siding for exterior finishes that outlast the original stucco in heat and clay-soil movement. We don’t list brands to impress. We name them because the quote includes the specific SKU, and the customer can look it up.

Common Home Additions & ADU Problems We See in Brentwood Homes

• Unengineered footings on expansive clay. Pouring new footings on the same unengineered clay profile as the 2000s slab, so the addition racks and cracks at door corners within a few seasons of Brentwood’s shrink-swell cycle. We see this in garages converted without soil reports, in room additions that matched the original slab depth, and in detached ADUs where the contractor treated the site like standard Bay Area fill.
• Skipped soil reports leading to mid-project change orders. Skipping the soil report to save on the scope of work, then hitting a mid-project change order when the inspector flags the footer depth. In Brentwood, this isn’t a rare occurrence - it’s a predictable outcome of building on Altamont-series clay without documentation.
• Builder-grade material repetition. Reusing builder-grade OSB sheathing and low-end exterior trim that was already aging out on the original tract home, so the new addition fails on the same timeline as the old one. We specify upgraded sheathing and James Hardie or equivalent siding as standard, not as an upgrade package.
• Heat-load miscalculations. Designing HVAC for coastal Bay Area conditions instead of Brentwood’s 105-112°F inland valley peaks, resulting in systems that can’t maintain setpoint in August and September. We size for the actual local design temperatures, not the generic California assumptions.

Pricing for Home Additions & ADU in Brentwood, CA

We Also Serve Cities Near Brentwood

Our crews work throughout the inland East Bay, including Brentwood, Antioch, Danville, San Ramon, and Livermore. Each city has its own soil profile, building stock era, and permit requirements, and we scope accordingly - the clay soils differ, the tract home vintages differ, and our quotes reflect those differences rather than applying a regional average.

FAQs - Home Additions & ADU in Brentwood