Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
Before
After
Before
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Before
AfterHow Brentwood’s Expansive Clay Soils Shape Every Addition We Build
Drive through any Brentwood subdivision built between 1995 and 2008 and you’ll see the same story: slab-on-grade tract homes that looked perfect on closing day, now showing hairline cracks along stucco corners, doors that won’t latch in August, and patios that tilt toward the house. Those homes sit on Altamont-series clay that shrinks hard under our 100°F summer heat and swells again when the Delta rains come. By the time a house hits the 15 to 25 year mark, that cycle has usually left its mark somewhere. Which means any addition or ADU we build in Brentwood has to start with what’s happening underneath the existing structure, not just what the new floor plan looks like on paper. This is home construction in a thermal pocket where it’s regularly 25 to 35 degrees hotter than the coastal cities 35 miles west. The soil here moves. The work has to account for that.
If you’re thinking about adding square footage or building an ADU on your Brentwood property, call Tom Barrett at (877) 610-1318. We’ll walk the site with you, look at how your existing slab is behaving, and give you a written scope of work before you spend a dime.
What Home Additions and ADUs Cost in Brentwood, CA
A full home addition in Brentwood typically runs $45,000 to $130,000. A detached ADU is usually $90,000 to $220,000 depending on size, site conditions, and whether we’re trenching new utilities through old orchard soil or connecting to existing lines. A garage conversion ADU lands between $30,000 and $80,000. Those are real numbers from this market, not generic ranges pulled from a national database. The single biggest wildcard in Brentwood is what’s under the topsoil. Much of east and south Brentwood was working farmland until the early 2000s, and when we trench for a new ADU’s water, sewer, or electrical, we routinely hit remnant agricultural drain tile, buried berms, and organically amended soil layers that drain unpredictably. That can change how we spec the foundation, and it’s why every price we give you comes with allowances in writing and a change order before anything changes.
| Project Type | Typical Brentwood Range |
|---|---|
| Home Addition | $45,000 - $130,000 |
| Detached ADU | $90,000 - $220,000 |
| Garage Conversion ADU | $30,000 - $80,000 |
| Junior ADU / Interior Conversion | $45,000 - $130,000 |
| Sunroom / Enclosed Patio | $45,000 - $110,000 |
Call (877) 610-1318 for a free estimate on your specific property. Estimates are always in writing, always itemized, and always free.
Why Brentwood Homeowners Choose Barrett & Sons for Additions
Tom Barrett founded this company in 1998, and his sons run crews on Brentwood job sites most days. That matters when you’re spending six figures on an addition. The name on the truck is the name that answers the phone, and the address and license number are published where anyone can look them up. Ask a neighbor on Sand Creek Road or in the Garin Ranch area. Better yet, look up the license yourself before you sign anything. We encourage it.
Around here, the last contractor most people used told them permits were optional. We pull permits and get inspections signed on every eligible job. The permit inspector works for you, not for me, and that’s exactly why I like him. If something isn’t right, I want it caught before the drywall goes up, not after the tile is set.
Every Barton Ranch or Apple Hill homeowner gets the same treatment: scope in writing, allowances in writing, change orders in writing. If the price changes, you know before the work changes. And if something’s not done right after we finish, the 90-Day Done Right Promise means one call brings Tom back.
Home Addition & ADU Services We Handle in Brentwood
ADU Design & Permitting
Brentwood’s city planning requirements for ADUs have their own quirks, from setback rules on narrow lots in older neighborhoods to utility capacity checks on properties that were once orchard land. We handle the design, the drawings, the submittal, and the inspections. You get a single point of contact from the first sketch to the final sign-off, and every permit is pulled before the work starts.
Garage Conversion ADU
Garage conversions are the most popular ADU route in 94513 because the existing structure is already there. But many of those garages were built in the 1995 to 2008 wave on slabs that have already moved. Before we convert, we check the slab elevation, the door openings, and the framing connections. It saves you from converting a garage and inheriting its cracks. A typical garage conversion in Brentwood runs $30,000 to $80,000.
New Detached ADU
Building a detached ADU on a Brentwood lot means trenching through that old orchard ground east of town or working with tighter soil conditions on the north side. We spec the foundation for the actual soil we find after excavation, not what the plan assumes. Detached ADUs here run $90,000 to $220,000, and the variance comes down to size, utilities, and site access.
Junior ADU & Interior Conversion
A junior ADU inside an existing Brentwood home converts underused space like a formal dining room or bonus room into a rentable or family unit. The cost generally falls in the $45,000 to $130,000 range depending on whether you’re adding a kitchenette, a separate entrance, or upgraded electrical. These projects move faster because there’s no foundation work, but we still check how the existing slab area is performing before we build over it.
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.
Sub-Services We Provide for Brentwood Properties
- Room Additions - Adding a bedroom, family room, or home office onto an existing Brentwood home, including foundation work matched to the original slab.
- Second Story Additions - Going up instead of out, which means verifying the first-floor framing and slab can carry the load before any walls come down.
- Sunrooms & Enclosed Patios - Enclosing Brentwood’s covered patios with properly engineered walls, insulated glass, and finished interiors, built to handle 100°F summers and wet winters.
- ADU Site Prep & Utilities - Trenching, grading, and utility connections, including dealing with residual agricultural drain tile and amended topsoil layers common across east Brentwood.
How Brentwood’s Climate Affects Every Addition
Brentwood sits east of the Diablo Range in a thermal pocket that regularly hits 100°F to 110°F in July and August. That heat doesn’t just make the work uncomfortable. It forces concrete pours into early morning windows, it expands framing and roofing materials, and it drives the shrink-swell cycle in the Altamont clay that underlies most of the city’s housing stock. When we pour a new foundation for an addition, we schedule it to beat the heat. When we spec windows, we think about thermal stress. When we tie a new roof into an old one, our Brentwood Roofing team accounts for how each material moves in that kind of heat. Those aren’t details a national estimate from a call center ever captures. They matter here, in this specific climate, on this specific soil.
Materials We Use on Brentwood Additions
We install windows and exterior products from manufacturers you can look up: Simonton, Ply Gem, PGT, and CGI. These aren’t mystery brands off a van. They’re name-brand materials with published specifications, real warranties, and performance data that matters in Brentwood’s heat. If your addition includes new Windows & Doors - Brentwood products, we’ll show you what we’re installing and why it’s rated for this climate before we order anything.
Foundation and Soil Conditions We See in Brentwood Homes
The compressed build-out of the late 1990s and early 2000s means Brentwood’s neighborhoods age in unison. That’s why we see the same pattern across entire subdivisions at the same time. Here’s what shows up most often when we’re called out for an addition or ADU job:
- Slab settlement and cracking from clay shrink-swell cycles. Brentwood’s Altamont-series clay shrinks dramatically in summer and re-swells with winter Delta rain. After 15 to 25 years, that movement shows up as slab cracks that affect the new structure we’re tying into.
- Stucco craze-cracking over OSB sheathing. The extreme temperature swings between Brentwood’s summer highs and winter lows stress stucco systems, and many homes built in the 1995 to 2008 window used OSB sheathing that moves more than plywood. We check adjoining walls before we attach a new structure.
- Remnant agricultural drainage beneath new trench work. East and south Brentwood were orchards and row-crop land until the early 2000s. Trenching for ADU utilities often exposes old drain tile and amended topsoil that changes how we spec foundation drainage and grading.
- Out-of-square door frames and heaving flatwork. The cumulative soil movement across whole neighborhoods means existing door frames, patios, and walkways are often slightly out of level. A room addition has to tie into those existing planes, and we measure carefully before we commit to a roofline.
Our Home Additions & ADU Process in Brentwood
When you call us about an addition or ADU, the first step is a site visit. Tom or one of his sons walks the property with you, looks at how the existing structure is sitting, and talks through what’s realistic for your lot. Then you get a written scope. Then we pull the permit. Then the work happens in phases, with a written change order before anything changes. No surprises at the end. That’s how we’ve run this company for over 29 years, and it’s why we’ve built a strong local reputation with homeowners across Home Additions & ADU in Brentwood.
We’re the same team that handles Kitchen Remodeling in Brentwood and Bathroom Remodeling in Brentwood. If your addition includes a new kitchen, bath, or laundry space, we can coordinate the entire job so you’re not juggling multiple contractors and multiple permits. That’s a real advantage when you’re living in the house during the work.
Do You Build the Same Quality in Oakley, Discovery Bay, and Antioch?
Yes. The same crew, the same written scopes, the same brands. We serve Brentwood, Oakley, Discovery Bay, Antioch, and Clayton from our San Francisco Bay Area scheduling office, with crews that work locally throughout the region. Whether you’re on a tract lot off Balfour Road in Brentwood or a larger parcel near Discovery Bay, you get the same Home Additions & ADU team, the same permits, and the same 90-Day Done Right Promise.
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
In most cases we can be on site for an initial walkthrough within a week of your call, and we can typically start construction within 4 to 8 weeks depending on the scope and how quickly Brentwood’s planning department processes the permit. The permit review time in Brentwood varies by project type and current workload, but we handle the entire submittal for you. Call (877) 610-1318 and we’ll give you a realistic timeline for your specific project.
We work on both. The older homes in central Brentwood and the newer tract homes in neighborhoods like Garin Ranch, Apple Hill, and Barton Ranch each have their own conditions. Older homes may have perimeter foundations that need verification before we add onto them. Newer ones sit on slabs that may have already moved. We assess each property individually and spec the foundation work for what we actually find.
Yes. We don’t use a one-size-fits-all foundation spec. Brentwood’s Altamont clay expands and contracts significantly with seasonal moisture changes, and we’ve seen how that affects slabs across the city. Our foundation design accounts for the soil we encounter during excavation, including any remnant agricultural material in former orchard areas. The permit inspector will verify the work at multiple stages.
We’re not the cheapest bid in Brentwood, and we don’t try to be. A home addition here runs $45,000 to $130,000, and a detached ADU runs $90,000 to $220,000. What you’re buying is a written scope, named materials like Simonton and PGT windows, permits pulled on every eligible job, and a 90-Day Done Right Promise in writing. If you have another written estimate, bring it in. We’ll give you a free second opinion, no charge.
Call us. The 90-Day Done Right Promise is written on every contract, not hidden in fine print. If something isn’t right, one call brings Tom back to make it right. We live and work in this community, and the last thing we want is a neighbor who can’t recommend us. Call (877) 610-1318 and we’ll schedule a visit.
Get a Free Written Estimate for Your Brentwood Addition or ADU
If you’re ready to add space in Brentwood, start with a free estimate. Tom Barrett or one of his sons will meet you on site, look at your property, and give you a written scope and pricing before you commit to anything. No pressure, no vague numbers, and no surprise invoices at the end. Call (877) 610-1318 today, or ask for Tom by name. He answers his own phone.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Brentwood since 1998.