Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Corte Madera
Adding square footage in Corte Madera means dealing with either compressible bay mud under the flat eastern neighborhoods or steep slope and fire-code requirements up toward Corte Madera Ridge - the same challenges we address with our Roofing in Corte Madera. Both demand a contractor who knows exactly which engineering and permitting path applies before you spend a dollar on design. A typical home addition in Corte Madera runs $45,000 to $130,000, and a new detached ADU usually lands between $90,000 and $220,000, depending on foundation conditions and access.
We’ve built additions and ADUs from the ranch-house flats off Tamalpais Drive to the hillside custom lots above Chapman Park, and we know what Marin County’s building department will flag before they flag it. If you’re weighing options for an aging ranch that’s settling or a steep lot that needs piers, call Tom Barrett’s crew at (877) 610-1318. We’ll come out, look at the actual site, and give you a written scope and a straight price.
Why Barrett & Sons Construction Is Corte Madera’s Preferred Home Additions & ADU Company
We’ve been owner-operated since 1998, and Tom Barrett still answers his own phone and walks every Corte Madera job site. His sons run the crews. That’s not a slogan; it’s how we work. When you call about a footing issue on a bay-fill lot near Madera Boulevard, the person who comes out knows bay mud, not just framing.
Corte Madera homeowners have seen plenty of contractors quote over the phone and show up with a different number. We don’t do that. Every allowance, every scope change, every price adjustment arrives as a written change order before work proceeds. The permit inspector signs off on every eligible job, so you’re never left wondering whether the addition was built to code. Our 90-Day Done Right Promise is written on the contract: if something’s not right, one call brings Tom back.
We serve all of Corte Madera’s 94925 and 94976 zip codes, and our scheduling is coordinated from the San Francisco Bay Area office with crews working locally in Marin. If you want to verify us before you sign anything, Tom’s name, address, and license number are published and searchable. Ask any neighbor on your street, or look it up yourself.
Our Home Additions & ADU Services in Corte Madera
ADU Design & Permitting
An ADU in Corte Madera isn’t just a set of plans; it’s a conversation with Marin County about floor elevation on the bay side or fire-safe assemblies on the hillside. We handle the site assessment, the design, the structural engineering, and the permit package as one scope so nothing gets kicked back for a missing drainage plan. On the flat eastern lots, finished-floor elevation and flood-zone compliance are the first things we check because those drive the entire foundation design.
Garage Conversion ADU
Many Corte Madera ranch homes built in the 1950s through 1970s have attached garages sitting on the same shallow slab as the house, and that slab has often settled or cracked as the bay mud consolidated. We also handle Windows & Doors - Corte Madera upgrades during these conversions. A garage conversion ADU in Corte Madera starts with a foundation assessment and usually includes slab repair, raising the finished floor, and addressing moisture that’s been wicking up for decades. Typical garage conversions here run $30,000 to $80,000, with the wide spread driven by whether the existing slab can be salvaged.
New Detached ADU
If you’re building a detached ADU in a Corte Madera backyard, the two big questions are access and ground conditions. On the east side near the bay, you’ll likely hit groundwater within a few feet of grade, which means dewatering during excavation and a more substantial footing system than a comparable project in Larkspur or upper Mill Valley. On hillside lots, expect piers or caissons and engineered drainage. A new detached ADU in Corte Madera typically lands between $90,000 and $220,000.
Junior ADU & Interior Conversion
A Junior ADU, or JADU, converts existing interior space like a large primary bedroom or a bonus room into an independent living unit with its own entrance and kitchenette. In Corte Madera’s ranch homes, this often means working around existing bearing walls and matching the home’s original framing era. If you’re already planning a Bathroom Remodeling in Corte Madera project, a JADU conversion can sometimes share the same plumbing wall and save real money on the combined scope.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in Corte Madera
For exterior work on Corte Madera additions, we install Techo-Bloc and Cambridge Pavers for hardscape and patio areas, and we use Quikrete products for foundations and flatwork where the job calls for a proven mix. These are brands you can look up, with published specs and warranties, not mystery materials off a van. When we quote a paver patio or a new concrete stem wall in Corte Madera, the material name is on the estimate, so you know exactly what you’re paying for before we start.
Common Home Additions & ADU Problems We See in Corte Madera Homes
- Bay mud settlement under slab foundations. The eastern neighborhoods of Corte Madera sit on historic bay fill that’s still consolidating. We regularly find cracked slabs, out-of-level floors, and doors that won’t close on ranch homes built between the 1950s and 1970s. Any addition tied to these foundations needs a soil report and a foundation plan that accounts for differential settlement.
- Standing groundwater during excavation. On the bay side, we’ve hit water less than three feet below grade in the middle of summer. Footings, trenches, and crawlspace work usually require active dewatering, which adds cost and time. Homeowners who get quotes from contractors used to hillside Marin towns often don’t see this line item coming.
- Persistent moisture and wood rot. Corte Madera sits in the coastal fog channel, which keeps relative humidity high year-round. Older framing, sill plates, and exterior sheathing often show rot and corrosion that’s hidden until the walls come open. We check for this early so it doesn’t become a surprise change order later.
- WUI fire code on hillside lots. Parcels rising toward Corte Madera Ridge fall under Marin County’s strict wildland-urban interface rules. That means Class A roofing, ember-resistant vents, and fire-rated wall assemblies on any addition or ADU. The design has to account for these from day one, or the permit review will stall.
Pricing for Home Additions & ADU in Corte Madera, CA
A full home addition in Corte Madera generally runs $45,000 to $130,000, while a detached ADU lands between $90,000 and $220,000 and a garage conversion between $30,000 and $80,000. What moves the number is what the site gives you to work with. Bay-fill lots with high water tables need deeper footings, dewatering, and often a structural mat or pinning; hillside lots need piers or caissons, engineered shear walls, and fire-rated assemblies. Square footage, finish level, and whether the existing foundation can be tied into also shift the final scope.
Every estimate we write is a line-item quote, not a ballpark. If something changes during the job, you get a written change order before we proceed. No surprise invoices at the end. Call (877) 610-1318 and we’ll schedule a free, no-pressure site walk. We also offer a Free Second Opinion on any written estimate you’ve already received.
We Also Serve Cities Near Corte Madera
While this page focuses on Home Additions & ADU in Corte Madera, our crews regularly work in Larkspur, Mill Valley, Tamalpais Valley, and Tamalpais-Homestead Valley. If you’re just over the town line, the same pricing structure and the same Home Additions & ADU team applies. Call and we’ll tell you honestly whether your parcel falls in our normal Marin service area.
Serving Corte Madera, CA - Our Local Coverage Area
We’re based in the Corte Madera 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 Corte Madera
A home addition in Corte Madera typically costs between $45,000 and $130,000, with site conditions being the biggest variable. Bay-fill lots with groundwater challenges and hillside lots with fire-code requirements both push costs toward the upper end. Call (877) 610-1318 for a written, line-item estimate on your specific parcel.
Yes. We pull permits on every eligible job and coordinate the inspections through Marin County’s building department. On the bay side, that means flood-zone and finished-floor elevation reviews; on hillside lots, it means WUI fire-code and slope-stability sign-offs. The permit inspector works for you, and we like it that way.
We can, but the foundation system will look different than on a dry hillside lot. Bay-fill parcels in eastern Corte Madera often hit groundwater within three feet of grade, which requires dewatering during excavation and a more substantial footing design. We’ll identify that in the site assessment before you commit to a design.
No, unless you change the scope. Every allowance and every adjustment comes to you as a written change order before any work proceeds. That’s the difference between us and the contractor who quoted one number on the phone and handed your neighbor a different bill at the door.
Every project is backed by our written 90-Day Done Right Promise. If something isn’t right, Tom comes back and makes it right. One call to (877) 610-1318 is all it takes. Beyond that, we stand behind our work because we live here and you’ll see us at the hardware store.
Get Your Free Corte Madera Home Addition or ADU Estimate
If you’re thinking about a room addition, a garage conversion, or a new detached ADU in Corte Madera, start with a real site walk. Give us a call at (877) 610-1318 and Tom or one of his sons will come out, look at your foundation, your drainage, and your permit path, and give you a written scope with real numbers. Free estimates, written change orders, and a name you can look up. That’s how we’ve worked since 1998.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Corte Madera and Marin County since 1998.