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 San Mateo
Most San Mateo homeowners don’t realize their property may sit on one of two very different soil types until the first foundation hole gets dug. West of El Camino Real, you’re likely on solid hillside ground. East of Highway 101, especially in the 94404 zip around Mariner’s Island, you’re building on bay fill that demands a whole different engineering approach. That split matters more than almost anything else when you’re adding square footage or planning an ADU in San Mateo. We’ve been navigating that divide long enough to know which side of the freeway your permit package needs to reflect. Call (877) 610-1318 and we’ll talk through your exact parcel.
Why Barrett & Sons Construction Is San Mateo’s Preferred Home Additions & ADU Company
Tom Barrett started this company in 1998 with his name on the truck, and his sons run crews today the same way he did then. When a San Mateo homeowner calls, a live person answers, not a call center. That matters when you’re three weeks into an ADU build and a question comes up about the soils report the city required. We’ve done enough work in the 94401 Craftsman neighborhoods near downtown and the ranch-house flats of 94403 to know what the inspectors look for before they show up. You can look up Tom’s license number, his address, his reputation. Nobody hides behind a franchise logo here.
Every job we start in San Mateo gets permits pulled and inspections signed. Every price change arrives as a written change order before work proceeds. And every finished project carries our 90-Day Done Right Promise: if something isn’t right, Tom comes back and fixes it. One call.
Our Home Additions & ADU Services in San Mateo
ADU Design & Permitting
San Mateo’s planning department has specific requirements for accessory dwelling units, and the paperwork is where most projects stall. We handle the full design and permit package for new detached ADUs, including the structural engineering the city asks for when your lot sits east of 101 on soft bay mud. If you’re in 94402 or 94403, your ADU plans may clear with standard footings, but we’ll verify that before you spend a dollar on construction. Our Home Additions & ADU team walks every San Mateo submission through plan check personally.
Garage Conversion ADU
Converting a garage into living space is the most cost-effective ADU path in San Mateo, and this city has plenty of 1940s and 1950s ranch homes with detached garages begging for a new life. The catch here is what’s hiding behind the drywall: many original garages in 94403 and 94401 have undersized foundations and no seismic hardware, which means the conversion triggers foundation upgrades before walls go up. We price that into the scope up front, in writing, so there’s no surprise invoice at the end.
New Detached ADU
A new detached ADU gives you the cleanest layout and the most rental flexibility, but in San Mateo it also brings the most site work. Depending on where you live, that could mean a straightforward slab on grade in the western hills, or an engineered grade-beam system with deep piers if you’re in the 94404 bayside flats. We coordinate the soils report, the structural design, and the site prep so the whole thing moves as one job, not three separate contractors pointing at each other.
Junior ADU & Interior Conversion
A junior ADU inside your existing footprint is the fastest way to add a rentable unit or a private space for family in San Mateo. Many postwar bungalows in the central flatlands have underused bonus rooms or oversized utility spaces that convert cleanly. We’ve done enough of these in the 1940s and 1960s housing stock to know where the original cripple walls and galvanized plumbing lines hide, and we scope the retrofit work before it becomes an argument. If the space connects to an existing kitchen or bath, we’ll coordinate with our Kitchen Remodeling in San Mateo and Bathroom Remodeling in San Mateo crews so finishes stay consistent front to back.
Room Additions & Second Story Additions
Adding a ground-floor room or going up with a second story changes how your San Mateo home sits on its lot, and that means the foundation conversation happens before the framing conversation. A second story addition on a 1950s ranch in 94403 almost always requires shear wall and anchor bolt upgrades that the original builder never installed. We handle the engineering, the permit, and the build as one contract with one point of contact.
Sunrooms & Enclosed Patios
San Mateo’s fog pattern means a poorly built sunroom turns into a damp, cold box by November. We design these spaces with proper insulation, drainage, and moisture barriers so they work as living space year round, not just in August. Whether you’re enclosing an existing patio in 94401 or adding a bump-out off the back of a Hillsborough-adjacent home, the envelope details make the difference.
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 San Mateo
We install Simonton and Ply Gem windows and doors on nearly every San Mateo addition and ADU we build, because these brands hold up to the marine air that rolls in off the Bay every morning. Nobody wants to replace sashes five years into a sunroom because the frames warped from moisture. We stock the common sizes locally and order the rest with lead times we confirm before you sign. You’ll know exactly what’s going into your home, and you can look up the warranty on any brand we quote.
Common Home Additions & ADU Problems We See in San Mateo Homes
- Original cripple-wall foundations failing seismic retrofit requirements. The postwar ranch homes in San Mateo’s central flatlands, especially around 94403, were built before California’s modern anchoring codes. Any addition or ADU triggers the need to bring those foundations up to current standards, and we see this on nearly every job we permit.
- Bay fill lots east of 101 needing engineered deep foundations. In the 94404 zip around Mariner’s Island and other bayside parcels, the city requires a soils report and structural foundation design as a matter of course. Standard spread footings that pass inspection two miles west simply won’t work here, and we plan for that cost from the first estimate.
- Persistent fog moisture accelerating wood rot on exterior connections. San Mateo’s mid-Peninsula position keeps north-facing siding, ledger boards, and fascia damp through the spring months. When we tie a new addition into an existing wall, we routinely find framing damage that other bids either miss or hide until it’s too late.
- Pre-1930s Craftsman and Spanish Revival homes hiding cast-iron plumbing. The older streetcar-era neighborhoods near downtown San Mateo, particularly in 94401, have original galvanized and cast-iron drain lines that can’t handle modern fixture loads. Any interior conversion or ADU that touches plumbing needs a plan for replacement, and we write that into the scope before demolition starts.
Pricing for Home Additions & ADU in San Mateo, CA
A home addition in San Mateo typically runs $45,000 to $130,000 depending on size, foundation work, and finishes. A new detached ADU on a standard lot west of El Camino Real generally falls between $90,000 and $220,000. A garage conversion ADU lands in the $30,000 to $80,000 range. Those numbers shift when your parcel is on bay fill and needs engineered foundations, which adds real cost but also real safety. We give you a written line-item estimate before you commit, and any price change during the job comes as a written change order first. Call (877) 610-1318 and we’ll walk your property and put numbers in writing.
We Also Serve Cities Near San Mateo
If you’re looking for Home Additions & ADU in San Mateo, we also run crews to San Mateo neighbors like Foster City, Belmont, Hillsborough, and Redwood Shores. Same crew members, same written change-order policy, same 90-Day Done Right Promise. Wherever your property sits on the Peninsula, we’ll bring the same site knowledge we apply to every bay-fill and hillside lot in the city.
Serving San Mateo, CA - Our Local Coverage Area
We’re based in the San Mateo 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 San Mateo
We schedule initial consultations in San Mateo within a few business days, and our estimate turnaround runs about a week for most addition and ADU scopes. Call (877) 610-1318 and you’ll talk to a live person who can lock in a site visit. We don’t do voicemail tag or out-of-area call centers.
Yes, we pull permits and manage inspections for ADUs and additions in 94401, 94402, 94403, and 94404. The 94404 zip east of Highway 101 requires a soils report and engineered foundation design, and we coordinate that as part of our permit package. The permit inspector works for you, not for us, which is exactly why we like having them involved.
A new detached ADU in San Mateo typically runs $90,000 to $220,000, which is comparable to Belmont and Foster City but often higher than similar work further south due to the foundation and seismic requirements in the bayside zip codes. Garage conversions run $30,000 to $80,000. We write every allowance and price change on paper before work starts, so the number you sign is the number you pay. Call (877) 610-1318 for a property-specific estimate.
If water is coming into a foundation hole or a tarp fails during storm season, we’ll get someone out to assess it fast. Live answers happen 24/7 at (877) 610-1318. For non-emergency warranty items, our 90-Day Done Right Promise means one call brings Tom back to make it right.
Every project carries the written 90-Day Done Right Promise: if it’s not done right, Tom Barrett comes back and fixes it, no fine print, no runaround. That’s on top of manufacturer warranties from brands like Simonton and Ply Gem. We stand behind the work because we live twenty minutes away and see you at the hardware store on Saturday.
Get Your San Mateo Home Addition or ADU Started Today
Whether you’re planning a garage conversion in the 94401 flats, a new detached ADU near Mariner’s Island, or a second story addition in the western hills, the first step is a conversation and a written estimate. Call Barrett & Sons Construction at (877) 610-1318. A live person answers. You’ll get straight answers, real numbers, and a scope in writing before a single nail moves.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Mateo with foundation-smart additions and ADUs since 1998.