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
Home additions and ADU construction in San Mateo typically run $45,000-$130,000 for a room addition and $90,000-$220,000 for a detached ADU, depending on which side of Highway 101 your lot sits on. The geology under your property changes the engineering, the permit path, and the final number more than almost anything else. Barrett & Sons Construction has built additions and ADUs across all four San Mateo zip codes: 94404, 94497, 94401, and 94402. If you’re weighing a garage conversion, a second story, or a brand-new detached ADU, call Tom Barrett’s team at (877) 610-1318 and we’ll give you a written scope and an honest price before anything gets torn open.
Key Takeaways:
- San Mateo building departments treat the bayside and hillside separately. Soils reports and engineered foundations are standard east of 101, seismic review is standard west of it.
- We write allowances into every San Mateo contract so old-home surprises like knob-and-tube wiring or rotted ledgers don’t become verbal change orders.
- Every price change is a written change order, signed before work proceeds. Permits pulled and inspections signed on every eligible job.
- Our work is backed by the 90-Day Done Right Promise: if it’s not done right, one call brings Tom Barrett back.
Why Barrett & Sons Construction Is San Mateo’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run crews today. That means the name on the truck is the name answering the phone (877) 610-1318, and the face you see at the permit counter is the same one you’ll see when we walk the job with the inspector. We’re not a franchise dispatching strangers. We’re a local Peninsula outfit that’s been pulling permits and passing inspections in San Mateo long enough to know which plan checker wants the soils report stamped before they’ll even look at your foundation drawing.
San Mateo homeowners have a specific worry when they hire a contractor for a big job like a Home Additions & ADU project: they’ve been burned by a phone quote that turned into a doorbell surprise. We handle that differently. Every scope, every allowance, and every price change arrives in writing before work happens. Tom Barrett publishes his name, his address, and his license number. Look it up before you sign anything. Ask anyone on your street about us. We’ve built a reputation in San Mateo on one principle: an allowance that isn’t on paper is a surprise waiting for a date.
We serve all of San Mateo, from the 1940s ranch houses in the flatlands to the hillside lots above Crystal Springs and the fill parcels out at Mariner’s Island. When the city’s building department requires a structural engineer’s foundation design, we don’t fight it. We bring the engineer in before we dig. That’s why inspections pass the first time more often than not.
Our Home Additions & ADU Services in San Mateo
ADU Design & Permitting
The permit path for an ADU in San Mateo depends entirely on where you live. In 94404, east of Highway 101, the city requires a soils report and often a structural engineer’s foundation design as a matter of course because the ground is bay fill with high water tables and real liquefaction risk. In 94402 and 94403 on the hillside side, plan check is more concerned with seismic tie-downs and slope stability. We handle the drawings, the engineer coordination, the permit submittal, and the inspections. You don’t have to learn the San Mateo building code; we already know it.
New Detached ADU
A detached ADU in San Mateo is a real construction project, not a shed with drywall. Expect $90,000-$220,000 depending on size, finishes, and foundation engineering. In the 94404 zip code, standard spread footings that would pass two miles west are non-starters. We design from the soils report first. On hillside lots, we add seismic hold-downs and often a structural engineer’s hillside review. Every ADU we build in San Mateo gets a permit pulled and an inspection signed before anyone moves in.
Garage Conversion ADU
Converting a garage in San Mateo usually runs $30,000-$80,000, and the wild card is what’s already in those walls. Many San Mateo garages were built before current electrical and seismic codes, so once the inspector sees what’s behind the sheetrock, the scope can shift. We write that into the contract as a written allowance, with a dollar figure, before we start. No surprises at final billing.
Junior ADU & Interior Conversion
A JADU inside your existing San Mateo home is the most cost-effective way to add rental income or space for family. These run on the lower end of the range because you’re working inside the existing footprint. The catch in older San Mateo homes is the same catch everywhere: 1940s-1960s wood-frame construction often hides original galvanized drain lines, undersized framing, and wiring that predates the current code. Once walls are open, the permit triggers upgrades. We plan for that with allowances written into the contract, not discovered at billing.
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
When your San Mateo addition calls for new windows and doors, we install brands you can look up: Simonton, Ply Gem, and PGT. These are manufacturer-backed products with published specs and warranties, not mystery brands off a van. We coordinate with San Mateo suppliers for faster turnaround on common sizes and styles, which keeps your project moving instead of waiting on a truck from out of state. If you’re adding a sunroom or enclosing a patio, the window and door package matters for both fog-driven moisture and heat loss. We spec products that hold up to San Mateo’s morning marine layer and afternoon sun.
Common Home Additions & ADU Problems We See in San Mateo Homes
- Fog-driven rot on north-facing ledgers and fascia. San Mateo’s persistent morning fog keeps north-facing wood siding, deck ledger boards, and fascia continuously damp through spring. We routinely open exterior walls to tie in a room addition and find framing damage that changes the structural scope. We write it up, get your signature, and repair it properly before closing the wall.
- Undersized anchor bolts and cripple-wall failure on postwar ranch homes. The 1940s-1960s flatland homes in San Mateo were built before modern seismic standards. Many still have original cripple-wall foundations that would fail a retrofit inspection today. When California law or a lender requires a retrofit, the addition trigger often forces the upgrade.
- Soft bay mud east of Highway 101. In 94404, including Mariner’s Island, the ground itself is the problem. Standard spread footings that work fine in 94402 simply don’t pass plan check here. The city wants a soils report and often a structural engineer’s foundation design. Skipping that step stalls the permit and the dig.
- Original galvanized and cast-iron drain lines in older homes. The streetcar-era Craftsman and Spanish Colonial Revival homes near downtown San Mateo (94401) often have lath-and-plaster interiors and original galvanized or cast-iron drain lines. Any gut renovation or addition that ties into that plumbing triggers replacement. We budget for it in writing.
Pricing for Home Additions & ADU in San Mateo, CA
Here’s what we tell San Mateo homeowners before any work starts: expect a room addition to land between $45,000 and $130,000. A garage conversion typically runs $30,000-$80,000. A detached ADU is $90,000-$220,000. The spread comes from foundation engineering, seismic requirements, and what’s hiding inside your existing walls. We give free estimates with a written scope, and we price allowances into the contract so you’re not paying for surprises you never approved. Call (877) 610-1318 to schedule a walkthrough at your San Mateo property.
| Project Type | Typical San Mateo Range |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
What actually moves the number in San Mateo more than square footage? The soils report. On a bayside lot in 94404, you’re paying for engineered deep-foundation or grade-beam design. On a hillside lot in 94402, you’re paying for seismic tie-downs and slope review. On a 1940s flatland ranch, you’re often paying for a cripple-wall retrofit before the addition can tie in. None of that is optional, and all of it gets written into your contract before we submit for permit.
We Also Serve Cities Near San Mateo
Our crews are based in the Peninsula and regularly work in Foster City, Belmont, Hillsborough, and Redwood Shores. We coordinate permits and inspections with each city’s building department, and we know which jurisdictions require what engineering. Scheduling for San Mateo jobs is handled from our Bay Area office, but the crews live and work locally.
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
A soils report is required as a matter of course in the 94404 zip code east of Highway 101 because the bay fill ground has high water tables and liquefaction risk. In western hillside zip codes like 94402 and 94403, a soils report may still be required depending on the lot’s slope and proximity to the San Andreas fault corridor, but the trigger is usually the structural engineer’s review rather than the soil itself. Call (877) 610-1318 and we’ll tell you what your specific parcel triggers before you spend money on engineering.
We write allowances into the contract before we start, with dollar figures for the likely unknowns: old wiring, rotted ledgers, galvanized drain lines, undersized framing. When we open a wall and find something unexpected, we stop, write a change order with the exact cost, and get your signature before touching another stud. You approve the number in writing, or we don’t do the work. That’s the whole system. Call (877) 610-1318 for a contract that works this way.
A permit means an inspector signs off on the work. In San Mateo, once walls are open, code upgrades that were deferred for decades become mandatory: seismic anchoring, electrical grounding, insulation, and waterproofing. We pull permits on every eligible job because the inspector is working for you, not for us. If a contractor tells you permits are optional, walk away. That’s how you end up with an uninsurable addition. Call (877) 610-1318 and we’ll show you what a permit packet for your San Mateo address actually includes.
Because the ground is different. Mariner’s Island sits on bay fill with soft mud and high water tables; the city requires a soils report and often an engineered grade-beam or deep-foundation design. The western hills near 94402 sit on more stable geology but close to the San Andreas corridor, so the concern shifts to seismic tie-downs and slope stability. A foundation that sails through plan check in 94402 may be rejected outright in 94404. We design from the soils report and the structural engineer’s review, never from a guess.
An allowance is a written dollar amount in your contract for work we know might be needed but can’t confirm until walls are open. In San Mateo, that means knob-and-tube wiring, rotted north-facing ledgers, original galvanized drains, and undersized framing. The allowance says: if we find X, it costs Y, and it’s already approved in writing. Without it, every discovery becomes a negotiation. With it, you know the number before we spend it. That’s how we do every San Mateo job. Call (877) 610-1318 for a written estimate with allowances spelled out.
Contact Barrett & Sons Construction in San Mateo
If you’re planning a home addition, second-story build, garage conversion, or detached ADU in San Mateo, call Tom Barrett’s team at (877) 610-1318. We’ll walk your property, listen to what you’re trying to build, and give you a written scope with real numbers. Free estimates, written line-item quotes, permits pulled, inspections passed, and the 90-Day Done Right Promise on every job. Senior and veteran homeowners get a standing 10% discount.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving the San Francisco Bay Area since 1998.