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 Carlos
Adding square footage to a San Carlos home is rarely a simple bolt-on job. Between the steep hillside lots west of Alameda de las Pulgas and the aging ranch homes near El Camino Real, nearly every addition or ADU we scope here involves some mix of soils reports, seismic retrofitting, and county-level permitting that flatland cities don’t face. That’s the work we’ve been doing since 1998, and it’s why we walk every San Carlos site before we put a number on paper. If you’re planning a room addition, second story, or detached ADU in the 94070, call (877) 610-1318 and we’ll give you an honest read on what the lot will actually require.
Why Barrett & Sons Construction Is San Carlos’s Preferred Home Additions & ADU Company
Tom Barrett started Barrett & Sons Construction in San Carlos in 1998, and he still answers his own phone. You won’t reach an out-of-state call center or a voicemail box at 2 a.m., and you won’t have a different project manager every other week. His sons run the crews, and Tom is on-site most days, including on San Carlos jobs. There’s a difference between a contractor who knows the streets here and one who has to map them. We know that a lot off Brittan Avenue handles drainage differently than one off Crestview Drive, and we know which hillside streets get wind-driven moisture off the Bay in the winter. That local knowledge goes into every quote we write for our Home Additions & ADU team.
Every Barrett & Sons job in San Carlos starts with a scope in writing, allowances in writing, and change orders in writing. The permit inspector works for you, not for us, which is exactly why we insist on pulling permits and signing inspections on every eligible job. We’ve had San Carlos customers show us quotes from other outfits that left off the soils report and the retaining wall permit entirely, then act surprised when the county stopped the job. We don’t work that way. Ask anyone on your street, or look up the license number yourself.
Our Home Additions & ADU Services in San Carlos
New Detached ADU
A detached ADU on a San Carlos hillside lot is a different animal than one on a flat lot in Redwood City. West of Alameda de las Pulgas, the clay-rich soils shrink and swell seasonally, which means your ADU’s foundation needs pier-and-grade-beam or deepened footings specified before the permit goes in, not discovered during excavation. We handle the full build, from grading and utilities through final inspection, and we’ll tell you upfront if your lot will need a soils report and a retaining wall permit.
Second Story Additions
Most San Carlos homes built between the 1940s and 1960s were never engineered for a second story. The original cripple-wall foundations and slab footings were sized for a single-level ranch, not an added floor. Before we frame anything upward, we check the existing foundation and seismic connection, because San Mateo County won’t give inspection sign-off on a second-story addition until the original house is stabilized. We’ll scope that work in writing before you sign.
ADU Site Prep & Utilities
Excavating for a detached ADU in San Carlos during the winter wet season, roughly November through March, can trigger a county stop-work order if there’s no erosion control plan for sediment runoff. The corridor wind off the Bay accelerates drying in the summer, but it also pushes moisture inland during winter storms. We time our pours and grade work around that pattern. If you’re on a hillside acreage property, we’ll also check septic and well setbacks before we draw the ADU footprint.
Garage Conversion ADU
Converting an attached garage into a junior ADU can be the fastest path to a rental unit or in-law space, but the original structure still needs to meet current code. On a 1950s ranch, that often means shear transfer hardware, new footings at the garage opening, and sometimes seismic retrofit of the existing cripple walls. We’ll tell you exactly what’s required before the first wall comes down, and every change is a written change order before work proceeds.
What happens when you call
- 1
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 Carlos
We stock and install brands you can look up, not mystery materials off a van. For ADU bathrooms and kitchens, we routinely work with Schluter waterproofing and Grohe fixtures. For enclosed patios and sunrooms, GAF roofing materials hold up to the afternoon wind that funnels through the San Carlos gap. Because we’re local, we can get parts for San Carlos jobs fast without waiting on a distributor two counties over. A live person answers our phone 24/7, so if a material shows up short or a question comes up mid-build, you reach us directly.
Common Home Additions & ADU Problems We See in San Carlos Homes
- Foundation step cracks on west-side hillside lots. The clay soils west of Alameda de las Pulgas swell in the wet season and shrink in the dry season, cracking slab and cripple-wall foundations in homes built on original 1950s-60s footings. Any addition on those lots needs a foundation designed for that movement.
- Stop-work orders for missing erosion control. Starting ADU excavation in November without a county-level erosion control plan is one of the most common reasons San Carlos projects stall. San Mateo County inspects for sediment runoff on exposed slopes.
- Second-story additions on un-retrofitted cripple walls. A 1958 ranch with original cripple walls will not pass inspection for a second-story addition until the existing house is seismically stabilized. Contractors who skip this step lose months and money.
- Heaving driveways and patios from seasonal soil movement. Flatwork poured on expansive clay without a proper base heaves within a few years on hillside lots. We specify base and drainage for the local soil, not for a generic California lot.
Pricing for Home Additions & ADU in San Carlos, CA
What you pay depends mostly on your lot. A detached ADU on a flat lot near El Camino Real is a very different job from one on a steep hillside west of Alameda de las Pulgas, where soils reports, pier footings, retaining wall permits, and longer equipment access all add cost. Here are the ranges we see in San Carlos:
| Project | Typical San Carlos Range |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
Every quote is a written line-item quote. No allowances that aren’t on paper. No change orders that show up at the end. Call (877) 610-1318 for a free estimate, and mention the $40 Off Any Repair Over $200 or the permanent 10% Senior & Veteran discount if it applies. Free second opinions on any written estimate, and no trip charge inside the county.
We Also Serve Cities Near San Carlos
Our San Carlos crews regularly work in Belmont, Redwood Shores, Redwood City, and North Fair Oaks. If you’re just over the city line, the same local knowledge applies, and the same written-scope process applies too. We know the Hillsdale and Redwood Shores terrain, and we’ll give you the same honest read on your lot before you sign anything.
Serving San Carlos, CA - Our Local Coverage Area
We’re based in the San Carlos 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 Carlos
A hillside ADU in San Carlos will almost always require a soils report, engineered pier-and-grade-beam or deepened footings, and often a retaining wall permit. The clay-rich soils west of Alameda de las Pulgas heave and shrink seasonally, which a flat-lot slab foundation is not designed to handle. The flats near El Camino Real may allow a standard slab-on-grade, but the hillside lots add $15,000-$40,000 or more in foundation and geotechnical work depending on slope and access. Call (877) 610-1318 and we’ll walk your lot before we quote, so you know exactly what’s required.
Setbacks rule the footprint. San Mateo County requires minimum distances between any new structure and your septic tank, leach field, and well, and those setbacks often dictate where an ADU can sit on a San Carlos acreage parcel. We locate your septic and well lines as part of the initial site walk, then draw the ADU footprint to fit the setbacks rather than designing a building that can’t be permitted. On hillside acreage, drainage and erosion control for the exposed slope become part of the same plan.
Pier-and-grade-beam is the foundation type used on San Carlos hillside lots because the expansive clay soil shrinks and swells with the seasons, and a standard slab will crack. The piers drill deep enough to reach stable soil below the movement zone, and the grade beam spans between them to carry the structure. On the hillside streets west of Alameda de las Pulgas, contractors who skip this often get called back within a few years to fix step cracks and heaving floors.
Sometimes, but don’t count on it. San Mateo County requires the existing structure to meet current code before sign-off on a conversion, and on a 1950s-60s San Carlos ranch with original cripple walls, that frequently means shear transfer hardware and some level of seismic retrofit at the garage connection. The scope depends on what we find when we open the walls and crawlspace. We’ll put any required retrofit in writing before you sign, and no change order happens without your sign-off. Call (877) 610-1318 for a free written estimate on your garage conversion.
The gap in the Coast Range funnels afternoon wind off the Bay, which dries concrete and stucco faster in the summer and pushes moisture inward during winter storms. We time exterior pours and stucco work around that pattern so materials cure correctly instead of cracking. For sunrooms and enclosed patios, we account for the wind load in the framing and glazing spec, because a room facing the corridor gets hit harder than one on a sheltered lot. Wind-driven moisture also means we pay extra attention to flashing and sealant on exterior walls.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Carlos since 1998.