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
A home addition or ADU in San Carlos typically runs $45,000 to $130,000 for an addition and $90,000 to $220,000 for a detached ADU. On hillside lots west of Alameda de las Pulgas, expect to add geotechnical work, pier-and-grade-beam foundations, and county grading permits that flatland projects in nearby Belmont or Redwood City rarely trigger. We’ve been building additions and detached units across San Carlos since 1998, and our crews know exactly which lots need a soils report before we ever put a number in front of you. Call (877) 610-1318 and Tom will walk you through what your specific property actually requires.
Why Barrett & Sons Construction Is San Carlos’s Preferred Home Additions & ADU Company
We’re not a franchise dispatching strangers in white vans. Barrett & Sons is Tom Barrett and his sons, on-site in San Carlos most days, and Tom’s name, address, and license number are published for you to verify before you sign anything. The 90-Day Done Right Promise is written into every contract: if something isn’t done right, one call brings Tom back to make it right.
San Carlos homeowners call us because we answer the phone, and because we know the hillside lots, the 1950s and 60s ranch homes, and the county permitting offices in Redwood City. The permit inspector works for you, not for me, and we treat every inspection like it’s protecting your house. That’s why we have a strong local reputation in the 94070 area from El Camino Real up into the western hills.
Our Home Additions & ADU Services in San Carlos
We scope every project before we quote it. That means walking the lot, reviewing any existing septic or well records, checking the slope, and identifying what the county will require for grading or erosion control. Then you get a written line-item quote with every scope item on paper. Here are the four services San Carlos residents ask about most.
ADU Design & Permitting
San Carlos sits on terrain that makes permitting the hardest part of an ADU, not the framing. Hillside lots west of Alameda de las Pulgas often need a soils report, engineered foundation drawings, and a grading permit from San Mateo County before the building permit can even be issued. We handle all of it, and every scope change along the way arrives as a written change order before work proceeds.
Garage Conversion ADU
Converting a 1950s garage in San Carlos usually costs $30,000 to $80,000. What pushes that number is what’s hiding in the slab and walls: many garages here were built with unreinforced footings and no vapor barrier, and some still have original electrical panels that won’t pass an ADU load calculation. We open the walls first, then price the conversion honestly.
New Detached ADU
A new detached ADU in San Carlos’s 94070 zip code typically runs $90,000 to $220,000. That’s a wide range because a flat lot off El Camino Real builds differently than a hillside lot near Crestview Drive. On the hillside, pier-and-grade-beam foundations and deepened footings are often mandatory, and we factor that into the quote the first time, not as a surprise later.
Junior ADU & Interior Conversion
A Junior ADU carved from an existing primary home is the most cost-effective way to add rentable or family space in San Carlos. These projects reuse existing floor area but still require a separate entrance, a small kitchen, and often a fire-rated separation wall. In 1960s ranchers with open floor plans, the layout often works better than homeowners expect.
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 install materials you can look up. On San Carlos ADUs and additions we regularly set Schluter waterproofing systems in wet areas, Sterling fixtures in secondary baths, and GAF roofing on new dormers and second-story additions. We don’t stock parts locally because most of this work is custom; what we do stock is the knowledge of which products hold up through San Carlos’s wet winters and windy summers.
Common Home Additions & ADU Problems We See in San Carlos Homes
- Expansive clay heaving slabs on hillside lots. Homes west of Alameda de las Pulgas sit on clay-rich soils that swell in winter and shrink in summer. A standard slab poured on those lots often cracks within two wet seasons. We specify pier-and-grade-beam foundations or deepened footings when the soils report calls for it, so your ADU doesn’t start failing before the paint dries.
- Septic and well setbacks killing a garage conversion at permit review. Many San Carlos homes built in the 1940s through 1960s still operate on septic systems. A detached ADU or second-story addition that encroaches on a leach field or well setback will get flagged by San Mateo County, and redesign costs thousands after the fact. We verify setbacks before we quote.
- Original knob-and-tube wiring and sagging ridge beams in 1950s structures. San Carlos’s housing stock is mostly post-WWII ranch and Cape Cod infill. Opening walls often reveals wiring that hasn’t been touched since Eisenhower was in office. We scope electrical and structural upgrades into the first quote, not a change order after demo starts.
- Missing county grading and erosion-control permits stalling excavation. San Mateo County requires separate erosion-control plans for any grading on slopes. If those aren’t filed with the building permit, the whole project stalls before footings are dug. We pull them up front.
Pricing for Home Additions & ADU in San Carlos, CA
In San Carlos, a home addition runs $45,000 to $130,000, a new detached ADU runs $90,000 to $220,000, and a garage conversion runs $30,000 to $80,000. Here are the current line items we quote in San Carlos:
| Project Type | San Carlos Price Range |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Garage Conversion | $30,000 - $80,000 |
What Changes the Price on a San Carlos ADU
The single biggest variable is your lot. A flat infill lot near El Camino Real builds fast and predictably. A hillside lot off Crestview Drive adds geotech reports, engineered pier-and-grade-beam foundations, county grading permits, and sometimes retaining walls, and those items can add $15,000 to $40,000 before a stick of lumber is delivered. We price those in the first quote, in writing, because an allowance that isn’t on paper is a surprise waiting for a date.
We Also Serve Cities Near San Carlos
Our Home Additions & ADU crews work across the mid-Peninsula. While this page focuses on San Carlos, we regularly build in Belmont, Redwood Shores, Redwood City, and North Fair Oaks, and we know how each city’s permit desk differs. No trip charge inside the county when you book an estimate.
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
Not for every lot, but for most hillside lots west of Alameda de las Pulgas, yes. San Mateo County will not issue a building permit for a detached ADU or second-story addition on expansive clay slopes without a geotechnical report. Skipping it means your initial quote is fiction. Call (877) 610-1318 and we’ll tell you honestly whether your lot triggers one before you spend a dollar.
They directly control your ADU’s footprint. San Mateo County requires specific horizontal distances between any new dwelling and an existing septic tank, leach field, or well. Many 1950s and 60s San Carlos lots have leach fields that limit an ADU to a corner of the yard you weren’t planning to use. We check your septic records before we draw the first plan, so you don’t redesign after permit review rejects it.
County jurisdiction and erosion control. Flat-lot infill near El Camino Real usually falls under standard city of San Carlos permitting. On hillside lots, San Mateo County requires a separate grading permit and an erosion-control plan, with specific winter wet-season restrictions from November through March. We time excavation and pour schedules around that, and we file the county permits up front so the excavation never stalls waiting on a missed filing.
Usually yes, but the cost depends on what’s inside the slab and walls. In San Carlos, many 1950s garages were built with thin, unreinforced concrete and original electrical panels that won’t carry an ADU’s load. A typical garage conversion here runs $30,000 to $80,000 depending on whether we’re replacing the slab, upgrading the panel, and adding a separate sewer connection. We scope it after opening the walls, never before.
Because the hillside forces engineering you can’t skip. Adding a second story on a flat lot near El Camino Real might require reinforcing a few load-bearing walls. The same addition on a slope west of Alameda de las Pulgas often requires pier-and-grade-beam footings, a soils report, shear wall upgrades, and county grading permits. That’s $15,000 to $40,000 in geotechnical and permit work before framing begins. Flatland prices don’t apply to hillside jobs, and anyone quoting flatland pricing on a hillside lot hasn’t looked at the soil.
Call Barrett & Sons for a Written Home Additions & ADU Estimate in San Carlos
We serve all of San Carlos, from the ranch-home flats near El Camino Real up into the steep lots west of Alameda de las Pulgas and the 94070 zip code. If you’re planning a detached ADU, a garage conversion, or a second-story addition, the first step is a free on-site walk with Tom or one of his sons. We’ll give you a written line-item quote with every scope, permit, and allowance on paper, so you know exactly what you’re paying before the first permit is pulled.
Call (877) 610-1318 to schedule your free estimate. $40 off any repair over $200, a free second opinion on any written estimate, a permanent 10% senior and veteran discount, and no trip charge inside the county. Tom answers the phone himself.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Carlos since 1998.