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San Mateo · Attached ADU
One shared wall, a full second home on your San Mateo lot.
An attached ADU grows off your San Mateo house — its own door, kitchen and bath — sized for a tight Peninsula side yard and matched to your Craftsman, Spanish-revival or mid-century home.
Trusted by homeowners across Sacramento & surrounding areas.
Attached ADU in San Mateo — fast facts
City
San Mateo
ADU type
Attached ADU
Contractor license
CSLB #1119373
Free estimate
(916) 677-9774
Last updated
Joined, not freestanding
What an attached ADU really is
An attached ADU shares exactly one wall with your existing San Mateo house. That single wall is the whole difference between this and a detached cottage in the backyard. Past it, the unit is a complete home — a private door off the side or rear, a real kitchen, a full bath, a bedroom a tenant reaches without walking through your living room. We lean toward attached units on the Peninsula because lots here are modest and land sells for a fortune. The new build ties into your roofline, your foundation and your wall line, and near the region's active faults that tie-in is a structural question, not a cosmetic one. Built right, you can't find the seam from the sidewalk.
Shares one wall; everything past it is fully independent
Private entrance, full kitchen, full bath, real bedroom
Ties into the existing roof, foundation and wall line
The wall between two homes
The shared-wall fire and sound assembly
Your shared wall stops being an ordinary interior wall the moment two separate dwellings sit on either side of it. Code wants a rated separation. Usually that's a one-hour fire assembly running floor to roof — the right gypsum layers, fireblocking at every penetration, and a sound-rated cavity so a tenant isn't living inside your TV. This is the part that surprises owners. Pull the finish off a 1920s Spanish-revival in San Mateo Park and you'll often find old wiring and board sheathing that won't carry the rating as-is. So the existing wall usually needs work too, not just the new side. We draw the assembly on the plans and build it to the listed spec, then the inspector signs off on a wall that does what the drawings claim.
One-hour rated separation between the two dwellings
Sound-rated cavity so neither household hears the other
Existing wall often needs upgrading to hit the rating
City of San Mateo permits
Permitting an attached ADU in San Mateo
Your permit files with the City of San Mateo — its own counter, its own pace. The city is ADU-friendly in policy and genuinely slower in practice than you'd hope, with fees that run high relative to anything inland. Set your clock to that. Attached units draw an extra look: reviewers study how the addition connects to your existing house, so your home's as-built condition matters more than it would on a detached job. If there's an unpermitted room nobody pulled paper on, it can surface here. State law keeps ADU review ministerial, so a complete set has to be issued — but a thin submittal in San Mateo buys correction cycles you'll feel in months, not weeks. Completeness is the whole game.
City of San Mateo review — friendly in policy, slow in practice
Reviewers study how the addition ties into your house
High fees; a complete set keeps review ministerial
Shared utilities, real savings
Why the shared wall costs you less
The reason an attached ADU usually pencils cheaper than a detached one is right in the name. You tap the foundation, the PG&E service and the water line already running to your house, instead of trenching the length of the lot to a freestanding cottage. That matters more here than almost anywhere — Bay Area detached units commonly run $400K to $750K and up, and the shared wall is how you knock a real chunk off that. One honest caveat: at 750 sq ft the City can require a separate water meter, and PG&E may want a panel upgrade before a second all-electric kitchen's load lands on your service. An attached unit also fits a tighter side yard than a detached one needs, which is the whole game on a narrow Hayward Park or Shoreview parcel.
Taps the existing foundation, PG&E service and water line
Cross the 750 sq ft line and a separate water meter can apply
Fits a tighter side yard than a detached build needs
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From design to move-in, we handle everything. Transparent pricing, on-time delivery, and uncompromising quality.
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Trusted by homeowners across Sacramento & surrounding areas.
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“Every ADU we build is designed to add long-term value and open new possibilities for your property. Let's build something great together.”
— Tim, ADU Specialist
Custom Designs
Built to fit your property & needs
Turnkey Process
We handle permits, plans & everything
Built to Last
Quality craftsmanship that stands the test
Financing Made Simple
$0 Down Options. Build Now, Pay Over Time.
You shouldn't have to drain your savings to build wealth. We partner with trusted ADU lenders to find financing that fits — many homeowners build with little to nothing out of pocket.
Home Equity (HELOC)
Tap your existing equity at competitive rates — the most popular way to fund an ADU.
Renovation Loans
Finance the build against your home's projected after-completion value.
ADU-Specific Financing
Specialized lenders who underwrite based on the rental income your ADU will produce.
Cash-Out Refinance
Roll the project into a new mortgage and keep one simple monthly payment.
Estimate Your Monthly Payment
See what financing your ADU could look like — and how rental income can help cover it.
$180,000Loan amount
Credit Score
Very good · 700–739
Estimated Monthly Payment
$1,320/mo
At 7.99% APR over 30 years (illustrative)
Often offset by $2,000–$3,500/mo in rental income
No prepayment penalties with most partners
Pre-qualification with no impact to your credit
Illustrative only — not a loan offer. Actual rates and terms are set by third-party lenders based on your qualifications.
Service areas
Where we build
Headquartered in the Greater Sacramento Area and serving homeowners from the Sierra foothills down through the Bay Area. Don't see your city? We probably still get there — just ask.
The most common things Sacramento homeowners ask before starting an ADU project.
An attached ADU shares one wall with your existing house. A detached unit stands alone in the yard. That shared wall is the whole difference — it lets the new unit borrow your foundation, your PG&E service and your water line, which usually makes it cheaper than a freestanding cottage on the same San Mateo lot. In a market where detached units commonly run $400K to $750K and up, that gap is real money. An attached unit also fits a narrower side yard, which matters on the modest Peninsula parcels around Hayward Park, Shoreview and Aragon where you don't have open ground to spare on all four sides.
With the City of San Mateo. The City handles ADU permits directly, and it's ADU-friendly in policy — but honestly slower in practice, with high fees. We handle the full submittal: site plan, floor plan, elevations, the structural tie-in and foundation details, Title-24, MEP, seismic design and PG&E coordination. State law keeps ADU review ministerial, so a complete, code-compliant set has to get issued. The whole game in San Mateo is turning in a clean, complete set so you're not stuck in correction cycles that drag for months instead of weeks.
It does, and on the Peninsula it isn't optional. San Mateo sits near the region's active faults, so seismic design shapes the structure rather than getting tacked on at the end. Your new unit gets engineered shear walls, hold-downs tying the frame to the foundation, and a load path detailed for ground shaking. On an attached unit the connection to your existing house is the critical point — it has to move with the main home in a quake rather than tearing away at the shared wall, which also happens to be your fire and sound separation. That junction is where the seismic detailing earns its keep.
Once two separate dwellings sit on either side of a wall, code wants it rated — typically a one-hour fire assembly running floor to roof, with the right gypsum layering and fireblocking at every penetration. We also build a sound-rated cavity into it so your tenant isn't listening to your living room. On older San Mateo homes the existing wall usually needs upgrading to carry the rating — original wiring and board sheathing won't do it as-is — and we catch that on the plans rather than mid-build, which is where it gets expensive.
Usually, and that's exactly why we steer people toward attached units here. San Mateo lots are modest and land is expensive, so siting an ADU is a tight-yard exercise from the start. A detached cottage needs clear ground and setbacks on all four sides — many Peninsula parcels just don't have that. An attached unit lives in the side-yard slot you already own, pressed against the house. On a narrow Baywood or Shoreview lot, that's often the difference between a unit that fits the parcel and one that doesn't pencil at all.
At and above 750 square feet the City can require a separate water meter for the ADU instead of letting it ride on your existing service. Below that, you usually share. PG&E may also want a panel upgrade before you add a second all-electric kitchen's load. We size the unit knowing where both lines sit, so you're not tripping a meter requirement to gain a handful of square feet you didn't actually need. On San Mateo's high fees, every avoidable trigger is worth dodging — a 612 sq ft unit can sidestep a cost a 760 sq ft one walks straight into.
That's the honest question, and the answer is usually yes — because of the rents. Bay Area ADU construction is a different planet from Sacramento; detached units commonly land $400K to $750K and up. But Peninsula rents are far higher too, and demand is strong and steady. An attached unit cuts the build cost by sharing your wall and utilities, and a quiet side unit with its own entrance rents fast in a market this tight on housing. We've seen owners treat the unit as a long-term income line that carries itself over the years they hold the property.
Straight answer: we're based in Sacramento, about two hours from San Mateo, and we extend out here for homeowners who want our craftsmanship and value, or for larger projects worth the drive. We're not pretending to be the corner outfit who's done a hundred jobs on your block. What we bring is design-build discipline, real seismic and shared-wall experience, and a pricing sensibility from a market where costs haven't gone fully vertical — applied to a Peninsula where local quotes can run wild. For the right project, that round trip pays for itself.
Longer than Sacramento, and I won't dress that up. San Mateo's permitting is ADU-friendly on paper but slow in practice, and the high-fee, strict-review reality adds weeks on top. A clean, complete attached set keeps you out of correction loops, which is the single biggest schedule lever here. Construction depends on the tie-in, the lot and the seismic work. We give you a real schedule after we see the parcel, the soil and how your house sits on it — not a generic number off a brochure.
That's the point of doing it right, and on the Peninsula it's most of the job. We match the siding or stucco, carry the elevation lines across, and tie the new roofline into your existing roof so the addition reads as part of the original home. On a Craftsman in Aragon, a Spanish-revival in San Mateo Park or a mid-century in Hillsdale, that match is what keeps the City, the neighbors and the reviewers satisfied. Someone walking past shouldn't be able to find the seam between the old house and the new unit.
Other services
Not quite the right fit?
Every Sacramento ADU project is different. Here are the other paths we build under one roof — and we're happy to talk through which one fits your lot best.
We treat every home like it's our own — quality work, clear communication, and happy homeowners.
PaulOwner · Future Generation
8 min read1,843 words8 sections
Building an Attached ADU in San Mateo, on the Peninsula
An attached ADU in San Mateo borrows one wall from your existing house and turns the rest into a separate, legal home — its own entrance, kitchen and bath. We're a Sacramento design-build contractor, license #1119373, about two hours east. That distance is the first honest thing here: we extend out to the Peninsula for owners who want our craftsmanship and value, or for projects big enough to earn the drive. What follows is how we think about attached units in San Mateo, and where it gets harder than the brochure admits.
01Section
What "attached" actually buys you
The word attached carries the whole concept. Your new unit shares precisely one wall with your existing San Mateo house and is otherwise a complete, independent home — a private door off the side or rear, a real kitchen with its own range and sink, a full bath, and a bedroom a tenant reaches without stepping into your space. That single shared wall is the only thing tethering the two homes. Everything else stands separate.
That one connection is why people pick attached over detached on the Peninsula. You're not setting a freestanding cottage at the back of a high-value lot and trenching utilities the length of the parcel. You grow off the house already standing, leaning on the foundation, panel and water line already there. On a San Mateo lot — modest in size, expensive by the square foot — that sidesteps a fair amount of the site's difficulty before you've poured anything.
