A free site visit, a check of what your lot allows and a written, line-item price.
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Adding a detached ADU, JADU, or garage conversion in San Mateo usually costs between $180,000 and $320,000 for a new backyard unit, while converting an attached garage into a JADU often runs $120,000 to $190,000. Those are the numbers most San Mateo homeowners need first, because they determine whether the project makes sense before a single drawing is produced. Our team has built ADUs across Hayward Park, Shoreview, Baywood, and the Aragon area, and we know how San Mateo’s zoning, setback rules, and older housing stock shape every decision. Call (831) 272-7212 to talk through your lot and get a written price before any work begins.
Key Takeaways
- Detached ADUs in San Mateo typically run $180,000 – $320,000; JADUs $120,000 – $190,000.
- San Mateo’s 35 – 45% lot coverage caps force many homeowners to remove an old garage entirely, not convert it.
- Plan-check turnaround in San Mateo currently runs 8 – 14 weeks depending on scope and zoning overlay.
- Every Sutton project carries a written price and the 365-Day Done Right Promise.
Why Sutton ADU San Francisco Is San Mateo’s Preferred ADU Builders Company
Miles Hartwell leads our ADU Builders team, and he’s spent enough time on San Mateo jobsites to know that the city’s permitting desk will not let a nonconforming lot slide. We bring that familiarity to every estimate we write. When a homeowner off South Norfolk Street asks about converting a pre-1960 garage, we already know Chapter 23.48 seismic retrofit triggers are part of the conversation, so the quote accounts for it up front rather than after plans get rejected.
Every technician who arrives in San Mateo is background-checked, identity-verified, and photo-badged under the Threshold Standard. We send their name and photo to you before the visit. You will never open your door to a stranger in an unmarked truck. The 365-Day Done Right Promise covers workmanship for one full calendar year from project completion, and if something fails, the second visit is free. Written pricing arrives before tools are lifted, always.
San Mateo homeowners tend to ask the same two questions: will the crew respect a dense neighborhood, and will the price change mid-build. The answers are yes and no, in that order. We keep sites clean because we work on six-thousand-square-foot lots where there is no room to hide a mess.
JADU in San Mateo
A JADU in San Mateo is a junior accessory dwelling unit, capped at 500 square feet, built within the existing walls of a single-family home, often from a converted attached garage, laundry room, or underused wing. For San Mateo homeowners with a 1920s bungalow near downtown or a 1950s rancher in the hills, a JADU is usually the fastest permitted path to rental income or multi-generational housing, and it costs substantially less than a detached unit because there is no new foundation, no new utility trenching, and often no new roof. In San Mateo’s market, an attached JADU conversion typically runs $120,000 to $190,000 depending on whether the existing space has a separate exterior entrance and how much seismic work the underlying structure needs.
Garage-to-JADU Conversion Realities
Many San Mateo garages built before 1960 have slab floors without moisture barriers and walls that were never strapped to the foundation. Converting one into a JADU means addressing those issues before finish work starts. We include that structural review in the written price, so it does not surface later as a change order.
When a JADU Beats a Detached ADU
If your lot already sits at or above the 35 percent lot coverage cap, a JADU built inside the existing home footprint does not add new covered ground. That can be the difference between a plan set that passes counter review and one that stalls. San Mateo’s Building Division has been efficient with JADU applications that respect the existing envelope, and we have seen smoother plan checks when the project avoids lot-line and setback triggers entirely.
ADU Plans in San Mateo
ADU plans in San Mateo are the architectural and structural drawings your project needs before the Building Division will even accept the permit application, and they must show setbacks, lot coverage calculations, stormwater management, and seismic retrofit scope where an existing structure is involved. For San Mateo homeowners, a full ADU plan set for a detached unit or garage conversion typically costs $8,000 to $18,000 depending on whether a structural engineer must assess an existing slab, and whether the property sits in an overlay district that requires additional review. This cost is included in the written price we provide, so you are not paying separately for drawings you cannot use.
What San Mateo Plan Check Actually Reviews
The reviewers look hardest at lot coverage math, the 4-foot side and rear setback compliance, and whether any demolition triggers parking replacement. On a 5,000 square foot lot, a 480 square foot ADU represents nearly 10 percent additional coverage, and many San Mateo homes already exceed their zoning cap. We calculate that before we draw anything, because a plan set that ignores coverage is a plan set that gets returned.
Site Plans That Survive Counter Review
We measure the existing garage footprint, note every mature tree within 10 feet, and mark the neighbor’s rear fence line with a 3-foot clear access notation when foundation work runs near the property boundary. Those details are not optional in San Mateo. They are the difference between a plan check that passes in ten weeks and one that stalls for four months.
Trusted Brands We Service in San Mateo
We work directly with Abodu, Plant Prefab, and Dvele panelized and modular systems, and we have installed Tuff Shed structures converted to conditioned living space where the budget or lot shape calls for it. For cladding and exterior envelope work, we specify James Hardie siding and trim, which holds up well in San Mateo’s damp winters and dry summers. We maintain direct relationships with these manufacturers, which means San Mateo homeowners get honest guidance about which product line fits their specific lot, not whichever unit we happen to have in a warehouse. Each brand has different lead times, foundation requirements, and finish levels, and we lay those options out plainly when we write your estimate.
Common ADU Builders Problems We See in San Mateo Homes
- Pre-1960 garages that fail seismic review. San Mateo Municipal Code Chapter 23.48 requires structural evaluation and often retrofit before conversion. We scope this during estimating, so the permit application includes the right engineering from day one.
- Lot coverage already over cap. Many 5,000 to 6,000 square foot San Mateo lots have primary homes that exceed the 35 percent coverage allowance. Adding a detached ADU without removing the old garage usually makes the numbers worse, not better.
- Party-wall and boundary friction. Foundation work within 3 feet of a shared property line requires a party-wall notice to the adjacent owner. Missing that step has triggered stop-work orders from the San Mateo Building Division, and the delay is measured in weeks, not days.
- Street encroachment violations. On narrow residential streets where parking is already tight, a construction dumpster or material delivery without a valid encroachment permit draws citations fast. We pull the encroachment permit as part of the project, and we schedule deliveries to minimize curb disruption.
Pricing for ADU Builders in San Mateo, CA
A detached ADU in San Mateo runs $180,000 to $320,000 before site work, and that range accounts for everything from a small 400 square foot studio to a 750 square foot one-bedroom with full kitchen and bath. In-law unit conversions that stay within the existing footprint usually land at $120,000 to $190,000. What moves the number in San Mateo specifically: whether you remove an old garage (demolition, slab re-pour, and parking reallocation typically add $25,000 to $40,000), whether the site needs deep excavation around mature trees, and whether your lot triggers overlay review. We give you a written, line-item price before work starts, and that price is the price. Call (831) 272-7212 for a free estimate.
One thing about San Mateo’s garage footprint worth knowing: many neighborhoods here have original detached garages that are so small and dated that conversion is not practical. A 9-foot-wide, single-car garage from 1948 does not become a legal 480 square foot ADU. It becomes a foundation. On one Hayward Park lot off South Norfolk Street, we demolished a 1950s single-car garage that occupied the only flat backyard corner outside the 4-foot side setback. We re-poured the slab with a 12-inch thickened edge to avoid deep excavation near a mature redwood, set a panelized shell in two days, and reallocated parking to the existing driveway. The 3-foot clear access path to the neighbor’s fence stayed open for the entire build. That is the kind of constraint that only shows up on a tight San Mateo lot, and we plan for it before the first truck arrives.
We Also Serve Cities Near San Mateo
Beyond San Mateo, our crews work regularly in San Mateo neighboring communities: Belmont, South San Francisco, Brisbane, and Redwood City. Each city has its own building division quirks, but the same written-price and photo-badged crew standards apply everywhere we go.
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 About ADU Builders in San Mateo
Yes, in most residential zones San Mateo requires that any parking space lost through demolition be replaced or reallocated on the lot, typically as a new driveway stall or a compact space with a curb ramp. The city will not waive this requirement just because the original garage was substandard. We map the replacement parking into the site plan before submittal, so the permit application addresses it in the first review. Call (831) 272-7212 to find out whether your specific lot triggers the requirement.
No, attached ADUs must be attached to your primary dwelling, not to a neighbor’s structure, and San Mateo’s side setback rules require a minimum 4-foot gap from the property line for new construction unless your lot predates the current code with a legal nonconforming structure. Building anything within 3 feet of the shared line also requires a party-wall notice to the adjacent owner before foundation work starts. We handle those notices and the structural separation details as part of the permit package.
Plan-check turnaround for a standard detached ADU in San Mateo currently runs 8 to 14 weeks from submittal to approval, with JADUs inside the existing footprint often moving faster because they avoid lot-line and coverage review triggers. Overlay districts, seismic retrofit review on existing structures, and parking replacement questions can push the timeline longer. We submit complete plan sets the first time so the clock does not restart on a resubmittal. Estimates are free, so call if you want a realistic timeline for your address.
Yes, construction noise in San Mateo is generally restricted to the hours between 8 a.m. and 6 p.m. on weekdays, with further limits on weekends and holidays. Demolition, concrete cutting, and heavy equipment fall under these rules. We schedule noisy phases within the allowed window and coordinate with neighbors on dense streets where the sound travels across narrow side yards, which keeps the project moving without citations or friction.
San Mateo requires stormwater management for new construction, which for a typical ADU means directing roof runoff to on-site pervious areas or a dry well, and sizing the new slab drainage so water does not sheet onto a neighbor’s property. Since San Mateo’s winters are wet, this matters more here than in drier Bay Area cities. Our plan sets include the drainage calculations the Building Division expects to see, and the 365-Day Done Right Promise covers the workmanship on those systems for a full year after completion.
Written by Miles Hartwell, Owner at Sutton ADU San Francisco, serving San Mateo homeowners with written pricing and photo-badged crews under the Threshold Standard.
