ADU in San Mateo, CA
Most San Mateo homeowners considering an ADU are looking at a detached one-car garage built between 1948 and 1965, or an underused side yard on a 5,000 to 7,500 square foot lot. The question is rarely whether an ADU makes sense here. It’s what the permit set has to account for: original 60-amp electrical panels, galvanized supply plumbing, FEMA flood-zone requirements in Shoreview, and marine humidity that punishes a poorly detailed building envelope. We handle all of it under one contract, with a written price before anything starts. Call (971) 497-2402 to schedule a free estimate.
Why San Mateo Homes Are Different
San Mateo’s post-WWII building boom produced thousands of 1940s to 1960s ranch homes and bungalows across flat neighborhoods like Beresford, Baywood, and Hillsdale. Those homes share a profile: stucco exteriors, detached single-car garages, 60-amp panels, and galvanized plumbing that’s now 60 to 80 years old. The good news is that the detached garage is exactly the conversion target California’s streamlined ADU law was designed to unlock. The complication is that the infrastructure feeding it rarely meets current code, and the permit set has to address that before construction starts.
Climate is the second variable. Low-lying eastern neighborhoods like Shoreview and North Shoreview absorb persistent marine humidity off the Bay, which makes vapor barriers and exterior envelope detailing on ADUs more critical than in Redwood City or Burlingame. And the eastern edge of the city carries FEMA Special Flood Hazard Area designations. A detached ADU in an AE zone may need an elevated or engineered foundation to meet Base Flood Elevation, a $15,000 to $30,000 adder that doesn’t apply a mile inland. We surface that at the design stage, not mid-permitting.
San Mateo also has a significant share of unpermitted in-law units, often converted garages or basement spaces, that now qualify for legalization under California’s ADU amnesty pathway. A legalization project is different from new construction: the scope includes documenting what exists, correcting code violations, and bringing the unit into the permit record. We do both regularly.
Our Main San Mateo Services
The two services San Mateo homeowners call about most are garage conversions and new detached ADUs. Both start with the same question: what can this lot legally support, and what will the permit set cost to produce.
Garage Conversion ADU in San Mateo
Most San Mateo ranch homes have a detached single-car garage with a slab that’s 4 inches thick and framing that’s structurally sound but never designed for habitation. Converting it means upgrading the electrical service, insulating the walls and roof, and usually replacing the roof. Done right, it’s the fastest path to a rentable unit because the shell already exists. We’ve designed and permitted hundreds of these across Beresford and Hillsdale, and the 365-Day Done Right Promise applies in writing before we start. Read the full service breakdown on our Garage Conversion ADU in San Mateo page.
New Detached ADU in San Mateo
A new detached ADU on a San Mateo lot means working within the city’s setback requirements and, in some eastern neighborhoods, flood-zone foundation rules. The design has to account for both. We specify and coordinate prefab systems from brands like Cover and Dvele where they fit the site, and we handle the permit set, foundation, utilities, and final inspection under a single written price. New Detached ADU in San Mateo covers the full scope, including what changes when the site is in an AE flood zone.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
ADU Design & Permitting in San Mateo
ADU design and permitting in San Mateo typically costs $15,000 to $30,000 depending on scope, and it’s the part of the project where a bad set of drawings causes the most expensive delays. San Mateo’s 1940s to 1960s housing stock means nearly every permit set includes an electrical service upgrade from the original 60-amp panel, and a meaningful share includes replacing galvanized supply plumbing. The permit set is the core product here: the drawings, the Title 24 energy compliance documentation, and the structural calculations that tell the city exactly what will be built. We’ve been producing them since 2015, and every one is quoted with a written price before work begins. You can see the full scope of ADU Design & Permitting we handle.
Site Assessment
Signs you need it
You own a San Mateo home built before 1970, and you don’t know whether your lot’s setbacks, flood zone, or existing utility service will support an ADU.
What it costs in San Mateo
Site assessment typically runs $1,500 to $4,500 when done as part of a full design and permitting contract.
How we do it
We document the lot, check the FEMA flood map, measure existing utility services, and produce a written feasibility summary with the specific constraints the permit set will have to engineer around.
Permit Application
Signs you need it
You have a design you like but no one has filed the building permit application with San Mateo’s planning department, and the clock is running on your construction schedule.
What it costs in San Mateo
Permit application and city submission typically runs $3,000 to $10,000 depending on how many review cycles the city requires.
How we do it
We prepare the full submittal package, file it with San Mateo, track the plan-check comments, and return corrections until the permit is issued.
What ADU Design & Permitting Costs in San Mateo
| Item | Typical range in San Mateo |
|---|---|
| Site assessment & feasibility | $1,500 - $4,500 |
| Full design & permit set | $15,000 - $30,000 |
| Permit application & city filing | $3,000 - $10,000 |
How long does ADU permitting take in San Mateo?
San Mateo’s streamlined ADU review typically takes 60 days for a complete application, but plan-check comments can extend that to 4 to 6 months if the drawings are incomplete. A clean permit set, filed correctly, shortens that materially. Call (971) 497-2402 for a written timeline on your specific project.
Will the city reject my ADU because of my home’s age?
No. San Mateo routinely permits ADUs on 1940s to 1960s ranch homes. The city’s concern is life-safety compliance, not building age: the electrical panel, plumbing, and foundation have to meet current code. Call (971) 497-2402 for a written scope.
Junior ADU & Interior Conversion in San Mateo
Junior ADUs and interior conversions in San Mateo typically cost $80,000 to $180,000 depending on whether the space already has a bathroom and a separate entrance. The classic candidate is an unpermitted in-law unit built into a 1950s garage or basement in Baywood or Beresford, where the plumbing exists but the permit never happened. Legalizing an existing unit under California’s amnesty pathway is often cheaper than building new, and it converts dead equity into a rent-producing asset with a documented permit record. The full Junior ADU & Interior Conversion scope covers everything we handle.
Basement ADU
Signs you need it
Your San Mateo home has a basement with existing ceiling height, and you’re either using it for storage or suspect the previous owner finished it without permits.
What it costs in San Mateo
Basement ADU conversions typically run $100,000 to $220,000 depending on egress windows, waterproofing, and whether the existing plumbing is salvageable.
How we do it
We document the existing conditions, design the legalization or conversion scope around the current plumbing and electrical, and file the permit set with San Mateo before construction starts.
Interior JADU
Signs you need it
You have an underused bedroom, den, or attached space inside your main home, and you want to rent it without changing the building footprint.
What it costs in San Mateo
Interior JADU conversions typically run $80,000 to $150,000, with the lower end applying when a bathroom and kitchenette are already roughed in.
How we do it
We add the required separate entrance or interior access, install a kitchenette and bathroom if needed, and file the JADU permit under San Mateo’s streamlined interior-conversion rules.
What Junior ADU & Interior Conversion Costs in San Mateo
| Item | Typical range in San Mateo |
|---|---|
| Interior JADU conversion | $80,000 - $150,000 |
| Basement ADU conversion | $100,000 - $220,000 |
| Garage-to-JADU legalization | $80,000 - $180,000 |
Can I legalize an unpermitted in-law unit in San Mateo?
Yes, in most cases. California’s ADU amnesty pathway lets San Mateo homeowners bring unpermitted in-law units into the permit record by correcting code violations and filing the proper documents. The process starts with a documented inspection. Call (971) 497-2402 for a written scope.
Does a JADU need its own electrical panel in San Mateo?
A JADU typically doesn’t need a separate panel, but the main panel almost always needs an upgrade from the original 60-amp service to handle the added load. We include that in the written price. Call (971) 497-2402 for specifics.
ADU Site Prep & Utilities in San Mateo
Site prep and utilities for a San Mateo ADU typically run $20,000 to $60,000, and the single largest line item is usually the electrical panel upgrade from a 1960s-era 60-amp service to a 200-amp panel. Galvanized supply plumbing is the second common replacement, particularly on Baywood and Beresford ranch homes. In Shoreview and North Shoreview, FEMA flood-zone designations can push foundation costs higher when Base Flood Elevation requires an elevated or engineered slab rather than standard slab-on-grade. We quote all of it as a written line-item price under Haven Standard Clause 1, before any work begins. The ADU Site Prep & Utilities page covers the full scope.
Electrical Panel Upgrade
Signs you need it
Your San Mateo home still has the original 60-amp panel, or the panel is full and the ADU will need circuits for a kitchenette, laundry, and mini-split heating.
What it costs in San Mateo
A 200-amp panel upgrade typically runs $3,000 to $8,000, assuming no service-drop replacement is required.
How we do it
We pull the electrical permit, coordinate the service drop with PG&E, install the new panel, and run the subfeed to the ADU as part of the same contract as the design and construction.
Foundation & Grading
Signs you need it
Your lot is in a FEMA AE flood zone in Shoreview or North Shoreview, or the existing garage slab is cracked and insufficient for habitable floor loads.
What it costs in San Mateo
Foundation and grading typically runs $10,000 to $30,000 for a standard slab, with flood-zone engineering adding $15,000 to $30,000 more.
How we do it
We verify the flood-zone designation, specify the foundation type the Base Flood Elevation requires, and include the grading plan in the permit set before filing.
What ADU Site Prep & Utilities Costs in San Mateo
| Item | Typical range in San Mateo |
|---|---|
| Electrical panel upgrade to 200A | $3,000 - $8,000 |
| Foundation & grading | $10,000 - $30,000 |
| Full site prep & utility package | $20,000 - $60,000 |
Does a Shoreview ADU really need a $30,000 flood-zone foundation?
It depends on the precise FEMA designation. Some Shoreview lots are in AE zones with Base Flood Elevation requirements that mandate an elevated or engineered foundation. Others are just outside the boundary. We check the flood map before we design anything. Call (971) 497-2402 for a written site assessment.
Can I keep my existing 60-amp panel for an ADU?
No. A 60-amp panel can’t support a main home plus an ADU with a kitchenette and mini-split. San Mateo’s current code requires a service upgrade, and we include it in the written price. Call (971) 497-2402 for a line-item quote.
Brands We Service in San Mateo
We specify and coordinate prefab and modular ADU systems from Cover, Dvele, and Boxabl within the same single contract as the design and construction. Parts and panels are ordered through regional suppliers, and we coordinate delivery to the San Mateo site on the construction schedule. See our brands for the full list.
Pricing for ADU in San Mateo, CA
| Service | Typical range in San Mateo |
|---|---|
| Site assessment & feasibility | $1,500 - $4,500 |
| Full design & permit set | $15,000 - $30,000 |
| Permit application & city filing | $3,000 - $10,000 |
| Interior JADU conversion | $80,000 - $150,000 |
| Basement ADU conversion | $100,000 - $220,000 |
| Garage-to-JADU legalization | $80,000 - $180,000 |
| Electrical panel upgrade to 200A | $3,000 - $8,000 |
| Foundation & grading | $10,000 - $30,000 |
| Full site prep & utility package | $20,000 - $60,000 |
What pushes a San Mateo ADU above the middle of these ranges is usually the flood-zone foundation work in Shoreview and North Shoreview, followed by the cost of replacing galvanized plumbing that’s reached the end of its service life. Every project we quote carries a written price before work begins, under Haven Standard Clause 1, so the number on the estimate is the number you pay.
Neighborhoods We Serve in San Mateo
We work across San Mateo’s flat ranch-home neighborhoods and hillside lots alike. Most of our projects here are in the 1940s to 1960s housing stock where garage conversions and JADUs are the natural ADU play.
- Beresford
- Baywood
- Hillsdale
- Shoreview
- North Shoreview
Response time for a site assessment in San Mateo is typically within 5 to 7 business days from the first call.
Service Area - Cities Near San Mateo
We serve the mid-Peninsula corridor around San Mateo, including Burlingame, CA, Belmont, CA, San Carlos, CA, and San Bruno, CA. The same written-price standard applies everywhere we work, and the same crews handle the jobs.
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 in San Mateo
A garage conversion ADU in San Mateo typically runs $150,000 to $250,000 all-in, drawings through final inspection. A new detached ADU runs $250,000 to $450,000. These ranges assume a written scope with no change orders, and every project is quoted with a written price before work begins. Call (971) 497-2402 for an exact quote on your property.
San Mateo follows California state ADU law, which does not require owner-occupancy for ADUs permitted after 2020. The city’s streamlined review process applies to both attached and detached ADUs, and JADUs have no owner-occupancy requirement either. Call (971) 497-2402 for current requirements on your specific lot.
Flood-zone rules do not block permits; they change the foundation design. A detached ADU in a FEMA AE zone may need an elevated or engineered foundation. We run the flood-map check during the site assessment so the permit set reflects the right foundation from day one. Call (971) 497-2402 to schedule a free estimate.
What happens when you call
- 1A real person answersNo phone trees - talk to a local adu pro.
- 2You get an upfront price rangeHonest numbers before anyone is dispatched. No surprises.
- 3We come out and measureLicensed & insured, at a time that suits you - the visit and the quote are both free.
- 4You get the price in writingLine-itemed, yours to keep, and nothing starts until you say go.
What San Mateo Customers Say
"The permit set and the build were one contract with one number. Our neighbours used two firms and spent a year watching them blame each other."
- Helena V."The rental income has covered the payment since the second month. They were realistic about the numbers rather than optimistic."
- Odette N."They walked us through setbacks and utility capacity before we paid anything, and told us one of our two options was not going to get approved."
- Mireille T.