Fast, Reliable Garage Conversion ADU Across San Mateo
A garage conversion ADU in San Mateo typically runs $185,000-$310,000 all-in, drawings through final inspection, with attached conversions at the lower end and detached work with flood-zone foundations at the higher. We carry the design, permit set, and construction under one written contract, so there’s no gap between who drew it and who built it. If you’re weighing a garage ADU on a 5,000-7,500 sq ft lot in Beresford, Baywood, or Hillsdale, call (971) 497-2402 and we’ll walk the numbers with you before you commit to anything.
Why Ellery ADU Studio Is San Mateo’s Preferred Garage Conversion ADU Company
Ellery ADU Studio has operated as a design-build studio since 2015, the year California’s ADU permitting landscape shifted. Our team works San Mateo neighborhoods weekly, which means we know what Shoreview’s FEMA maps do to a foundation line item, what a 1950s Beresford garage slab actually measures, and which San Mateo planning counter questions get answered on the first submittal. We don’t guess from across the Bay.
San Mateo homeowners who call us get a written price before work begins. That’s not a promo line. It’s Haven Standard Clause 1, and it’s in the contract. Under Clause 5 (No Surprises), the scope locks at signing. No door-side upsells, no scope creep. Nadia Ellery built the studio around this because the designer-versus-builder blame gap is where most San Mateo ADU projects quietly die.
We also answer the phone. A live person picks up 24/7 at (971) 497-2402. If you already have a written estimate from another firm, bring it in. We’ll give you a free second opinion and show you exactly where the numbers hold and where they don’t.
Every project under the Haven Standard includes a documented photo record on every visit. You’ll see what we found and what we did, not a summary of how it felt. And if the finished ADU is not built to the permitted drawings, the 365-Day Done Right Promise puts the fix in writing before the project starts.
Our Garage Conversion ADU Services in San Mateo
San Mateo’s post-WWII housing stock gives us a lot of the same garages to work with: detached one-cars on ranch lots, attached two-cars on 2000s-era subdivisions, and a surprising number of carports that are half a step from becoming rentable square footage. Here’s what we do most.
Attached Garage Conversion
Attached garage conversions in San Mateo usually run $165,000-$240,000 because the existing roofline and foundation do a lot of the work. The catch on newer subdivisions like Bay Meadows is the builder warranty and the CC&Rs. We read the architectural rules before we frame anything. On a 2018 Lennar home, a partial attached conversion that stays within the original footprint often sails through, but we flag the warranty implications in the permit set so the city and the builder both see what’s happening.
Detached Garage Conversion
San Mateo’s 1940s-1960s detached one-car garages are the classic ADU conversion target, and they typically land between $210,000 and $310,000. The spread is driven by the flood map. In Shoreview and North Shoreview, a FEMA AE zone designation can force an engineered foundation with flood vents and an elevated stem wall. That’s a $15,000-$30,000 adder that doesn’t apply even a mile inland. If you’re on the eastern edge of the city, we check the flood map before we touch the design.
Carport Conversion
A carport conversion in San Mateo runs $140,000-$210,000, but only if the structure has the bones for it. Many Beresford carports are open on three sides with no slab to speak of, and turning that into a conditioned ADU means a new foundation, full envelope, and often a panel upgrade. In a city where a well-built ADU rents for $3,000-$4,500/month, the math still works. But we’ll tell you upfront whether you’re looking at a conversion or a rebuild.
Garage Partition & Bathroom Add
For San Mateo homeowners who want to keep part of the garage, we partition. A one-bedroom JADU with a bathroom added off the existing garage typically runs $120,000-$180,000. This is the sharpest return on the Peninsula, because you keep storage and car space while creating a rentable unit. The catch is usually the 60-amp panel and galvanized supply plumbing, both common in 1950s San Mateo homes. We build the upgrade into the written scope so it’s in the price, not a surprise.
Garage JADU
A garage JADU in San Mateo runs $110,000-$170,000, and it’s the fastest permitted path to rental income. Junior ADUs cap at 500 sq ft and can share systems with the main house, which is why they pencil out well on flat lots in Hillsdale and Beresford. The trade-off is the ownership requirement: the owner must live in either the main house or the JADU. For San Mateo homeowners planning to stay put, that’s usually not a problem. We’ll lay out the JADU versus full ADU numbers side by side so you can decide with real dollars, not guesses.
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 Mateo
Our Garage Conversion ADU team specifies and coordinates prefab and modular systems from Abodu, Cover, Dvele, and Mighty Buildings within the same single contract. That means if you’ve been looking at a Plant Prefab module or a Cover unit, we can fold it into a design-build scope instead of sending you back and forth between a module rep and a separate builder. For San Mateo customers, coordinating factory-built components also shortens the on-site framing window, which matters when you’re working in a tight Baywood lot with neighbors on both sides.
Common Garage Conversion ADU Problems We See in San Mateo Homes
- FEMA flood-zone foundations on the eastern edge. Shoreview and North Shoreview carry AE zone designations that can require engineered foundations meeting Base Flood Elevation. Homeowners who order a prefab shell before checking the map end up with a $15,000-$30,000 fix and a module sitting on a trailer.
- Original 60-amp electrical panels. The dominant 1940s-1960s stock across San Mateo was wired for a different century. Adding a one-bedroom ADU almost always means a panel heavy-up and often a service line replacement. We put it in the written price, not on a change order.
- Marine humidity rot at the sill plate. San Mateo’s bay-facing position means low-lying neighborhoods like Shoreview absorb persistent damp from both the Pacific and the Bay. Untreated sill plates against old concrete rot in years, not decades. Vapor barriers and pressure-treated breaks cost about $2,000 on a conversion, and we won’t omit them to sharpen a quote.
- HOA rules newer subdivisions never advertised. Some 2000s-era San Mateo HOAs flat-out ban detached ADUs. Others restrict exterior changes to garages. We read the CC&Rs before framing, because a stop-work order is the most expensive document in the project.
Pricing for Garage Conversion ADU in San Mateo, CA
Here’s where the money goes in San Mateo, in real ranges.
| Scope | Typical Range (All-In) | Notes |
|---|---|---|
| Attached garage conversion | $165,000-$240,000 | Existing roofline and slab carry the load if structurally sound. |
| Detached garage conversion (inland) | $210,000-$280,000 | New foundation, envelope, and separate utility feeds. |
| Detached garage conversion (FEMA AE zone) | $245,000-$310,000 | Engineered elevated foundation, flood vents, BFE compliance. |
| Carport conversion | $140,000-$210,000 | Range depends on whether the slab and structure are salvageable. |
| Garage JADU | $110,000-$170,000 | 500 sq ft cap, can share systems with main house. |
| Garage partition & bathroom add | $120,000-$180,000 | Keeps partial garage; new bath and rated separation. |
The biggest swing factor in San Mateo is your flood map, not your square footage. A mile inland changes the foundation line item by five figures. The second factor is your panel: a 2010s home with a 200-amp service skips the heavy-up, while a 1955 ranch in Beresford eats a $4,000-$8,000 electrical upgrade. Every project is quoted with a written price before any work begins, per Haven Standard Clause 1. Call (971) 497-2402 and we’ll give you a line-item number for your specific garage, lot, and flood zone. Estimates are free.
We Also Serve Cities Near San Mateo
Ellery ADU Studio serves the whole mid-Peninsula. If your garage sits just over the line in Burlingame, Belmont, San Carlos, or San Bruno, the same Garage Conversion ADU in San Mateo team handles it with the same written-price standard. Dispatch and scheduling are coordinated from our San Francisco Bay Area office, and crews serve San Mateo locally.
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 - Garage Conversion ADU in San Mateo
No. A one-bedroom ADU in a 1950s San Mateo home with an original 60-amp panel almost always requires a panel heavy-up to at least 100 amps, and often a service line replacement. We include that in the written scope from day one, typically $4,000-$8,000 depending on your utility drop and meter location. Call (971) 497-2402 to get the exact number for your address before you spend anything else.
Norfolk Street in Shoreview sits inside the FEMA AE Special Flood Hazard Area, which means a detached garage ADU there is likely to require an engineered foundation meeting Base Flood Elevation. We just wrapped a detached garage conversion off Gresham Avenue in Shoreview where the original 1950s one-car had a slab three inches below Base Flood Elevation. The owner had already ordered a Plant Prefab module before we flagged the FEMA AE zone, and we had to rework the foundation for flood vents and an elevated stem wall. We ended up converting an attic dormer into a permitted JADU instead and kept the prefab shell for the garage. The lesson: check the flood map before design, not after. We do that as step one for every Shoreview and North Shoreview project. Call (971) 497-2402 for a flood-zone read on your specific parcel.
Yes, in most cases. California’s ADU amnesty pathway lets San Mateo homeowners legalize unpermitted converted garages or basement spaces that were built before the current code cycle, as long as they’re brought up to health and safety standards. San Mateo has a notable share of these, and legalization projects are as common for us as new construction. The process typically runs $80,000-$150,000 depending on what the original builder skipped. We document everything for the permit set, and the finished unit gets a legal certificate of occupancy, which is what actually matters for insurance and resale. Call (971) 497-2402 and we’ll tell you if your in-law unit is a legalization candidate.
Not automatically, but the way you scope it matters. Lennar’s structural warranty typically covers the home’s original structural components for 10 years, and a partial attached garage conversion that stays within the original footprint and doesn’t modify load-bearing walls or the roof structure usually doesn’t void it. But the moment you cut into a shear wall or modify the foundation, the original builder can argue that your work compromised what they warranted. We document the existing conditions in the permit set and keep the conversion inside the original envelope where possible. For Bay Meadows specifically, we also check the HOA’s architectural rules, because many Bay Meadows phases have design review requirements that apply even to attached conversions. Call (971) 497-2402 and we’ll review your CC&Rs and warranty language before you sign anything.
No, usually not. Beresford’s older homes generally predate HOA formation, so there’s no architectural review committee to answer to. A carport conversion on a 1950s Beresford lot is governed by the city of San Mateo’s ADU ordinance and the California state streamlined ADU law, not by private covenants. That’s one reason Beresford carport and garage conversions move through permitting faster than Bay Meadows or other newer subdivisions. The cleaner the title chain and the older the lot, the fewer cooks in the kitchen. Call (971) 497-2402 for a property-specific read on what governs your Beresford address.
If you’re weighing a garage conversion ADU in San Mateo, start with the map and the panel, not the finishes. Call (971) 497-2402 to schedule a free estimate, or bring us your existing written estimate for a free second opinion. We’ll give you a written price before any work starts, with the flood-zone math, the electrical scope, and the warranty implications all in the same document.
Written by Nadia Ellery, Owner at Ellery ADU Studio, serving San Mateo since 2015.