Where it can go
Three ways an ADU fits on a San Mateo lot.
Tap one to see San Mateo's rule for it.
Field Notes · Guide · Published Oct 17, 2026
ADUs in San Mateo
A small home on your lot, for a parent, a grown child or rental income. What the city allows, what to do first, and what families ask us.
An attached ADU we built in Cupertino
Who is it for?
Pick one. Everything below changes with it, except San Mateo’s rules.
If this is you
Mom or Dad is ready to live closer.
Your grown kid wants a place of their own.
You'd like the lot to earn a little.
- You own a house in San Mateo.
- They want their own door and kitchen, and you want them a few steps away.
- Stairs are getting harder, or soon will be.
- You'd like help nearby without moving them into the spare room.
- They want privacy and a front door of their own.
- You'd rather they stay close than rent across town.
- They should have their own address and their own mail.
- You'd rent month to month or longer, not by the night.
- You plan to keep the house for years.
- You have room out back, or a garage you don't park in.
Where it could go
Out backGood fit
A new backyard ADU, 4 ft from the side and rear lot lines. In San Mateo the walls can rise to 18 ft and the peak to 24 ft, and the first 800 sq ft doesn't count against your floor area limit.
A front door of their own and a little garden. Keep it to one floor.
The most privacy: its own walls, door and address.
The most private for a tenant, with its own way in.
Inside the houseGood fit
A junior ADU, up to 500 sq ft inside your walls, with its own outside door and a small kitchen. In San Mateo, if it shares your bathroom, an owner has to live on the property.
The closest of all, when your parent wants to be right there.
The closest, with the least privacy. Fine for a few years.
Works as a rental with its own bathroom, or if you live in the house.
The garageGood fit
In San Mateo, a garage converted in the same footprint and height needs no setbacks, and you don't have to replace the parking.
One level, no stairs, a few steps from your door.
Their own space without building new, if the garage holds boxes, not cars.
A good rental if the garage holds boxes, not cars.
Our advice
Plan it for your parent.
Plan it for your grown kid.
Plan it for a tenant.
- One floorWith the bedroom close to the bathroom.
- No step at the doorAnd a level, lit path from your door to theirs.
- Wide doorwaysRoom for a walker or a wheelchair later.
- A shower with no curbWith wood backing in the walls for grab bars.
- Lever handlesOn doors and faucets, easy on the hands.
- A window toward your houseSo you can see each other's lights on.
- A door that faces awayFrom your windows, so they come and go in private.
- Quiet shared wallsExtra sound insulation wherever the two homes touch.
- Their own laundryHookups inside the unit.
- A place to workA desk nook with good light for working from home.
- Their own controlsHeating, cooling and water heater of their own.
- Room for gearA spot for a bike and some storage.
- Its own way inA path, a front door and a mailbox for its own address.
- Its own systemsWater heater, heating and cooling, like our Cupertino ADU.
- Laundry insideA washer and dryer in the unit.
- Tough finishesFloors and counters that clean up between tenants.
- A little outdoor spaceA patio or a corner of yard that's theirs.
- A parking spot, if you canNot required, but up to two are allowed.
What to do first
Start here, before anyone draws a plan.
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This week
Talk it through with your parent.
Do they need one level, a full kitchen, their own bathroom? Those answers decide between a junior ADU inside the house and a full ADU.
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This week
Agree on the ground rules.
Privacy, guests, who pays which bills, how long they'll stay. A few answers now make the design simpler.
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This week
Check the rental rules.
In San Mateo an ADU or junior ADU can't be rented for less than 30 days, and it can't be sold apart from the house. You don't have to live on site, unless a junior ADU shares your bathroom.
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This weekend
Walk the lot with your parent in mind.
Look for a level route from your door to theirs, with no steps. Measure big tree trunks 4½ ft above the ground. In San Mateo an oak 10 inches across, or any other tree 15 inches across, is a heritage tree, and removing it or cutting big roots near it takes a city permit.
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This weekend
Walk the lot for privacy.
Picture where their door and path could go, so they come and go without passing your windows. Measure big tree trunks 4½ ft above the ground. In San Mateo an oak 10 inches across, or any other tree 15 inches across, is a heritage tree, and removing it or cutting big roots near it takes a city permit.
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This weekend
Walk the lot as a tenant would.
Find a path from the street to their door, and a spot where a car could park. Measure big tree trunks 4½ ft above the ground. In San Mateo an oak 10 inches across, or any other tree 15 inches across, is a heritage tree, and removing it or cutting big roots near it takes a city permit.
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Next week
Get free advice on a one-level plan.
Bring your parent's needs to a free one-hour online consult with the San Mateo County ADU Resource Center, or stop by the city's code counter on a weekday between 8:30 and 11:30 am.
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Next week
Get free advice on privacy.
Ask the San Mateo County ADU Resource Center's free one-hour online consult where a private door and path could go, or stop by the city's code counter on a weekday between 8:30 and 11:30 am.
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Next week
Get free advice on renting it.
Ask the San Mateo County ADU Resource Center's free one-hour online consult about renting an ADU, or stop by the city's code counter on a weekday between 8:30 and 11:30 am.
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Before drawings
Plan for the years ahead.
Ask for one level, an entry with no step, wide doorways, a shower with no curb and wood backing in the walls for grab bars. It's easy to add now and hard to add later.
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Before drawings
Plan for privacy and sound.
In San Mateo an ADU attached to the house needs fire-rated walls and sound control between the two; a junior ADU doesn't. A backyard ADU keeps the most distance.
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Before drawings
Plan the tenant's way in.
Every ADU needs its own outside door, and San Mateo gives it its own address. Parking isn't required, but you can add up to two spaces.
What's different in San Mateo
The city's rules, in plain words.
Checked against San Mateo's ADU code (in effect since March 4, 2026) and state law on October 15, 2026. The same for every family.
One of each
A lot with a single-family house can have a junior ADU, an attached or converted ADU (a garage, for example), and a new detached ADU, each meeting the city's standards.
San Mateo code 27.19.0604 ft
Side and rear setback for a new ADU. A garage converted in place needs none.
San Mateo code 27.19.06018 / 24 ft
Detached ADU height: top of the walls, then roof peak, measured from the ground as it is today.
San Mateo code 27.19.06012 ft + 45°
Over 800 sq ft, a new detached ADU must also fit under the daylight plane: 12 ft up at the side and rear lines, then sloping in.
San Mateo code 27.19.060(g)800 sq ft
ADU space that doesn't count against your floor area limit. A new ADU can't be bigger than your house.
San Mateo code 27.19.060No parking
None required for an ADU or junior ADU, and none to replace when the garage becomes the ADU.
San Mateo code 27.19.06060 days
Once your application is complete, the city must approve or deny it within 60 days. It has 15 business days to tell you what's missing.
State law, every city10 in.
Oak trunk size that makes it a heritage tree. For other trees it's 15 in. Both measured 4½ ft up.
San Mateo code 13.40.03030 days
Shortest rental allowed if you ever rent it out. It can't be sold apart from the house.
San Mateo code 27.19.030Sources, checked October 15, 2026
- City of San Mateo Municipal Code, Chapter 27.19, ADUs and JADUs (as amended by Ord. 2026-02, in effect March 4, 2026)
- City of San Mateo, Single-Family ADU and JADU Standards (updated October 7, 2026)
- City of San Mateo, Pre-Approved ADU Plans Guide (revised January 13, 2025)
- City of San Mateo Municipal Code 13.40.030, protected and heritage trees
- California HCD, ADU Handbook (March 2026)
- California State Board of Equalization, new construction and property tax
- San Mateo County ADU Resource Center (free consults, typical timeline)
Questions families ask
What families ask us next.
Will our property taxes go up?
Only for the new part. The San Mateo County assessor adds the value the ADU brings. Your house and land keep their current assessed value.
State Board of EqualizationCan we rent it by the night?
No. San Mateo doesn't allow an ADU or a junior ADU to be rented for less than 30 days in a row.
San Mateo code 27.19.030Will it get its own address?
Yes. San Mateo gives every ADU and junior ADU its own address, as part of the building permit.
San Mateo code 27.19.050(e)Does it need its own entrance?
Yes. In San Mateo every ADU and junior ADU needs its own outside door. A door into the house is allowed but not required, unless a junior ADU shares your bathroom.
San Mateo code 27.19.060(e)Do we have to add parking?
No. San Mateo requires no parking for an ADU or a junior ADU, and you don't have to replace a garage you convert. You can add up to two spaces if you want them.
San Mateo code 27.19.060(f)How long does it take?
State law gives San Mateo 15 business days to tell you if your application is complete, then 60 days to approve or deny it. The county's ADU center says most projects take one to two years from the first idea to move-in, with six to twelve months of building.
County ADU Resource CenterDo we have to live on the property?
Not for an ADU. In San Mateo, for a junior ADU that shares your bathroom, yes: an owner has to live in the house or in the junior ADU.
San Mateo code 27.19.080(n)Could we rent it out later?
Yes, for 30 days at a time or longer. San Mateo doesn't let you sell it apart from the house.
San Mateo code 27.19.030Can we sell the ADU on its own later?
No. In San Mateo an ADU or junior ADU can't be sold apart from the house.
San Mateo code 27.19.030Can there be a door between our house and the ADU?
Yes, if you want one. In San Mateo an ADU must have its own outside door, and a door into the house is allowed but not required.
San Mateo code 27.19.060(e)Can it be bigger than 800 sq ft?
Yes, if your lot's floor area limit has room. In San Mateo a new ADU over 800 sq ft counts toward that limit, has to fit under the daylight plane and can't be bigger than your house. These size rules don't apply when the ADU is made inside walls that already stand, like a garage.
San Mateo code 27.19.060(b), (g)Does it need its own meters?
Not always. In San Mateo an ADU made from existing space doesn't need its own water, sewer or power connection. For a newly built ADU, the city may require new or separate ones.
San Mateo code 27.19.050(c)Can the driveway stay as parking?
Yes. In San Mateo, if the garage becomes the ADU, the driveway and curb cut can stay as parking. You can have up to two spaces for the ADU, though none are required.
San Mateo code 27.19.060(f)Can we rent out a junior ADU?
Yes, for 30 days or longer, if it has its own bathroom or you live in the house. In San Mateo a junior ADU that shares your bathroom needs an owner living on the property.
San Mateo code 27.19.080(n)Do we need fire sprinklers?
Usually not in San Mateo, if your house doesn't have them. The Fire Marshal decides, and things like a long driveway or weak hydrant coverage can change the answer.
City ADU standardsCan we use a ready-made plan?
Yes. San Mateo keeps a gallery of pre-approved ADU plans you can pick from, but the design can't be changed.
City pre-approved plans guideDoes a backyard ADU need solar panels?
A new detached ADU does, under the state energy code. San Mateo lets the panels go on your house instead of the ADU.
City ADU standardsJobs like yours
Two ADUs we’ve built.
Closest to a parent's unit: our Cupertino ADU, attached to the main house with its own entrance and systems.
Closest to a place of their own: a garage turned into a full home, with direct access to its own backyard.
Both are complete small homes, each with a kitchen and a bath of its own.
San Francisco · Garage conversion · 2024A garage turned into a full homeA kitchen, a bedroom, a full bath and direct access to a rebuilt backyard.See how it was built
Cupertino · Attached ADU · 2020Attached, with a door of its ownIts own entrance, kitchenette, bath, water heater and heating and cooling.See the project
Talk to us
Planning an ADU in San Mateo?
Tell us who it's for and where it might go. We'll look at your lot and reply within one business day.
- Call(650) 667-9493
- Emailinfo@amiari.com
- LicenseCA General Contractor #1138009