If you own a rental in San Mateo, the city's just-cause clock starts a month earlier than the state's, and it comes with a relocation payment on no-fault notices. Miss a step and the notice is void. On the development side, San Mateo caps a split lot at two units including ADUs, so run the numbers on two units per lot, not three.
Adding an ADU
In San Mateo
In October 2024, HCD reviewed San Mateo's ADU ordinance (Ordinance No. 2022-02) and found it didn't comply with state law, including a rule barring ADUs from the front setback. State law doesn't allow front setbacks to block an 800-square-foot ADU with four-foot side and rear setbacks.[1]
State law, everywhere in California
A city has 60 days to approve or deny a complete ADU or junior ADU (JADU) permit application.[2]
Lot coverage, floor area ratio, and front setback rules can't be used to block an ADU of at least 800 square feet with four-foot side and rear setbacks.[2]
A city can't set a height limit below 16 feet for a detached ADU, or below 18 feet within a half mile of a major transit stop or high-quality transit corridor. Attached ADUs can go to 25 feet or the main home's zoning height, whichever is lower.[2]
A JADU is up to 500 square feet of interior space inside the main home. Owner occupancy can't be required for an ADU, but it is required for a JADU that shares a bathroom with the main home.[2]
No impact fees on an ADU of 750 square feet or less. Above that, fees have to be proportional to the main home.[2]
On a lot with an existing apartment building, state law allows up to eight detached ADUs (never more than the number of existing units), plus conversions of non-living space.[2]
Lot splits and second homes
In San Mateo
On a lot created by an urban lot split, the city allows a maximum of two dwelling units per parcel, counting primary units, SB 9 units, ADUs, and JADUs together.[3]
The smaller parcel can't be less than 40% of the original lot, and each parcel has to be at least 1,200 square feet.[3]
A flag lot or landlocked parcel needs at least a 10-foot access strip, or 20 feet if the strip is longer than 150 feet.[3]
Demolishing 50% or more of a structure's exterior walls or roof requires a ministerial Special Use Permit.[3]
State law (SB 9)
SB 9 applies to lots zoned single-family. It allows two primary homes on one lot, a split into two lots, or both, with staff-level approval and no public hearing.[4]
Local rules can't stop two homes of at least 800 square feet each, or a split into two lots of at least 1,200 square feet. Side and rear setbacks are capped at four feet.[4]
Parking is capped at one space per unit, and none can be required within a half mile walk of a high-quality transit corridor or major transit stop, or within a block of a car-share vehicle.[4]
To split a lot, you sign an affidavit that you intend to live in one of the units as your main home for at least three years. Any rental has to be for more than 30 days.[4]
SB 9 doesn't apply if the project would demolish or alter rent-restricted housing, rent-controlled housing, or housing a tenant has lived in during the last three years. It also excludes historic properties, lots with an Ellis Act withdrawal in the last 15 years, lots already created by an SB 9 split, and lots next to one the same owner split.[4]
The ceiling is four units total. After a split, the city has to allow two units on each new lot. The city must decide on a complete application within 60 days.[4]
Rent control and AB 1482
In San Mateo
San Mateo applies no-fault just-cause protections after 11 months of tenancy, a month earlier than state law.[5]
On a no-fault termination, the owner owes relocation assistance equal to one month's rent regardless of the tenant's income, with half due within 15 calendar days of the notice. Failing to strictly comply makes the notice void.[5]
There is no separate city rent cap in this section of the code. For units AB 1482 covers, the state cap applies.[5],[6]
State law (AB 1482)
Under the Tenant Protection Act (AB 1482), rent can't go up more than 5% plus the change in the cost of living, or 10%, whichever is lower, in a 12-month period.[6]
After a tenant has lived in a unit for 12 months, the landlord needs a listed reason to end the tenancy, either at-fault (like nonpayment) or no-fault (like owner move-in or a substantial remodel).[6]
Exempt: units with a certificate of occupancy issued in the last 15 years (a rolling date), single-family homes not owned by a corporation or REIT, a duplex where the owner lives in the other unit, and units already under a stricter local rent control law.[6]
What is your San Mateo home worth, and what could it become?
I spent my career on the investment side, helping operate more than 7,000 units. I'll give you a valuation built from recent sales, and a straight read on what your lot allows: an ADU, a split, or neither.
San Mateo questions, answered
How many units can I build after an SB 9 lot split in San Mateo?+
Two per resulting parcel, counting primary units, ADUs, and JADUs together, under San Mateo Municipal Code 27.21.140.[3]
Does a San Mateo landlord have to pay relocation on a no-fault eviction?+
Yes. The city requires one month's rent, regardless of tenant income, with half paid within 15 calendar days of the notice. Just-cause protection starts after 11 months of tenancy.[5]
Is there rent control in San Mateo?+
We did not find a city rent cap. The state Tenant Protection Act (AB 1482) caps increases on covered units at 5% plus the change in cost of living, or 10%, whichever is lower.[6]
- [1]HCD (posted by the City of San Mateo), “Review of San Mateo's ADU Ordinance, October 18, 2024”
- [2]California Department of Housing and Community Development (HCD), “ADU Handbook (March 2026 update)”
- [3]City of San Mateo, “Municipal Code 27.21.140, Urban Lot Split Development Standards”
- [4]California Department of Housing and Community Development (HCD), “Housing Law Fact Sheet: Duplexes and Lot Splits (SB 9), April 2026”
- [5]City of San Mateo, “Municipal Code 10.100.020, No-Fault Just Cause Evictions; Relocation Assistance”
- [6]California Attorney General, “Landlord-Tenant Issues: Tenant Protection Act”
Official San Mateo pages
This page is general information, not legal or tax advice. Rules change, and cities amend their codes after state reviews. Confirm current requirements with the city and a qualified professional before you buy, build, or change a tenancy. Erica Walters, REALTOR®, CalDRE# 02239622, Compass.