In Foster City, the details that sink SB 9 deals aren't the big ones, they're frontage and utilities. A split needs 20 feet of frontage and separate utility meters for each parcel, and a detached SB 9 unit tops out at 16 feet. Check those three things before you pay an architect.
Adding an ADU
In Foster City
Foster City counts ADUs toward its SB 9 limit: four units total on a lot, and two on a newly created parcel.[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 Foster City
SB 9 is implemented through Chapter 17.79 of the zoning code and applies to R-1 and R-1/PD parcels.[1]
A detached SB 9 unit can be up to 16 feet tall and an attached one up to 25 feet. Side and rear setbacks are four feet; the front setback follows the zoning district.[1]
A lot split needs a minimum frontage of 20 feet, parcels of at least 1,200 square feet, and separate utility meters for each parcel connected to the public sewer.[1]
Applications require a parcel map for splits and a recorded covenant prohibiting further subdivision and short-term rentals. The city decides within 60 calendar days of a complete application.[1]
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.[3]
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.[3]
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.[3]
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.[3]
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.[3]
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.[3]
Rent control and AB 1482
In Foster City
We did not find a city rent cap or rent stabilization ordinance in our review of official city sources. For units AB 1482 covers, the state cap applies.[4]
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.[4]
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).[4]
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.[4]
What is your Foster City 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.
Foster City questions, answered
What frontage do I need to split a lot in Foster City?+
At least 20 feet, along with parcels of at least 1,200 square feet and separate utility meters for each parcel.[1]
How tall can an SB 9 unit be in Foster City?+
Up to 16 feet for a detached unit and 25 feet for an attached unit.[1]
Does Foster City have rent control?+
We did not find a local rent cap in official city sources. For units the state Tenant Protection Act covers, increases are capped at 5% plus the change in cost of living, or 10%, whichever is lower.[4]
- [1]City of Foster City, “California Senate Bill (SB) 9”
- [2]California Department of Housing and Community Development (HCD), “ADU Handbook (March 2026 update)”
- [3]California Department of Housing and Community Development (HCD), “Housing Law Fact Sheet: Duplexes and Lot Splits (SB 9), April 2026”
- [4]California Attorney General, “Landlord-Tenant Issues: Tenant Protection Act”
Official Foster City 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.