In Sunnyvale, 800 square feet is the number that decides your timeline. At or under it, a new ADU on a single-family lot needs only a building permit. Go bigger and you add a planning permit first. If you're buying a home with an unpermitted unit, the city has a legalization path for units built before 2020. Price that in, don't ignore it.
Adding an ADU
In Sunnyvale
A single-family lot can have up to three ADUs: one new construction, one conversion, and one JADU, but not both a conversion ADU inside the main home and an attached new-construction ADU.[1]
New-construction ADUs up to 800 square feet on single-family lots qualify for streamlined review with a building permit only. Larger ones need a Miscellaneous Plan Permit first.[1]
On existing multifamily lots, up to eight detached ADUs (not more than the existing number of units) and conversions of at least one and up to 25% of the existing unit count.[1]
An ADU or JADU built inside a permitted structure before January 1, 2020 can apply to be legalized, and owners of unpermitted pre-2020 ADUs can request five-year deferred enforcement.[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 Sunnyvale
We did not find a Sunnyvale SB 9 summary in official city sources. The state SB 9 rules above set the floor.[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.[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 Sunnyvale
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]
Sunnyvale has discussed local tenant protections such as relocation assistance, but we could not confirm current terms from an official city source, so we've left them out. Ask the city's Housing Division before you rely on any summary.[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 Sunnyvale 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.
Sunnyvale questions, answered
How many ADUs can I build on a Sunnyvale single-family lot?+
Up to three: one new construction, one conversion, and one JADU, with a limit on combining a conversion inside the main home and an attached new ADU.[1]
When does a Sunnyvale ADU need a planning permit?+
When a new ADU on a single-family lot is larger than 800 square feet. Up to 800 square feet needs only a building permit.[1]
Can I legalize an unpermitted ADU in Sunnyvale?+
If it was built inside a permitted structure before January 1, 2020, you can apply to legalize it.[1]
- [1]City of Sunnyvale, “Accessory Dwelling Units”
- [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 Sunnyvale 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.