Blog > Buying a Silicon Valley home with an ADU, or planning one
property nerd guide · buyers
Thinking about a Silicon Valley home with a backyard cottage, or one where you could add one? Here are California's ADU rules in plain English, checked against the state's latest ADU Handbook, plus our checklist for evaluating an ADU that's already built.
By the Property Nerds® of the Boyenga Team at Compass · NextGenAgents™
the short version
State law sets the floor for ADUs in every Silicon Valley city
An accessory dwelling unit, or ADU, is a second home on a residential lot with its own living, sleeping, cooking and bathroom facilities. A junior ADU, or JADU, is a smaller unit carved out of the main house.
California's ADU law has changed many times in recent years, and in 2024 its code sections were renumbered (they now run from Government Code §66310 to §66342). The state housing department, HCD, updated its ADU Handbook in March 2026, and everything below comes from that handbook or the statute. State law sets minimum requirements that cities must honor; cities can choose to be more generous, so always check the local ordinance too.
detached ADU allowed
permit decision
no impact fees up to
JADU maximum
the rules
California ADU rules in plain English
What fits on a single-family lot
Cities must allow at least one ADU converted from existing space, one JADU and one newly built detached ADU, together on the same lot (Government Code §66323).
Size and setbacks
A detached ADU of up to 800 sq ft with four-foot side and rear setbacks must be allowed. Local size caps can't be below 850 sq ft, or 1,000 sq ft for an ADU with more than one bedroom.
Height
A detached ADU can be at least 16 feet tall, or 18 feet within a half mile of a major transit stop or high-quality transit corridor, plus up to two feet to match the main roof's pitch.
Owner-occupancy
Cities can't require the owner to live on site for an ADU. A JADU that shares sanitation facilities with the house does require the owner to live in the house or the JADU.
Rentals
ADUs built under §66323 and all JADUs must be rented for terms longer than 30 days, and cities may apply the same rule to other ADUs.
Separate sale
An ADU generally can't be sold apart from the main house unless the city has adopted an ordinance allowing ADU condominiums. A JADU can't be sold separately.
| Feature | ADU | JADU |
|---|---|---|
| Where | Attached, detached, or converted from a garage or other space | Entirely within the walls of the house, including an attached garage |
| Size | 800 sq ft detached allowed; larger by local rule | Up to 500 sq ft of interior livable space |
| Kitchen and bath | Complete, independent living facilities | An efficiency kitchen; may share a bathroom with the house |
| Owner lives on site | — not required | ✓ required if sanitation is shared |
| Parking | No more than one space, and none in many cases | None required |
| Impact fees | None up to 750 sq ft; proportional above | None up to 500 sq ft |
permits, fees, parking
Timelines, fees and parking
A permitting agency has 15 business days to tell you whether an ADU application is complete, and 60 days from a complete application to approve or deny it. If it misses the 60 days, the application is deemed approved. Every city also had to set up a program for pre-approved ADU plans by January 1, 2025, which can shorten design time.
Impact fees can't be charged on an ADU of 750 sq ft or less of interior livable space; above that, they are proportional to the size of the main house. School districts may charge fees on units over 500 sq ft. An ADU converted from existing space isn't treated as a new residential use for water and sewer connection fees unless it's built along with a new house.
Parking is capped at one space per ADU or per bedroom, whichever is less, and a garage converted to an ADU doesn't have to be replaced. No parking can be required within a half-mile walk of public transit, in an architecturally and historically significant district or for an ADU inside the existing house.
buyer checklist
Evaluating a home with an existing ADU
- Permits. Ask for the building permit and the final inspection sign-off. An ADU needs a certificate of occupancy before anyone lives in it.
- Square footage. Confirm the interior livable space, since state and local size rules and fees are based on it.
- Utilities. Find out whether water, gas and electricity are shared or separately metered, and how the sewer connects.
- Parking. Check where the ADU's occupant parks and whether the garage was converted.
- Egress. Look for exterior access and bedroom windows large enough to exit in an emergency, and have your inspector check smoke and carbon monoxide alarms.
- Current occupants. If the ADU is rented, read the lease and ask about local tenant rules before you plan to move in or remodel.
- HOA rules. State law voids CC&Rs that effectively prohibit or unreasonably restrict ADUs on single-family lots, but reasonable design rules can apply.
- Financing. Some FHA, Fannie Mae and Freddie Mac programs can count ADU rental income. Ask your lender early.
property nerd note
A unit built without permits isn't always a dead end. State law bars a city from denying a permit for an unpermitted ADU built before January 1, 2020 just because it breaks building standards or ADU rules, unless fixing the problem is needed for health and safety or the building is substandard. Price that path into your offer, and don't count unpermitted space as if it were finished.
nextgenagents™
How the Boyenga Team helps
Permit detective work
We pull the permit history and match it to what's on the ground before you write an offer.
Project know-how
Our team manages restoration and pre-listing projects, so we can talk through scope, contractors and timing with you.
Honest comps
As NextGenAgents™ we compare homes with permitted ADUs to homes with permitted ADUs, so the price reflects what's really there.
talk to a property nerd
Looking for a home with room for a second unit?
Tell us how you'd use it: family, guests, an office or rental income. We'll help you find lots and homes that fit, and check the permits before you commit.
faq
ADUs for Silicon Valley buyers: FAQ
Do I have to live on the property if it has an ADU?
No. Cities can't require owner-occupancy for an ADU. A JADU that shares sanitation facilities with the main house does require the owner to live in the house or the JADU.
Can I rent out an ADU on a short-term basis?
ADUs built under Government Code §66323 and all JADUs must be rented for more than 30 days, and cities may require the same of other ADUs. Check your city's rules.
How long does an ADU permit take?
The agency has 15 business days to confirm the application is complete and 60 days to approve or deny a complete application. Design and construction take additional time.
Can I use SB 9 and ADU law together?
Yes, with limits. On a lot that isn't split, SB 9 units and ADUs can be combined. After an SB 9 lot split, each new lot is limited to two units. See our SB 9 page.
Sources: California Department of Housing and Community Development, ADU Handbook, March 2026 (hcd.ca.gov/building-standards/adu/handbook) · California Government Code §§66310–66342 and §65852.27 · Civil Code §§714.3 and 4751. General information, not legal advice. Cities can adopt more generous rules; check the local ADU ordinance for any address.