ADU Rules and Resources by City in Silicon Valley (2026): Pre-Approved Plans and Where to Start

Thinking about adding an ADU (an accessory dwelling unit: a backyard cottage, a garage conversion or an in-law unit), or buying a home that has one? State law sets the floor for what cities must allow, and each city handles the permit, the plan review and its own add-ons. Here’s where to start in each of the 31 cities we cover, which ones offer pre-approved plans, and how often the houses that sold here already had an ADU.

The short answer

• State law comes first. The California Department of Housing and Community Development (HCD) publishes the ADU Handbook (updated March 2026), its guide to state law on accessory dwelling units (ADUs) and junior ADUs (JADUs), with an addendum that summarizes the changes to state ADU law in effect on January 1, 2026. Read the state guide.
• Pre-approved or pre-reviewed ADU plans are offered in at least 18 of the 31 cities, per their pages: Campbell, Cupertino, Gilroy, Los Altos Hills, Los Gatos, Milpitas, Monte Sereno, Morgan Hill, Mountain View, Palo Alto, San Jose, Santa Clara, Sunnyvale, Atherton, Foster City, Hillsborough, San Carlos and San Mateo. Burlingame, Daly City, Menlo Park, San Bruno, South San Francisco and Woodside have set up a program but haven’t posted plans yet.
• ADUs already on the market: 9% of the 21,632 houses that sold in the two years to October 27, 2026 had listings mentioning an ADU or in-law unit; highest in Atherton (33%), Woodside (26%), Monte Sereno (24%), Los Altos Hills (20%) and East Palo Alto (17%).
• Start with the city page, then a pre-application meeting or the city’s checklist; fees, utilities and fire rules vary by city and lot.

Santa Clara County

San Mateo County

“Sales with an ADU” counts house listings (MLS property type “single family home”) whose description mentions an ADU, in-law unit, guest house or similar, in the two years to October 27, 2026; descriptions undercount. “—” means fewer than 20 house sales.

Good to know, city by city

• Campbell: The city's Express Plan Check for ADUs was temporarily suspended (update dated August 4, 2025); the update points to the pre-approved plan gallery.
• Campbell: The Building page also links an ADU cost calculator.
• Cupertino: On a single-family lot, an ADU can be up to 850 sq ft for a studio or one-bedroom and up to 1,000 sq ft with two or more bedrooms.
• Gilroy: With a pre-approved plan, permits may be issued within 15 working days if the site-specific plan and required documents are accurate and complete.
• Gilroy: Pre-approved plans are limited to R1 and R2 zones (no hillside or Wildland Urban Interface areas) and cannot be revised, though they can be mirrored or oriented in any direction.
• Los Altos Hills: The Town says its pre-approved plans have already been approved by the California Department of Housing and Community Development and get a streamlined review, but still need a planning consultation on placement on the lot.
• Los Altos Hills: ADU height limit is 16 ft, or up to 19 ft with an on-site parking space and 30-ft side and rear setbacks.
• Los Gatos: The Town's ADU Plans Gallery offers several plans pre-approved by the California Department of Housing and Community Development; modifications are not allowed, however minor.
• Los Gatos: New ADUs and JADUs go through a single application form and a single plan review to obtain a building permit.
• Milpitas: Pre-approved plans over 800 sq ft need a minor site development permit from Planning before the building permit, and any modification disqualifies the plan from the program.
• Milpitas: The city offers free 'ADU Monday' consultations with staff and a Safe ADU Legalization Program for existing unpermitted units.
• Morgan Hill: Gallery plans marked pre-approved have been pre-reviewed for building code by a local, state or federal government; all plans, even pre-approved ones, still need site planning and permitting for your property.
• Morgan Hill: Detached and attached ADU checklists are posted, including Spanish versions.
• Palo Alto: Plans in the city's ADU Plans Gallery have been pre-reviewed by City staff for code compliance; selecting one 'will significantly expedite the review process,' though a site-specific permit is still needed.
• Palo Alto: The city's ADU/JADU Handbook (2025) summarizes the local rules in Municipal Code Chapter 18.09.
• Burlingame: Burlingame's AB 1332 pre-approval program aims to cut permit review to 30 days, but its page lists no approved plans yet ('None at this time').
• Burlingame: A building permit is required for a new ADU or JADU, and applications go directly to the Building Division.
• Daly City: The city is accepting designer applications to pre-review model detached ADUs, but its plan list still says 'Coming Soon'; homeowners can join a Preapproved ADU Interest List.
• Daly City: To use a pre-reviewed plan you must contract with the designer who created it, and modifications are not permitted.
• Foster City: Free pre-reviewed detached ADU designs from HEART (GLADUR plans) can be downloaded through the city's site, and a 1,123 sq ft ADORE Homes detached plan is also pre-approved.
• Foster City: Pre-approved plans are 'only about 80% complete' and always need property-specific changes made with the original designer.
• Hillsborough: ADUs of 800 sq ft or less, no taller than 16 ft, with side and rear setbacks of at least 4 ft (new detached, or within an existing structure) can go straight to the Building Division under the 'State Exempt' path.
• Hillsborough: The Town has an ADU Specialist for questions: adu@hillsborough.net.
• Menlo Park: The City Council adopted an updated ADU ordinance on June 9, 2026, effective July 10.
• Menlo Park: Menlo Park runs an ADU preapproval process for designers, but its homeowner page says 'There are no available ADU plans at this time.'
• San Bruno: San Bruno launched a Pre-Approved ADU Program under AB 1332, but no approved plans are posted on its page yet.
• San Bruno: Owners of unpermitted units can use the city's AB 2533 amnesty documentation to legalize them.
• San Carlos: No impact fees are collected on an ADU or JADU under 750 sq ft.
• San Carlos: The city is updating its ADU ordinance to match state law, with a Planning and Transportation Commission recommendation scheduled for December 7, 2026.
• San Mateo: The flat permit fee for a new ADU is $2,831.57 per unit ($1,454.02 for a JADU).
• San Mateo: Handouts and an inspection checklist are posted for legalizing unpermitted ADUs and JADUs under AB 2533.
• Woodside: ADUs can be up to 1,500 square feet.
• Woodside: The Town's pre-approved plan inventory is empty ('No Pre-Approved ADU plans have been added', updated July 8, 2025); it suggests the county ADU Resource Center's pre-designed plans and checking with Planning first.

Before you plan an ADU

• Check the lot first: setbacks, easements, trees, utilities and fire or flood zones shape what fits; see Flood & Fire Zones.
• Count every cost: design, permits, impact or utility fees and construction; a garage conversion and a new detached unit are very different budgets.
• Buying a home with an ADU? Ask for the permits and final inspection; an unpermitted unit can affect value, insurance and financing. Our Disclosure Decoder covers what to look for.

How we checked

Each city’s page and pre-approved plan program come from the city’s own website, read on October 29, 2026; where a page couldn’t be confirmed we say “not stated”. Rules change, and your lot’s own conditions decide what you can build, so the city is the final word. This is general information, not legal or design advice.

Want help with an ADU property?

We’ll help you find homes with ADUs or lots that can take one, and look at what similar homes sold for. Call (408) 373-1660 or (650) 383-8606, or book a call. Commissions are negotiable, and we offer bundled services, so ask us what’s included.