What the City of San Mateo allows in 2026: sizes, heights, parking, fees, rentals, JADUs, and how a factory-built ADU gets permitted.
Yes. A single-family lot in the City of San Mateo can add an attached or conversion ADU, a detached ADU, and a junior ADU. No parking is required, and a compliant ADU needs only a building permit.
San Mateo ADU rules: what fits on one lot?
San Mateo updated its ADU rules effective March 4, 2026. The city sets no fixed maximum size: the first 800 sq ft of an ADU don't count toward your lot's floor area limit.
Detached ADU: 18 ft to the top plate and 24 ft to the peak, enough for two stories, with 4 ft side and rear setbacks. Junior ADU: up to 500 sq ft inside the house.
No parking, no hearing. The city requires no parking for any ADU, anywhere in the city. A code-compliant project is approved ministerially, with only a building permit, and the permit can't be appealed.
How much does an ADU cost in San Mateo?
The city permit fee is $2,831.57 for a new ADU, or $1,455.43 for an eligible prefab ADU. Up to 750 sq ft, there are no impact fees. Our price is one all-in number.
Launch price through Dec 31, 2026. Everything from permits to keys. Not included: solar, furniture, upgrades. What's included
Plan for the sewer charge. A detached ADU pays a sewer capacity charge of $5.87 per sq ft, about $2,818 at 480 sq ft. Ask Public Works for the figure for your address; we confirm which city charges your lot triggers during the free lot check.
A Bay Modular project, July 2021.
How long does an ADU take in San Mateo?
The city has 15 business days to confirm your application is complete and 60 days to decide. Our projects take 4 to 6 months in total, from first call to keys, permits included.
Before you apply, fill out the ABAG ADU survey: the city requires it for an ADU building permit. Our homes are factory-built to HCD-approved plans (HCD #1585366) in our Lathrop factory. What factory-built means
San Mateo ADU FAQ
San Mateo ADU rules in full›
Units: on a single-family lot, one attached or conversion ADU, one detached ADU, and one junior ADU. Size: no fixed maximum. The first 800 sq ft of ADU living area don't count toward the lot's floor area ratio (FAR), and an ADU can't be larger than the main home, though an ADU of up to 800 sq ft is still allowed next to a smaller home. A conversion has no size limit inside existing walls and can grow up to 150 sq ft for an entrance or exit.
Height: detached 18 ft to the top plate and 24 ft to the peak; attached 24 ft and 32 ft, measured from existing grade. Setbacks: 4 ft from side and rear lot lines. A garage converted in its existing footprint needs none. Parking: none required for any ADU or JADU, and no replacement when you convert or remove a garage.
Source: SMMC 27.19.060 (Ord. 2026-02); City Single Family ADU/JADU Handout, updated October 7, 2026.
San Mateo fees to plan for›
City ADU permit fee (July 1, 2026 to June 30, 2027): $2,831.57 for a new ADU, $2,031.12 for a remodel, $1,454.02 for a JADU, and $1,455.43 for an eligible manufactured, prefab, or pre-approved ADU. Ask the Building Division if yours qualifies. Impact fees: $0 at 750 sq ft or less; larger ADUs pay in proportion to the main house.
Sewer capacity charge: $5.87 per sq ft for a detached ADU, about $2,818 at 480 sq ft (our math). Conversions don't pay it. School districts may charge on ADUs over 500 sq ft. See our 2026 California ADU cost guide.
Sources: City of San Mateo Comprehensive Fee Schedule FY 2026-27; SMMC 27.19.050; Gov. Code § 66311.5.
Renting, junior ADUs, and the permit steps›
Renting: 30 consecutive days or more; the ADU can't be sold separately from the house except as state law allows (Gov. Code § 66341). Owner-occupancy isn't required for an ADU (Gov. Code § 66315). JADU: up to 500 sq ft inside the house, including an attached garage, with an efficiency kitchen and its own exterior entrance. If it shares a bathroom with the house, the owner must live on the property.
Steps: 1. Check your lot (zoning, flood zone, heritage trees). 2. Prepare plans and fill out the ABAG ADU survey; the city requires its confirmation email with the permit application. 3. Apply in the city's Online Permit Center. 4. Build, inspect, and get the final sign-off.
Our homes are factory-built to HCD-approved plans (HCD #1585366) in our Lathrop factory, and we're a licensed California contractor (CSLB #1112074). What factory-built means
Can I build an ADU in San Mateo without a public hearing?›
Yes, if it meets the objective standards in SMMC Chapter 27.19. A compliant ADU needs only a building permit, with no public notice, and the permit can't be appealed.
Can I build a two-story ADU in San Mateo?›
Yes. A detached ADU can be 18 feet to the top plate line and 24 feet to the roof peak. Two-story ADUs face extra rules on second-story windows and decks.
Can I Airbnb my ADU in San Mateo?›
No. City code bans renting an ADU or JADU for less than 30 consecutive days. Monthly or longer rentals are allowed, and you don't have to live on the property.
Are San Mateo County ADU rules the same as the city's?›
No. County rules, including its coastal-zone chapter, apply only in unincorporated areas. Inside city limits, Chapter 27.19 of the city code governs.
We check your lot, tell you which model fits, and give you a fixed all-in price. You decide after that.
About Julia B. Julia B is a marketing specialist at Bay Modular, a licensed California ADU builder (CSLB #1112074) with its own factory in Lathrop. She writes about what ADUs really cost and how to avoid surprises.
Sources, city contacts, more guides
More: Models · San Jose · Sunnyvale · Palo Alto · San Francisco guides
- City of San Mateo, Accessory Dwelling Unit (ADU) page
- San Mateo Municipal Code Chapter 27.19, Accessory Dwelling Units
- Single Family ADU/JADU Handout, updated October 7, 2026
- ADU Development Standards Table, revised August 31, 2026
- Comprehensive Fee Schedule, July 1, 2026 to June 30, 2027
- ADU/JADU Affordability Survey (ABAG)
- San Mateo County, ADUs in unincorporated areas
- California HCD, ADU Handbook (March 2026)
Sources checked October 26, 2026. City of San Mateo, 330 W. 20th Ave. · Planning: (650) 522-7212, planning@cityofsanmateo.org · Building: (650) 522-7172, building.info@cityofsanmateo.org
General information, not legal, tax, or financial advice. Confirm details for your address with the City of San Mateo Planning and Building Divisions.
