The City of San Mateo (not San Mateo County) approves code-compliant ADUs and JADUs with a ministerial building permit under San Mateo Municipal Code Chapter 27.19, last amended by Ordinance 2026-02 effective March 4, 2026. A detached ADU may rise 18 ft to the top plate and 24 ft to the roof peak with 4-ft side and rear setbacks, the first 800 sq ft of ADU floor area does not count toward the lot's FAR, and a JADU is capped at 500 sq ft. Since March 2026 the city requires no off-street parking for any ADU or JADU, and a single-family lot may combine a JADU, an attached or conversion ADU and a new detached ADU. Building permit fees are flat for FY2026-27: $2,831.57 for a new ADU, $2,031.12 for a conversion, $1,455.43 for a prefab or pre-approved-plan ADU and $1,454.02 for a JADU, and the state's 60-day review clock applies.
Zoning & Size Requirements [1][2][3][4][5]
Yes — San Mateo allows ADUs. A detached ADU can typically be up to 800 sq ft under current California and San Mateo zoning rules.
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| Allowed Zones | Every zoning district that allows residential use, on any lot with an existing or proposed single-family or multifamily dwelling (SMMC 27.19.050); JADUs only on lots with a single-family dwelling, not condos or multifamily (27.19.080(b)). Confirm your zone with the city's Property Zone Look-Up tool |
| Max Size (Detached) | No fixed cap: up to the FAR of the underlying zone and no larger than the primary home. The first 800 sq ft of ADU living area is exempt from FAR, and an 800 sq ft state-exempt ADU is always allowed even when the lot's floor area is used up or the main house is under 800 sq ft (SMMC 27.19.060(b)) |
| Max Size (Attached) | Same as detached: governed by the zone's FAR, no larger than the primary home, first 800 sq ft FAR-exempt. A conversion ADU inside an existing house or accessory structure has no size limit and may add up to 150 sq ft for entry and egress (27.19.060(b)(2)) |
| Max Size (JADU) | 500 sq ft of living area, inside the walls of an existing or proposed single-family home or its attached garage; a detached garage or shed cannot become a JADU (27.19.080(e)-(f)) |
| Max Height | Detached ADU: 18 ft to top plate and 24 ft to roof peak from existing grade. Attached ADU: 24 ft to plate, 32 ft to peak (27.19.060(c)). Non-exempt ADUs must also fit a daylight plane that starts 12 ft above the side and rear lot lines and rises at 45 degrees; state-exempt ADUs (800 sq ft or less with 4-ft setbacks) are not subject to it |
| Max Units | Single-family lot: 1 JADU + 1 attached or conversion ADU + 1 new detached ADU may be combined (27.19.060(a)). Existing multifamily: up to 8 new detached ADUs (not more than the number of existing units) plus conversions of non-living space equal to at least 1 and up to 25% of existing units; proposed multifamily: 2 detached ADUs (27.19.070(b)) |
Setback Requirements [1][2][3][4][5]
San Mateo requires a rear setback of about 4 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.
| Rear Setback | 4 ft for new construction; none for a conversion rebuilt to the same footprint and height, or for a detached ADU in the rear third of the lot that meets the R1 accessory-structure rules in SMMC 27.70 (27.19.060(d)) |
| Side Setback | 4 ft for new construction, including an ADU built above an existing garage; none for same-footprint conversions or rear-third units that comply with Chapter 27.70 (27.19.060(d)) |
| Front Setback | The zone's front setback applies to a new ADU over 800 sq ft; no front setback is required for a state-exempt ADU of 800 sq ft or less (27.19.060(d)(3)). JADUs follow the underlying zone's setbacks |
| Parking Required | None required for any ADU or JADU since March 4, 2026 (SMMC 27.19.060(f)); no replacement parking for the main house when a garage, carport or driveway space is converted or demolished for an ADU. Voluntary parking is allowed: up to 2 stalls per unit, tandem OK, may sit in setbacks, and the driveway may widen to 20 ft. The former Parking Exempt ADU/JADU Map is no longer needed |
| Owner Occupancy | Not required for ADUs. Required for a JADU only when it shares a bathroom with the main home; a JADU with its own bathroom has no owner-occupancy requirement (27.19.080(n)). Every JADU needs a deed restriction recorded before permit issuance (27.19.090) |
| Min Lot Size | No minimum lot size |
Permit Process & Fees [6][7][8][9][10][11]
An ADU permit in San Mateo goes through a ministerial, by-right review. Typical timeline: 60 days from a complete application for the city to approve or deny, or the ADU is deemed approved (state law, incl. SB 543 from Jan 2026). The city publishes no shorter target for pre-approved plans; expect at least one correction cycle. Permit fees: Flat ADU building permit fees, FY2026-27 Comprehensive Fee Schedule p. 2.5: $2,831.57 new-construction ADU; $2,031.12 ADU remodel/conversion; $1,455.43 eligible manufactured, prefab or pre-approved-plan ADU; $1,454.02 JADU. A detached ADU also pays a wastewater capacity charge of $5.87 per sq ft (Public Works schedule p. 9.5), about $4,700 for 800 sq ft; conversions are exempt (27.19.050(d)). School district developer fees may apply.
| Approval Process | Ministerial building permit only; no planning application or public notice for a code-compliant unit (27.19.040). In order: 1) confirm zoning, FEMA flood zone and heritage trees; 2) optional free Code Consultation Counter visit, weekdays 8:30-11:30am; 3) register on the Customer Self-Service (CSS) portal and upload a PDF plan set per the Residential Plan Submittal Guide (site plan, floor/roof/elevation plans, Title 24 energy forms, CALGreen checklist); 4) building, fire (San Mateo Consolidated), public works and utility review; 5) separate address assignment and fee payment; 6) permit issued, then inspections. A unit that misses an objective standard (height, setback, daylight plane, deck size) can instead seek discretionary Zoning Administrator approval under 27.19.045 with public notice |
| Review Timeline | 60 days from a complete application for the city to approve or deny, or the ADU is deemed approved (state law, incl. SB 543 from Jan 2026). The city publishes no shorter target for pre-approved plans; expect at least one correction cycle |
| Permit Fees | Flat ADU building permit fees, FY2026-27 Comprehensive Fee Schedule p. 2.5: $2,831.57 new-construction ADU; $2,031.12 ADU remodel/conversion; $1,455.43 eligible manufactured, prefab or pre-approved-plan ADU; $1,454.02 JADU. A detached ADU also pays a wastewater capacity charge of $5.87 per sq ft (Public Works schedule p. 9.5), about $4,700 for 800 sq ft; conversions are exempt (27.19.050(d)). School district developer fees may apply |
| Impact Fees | None for ADUs of 750 sq ft or less; a larger ADU pays impact fees in proportion to the primary home's floor area (SMMC 27.19.050(b)). Utility connection and capacity charges for a new detached unit are proportional to its burden, and the ADU is not treated as a new residential use unless built with a new primary home |
| Plan Check | Electronic plan check through the CSS portal (PDF sheets, 11x17 minimum). Fire sprinklers are required only if the main house is sprinklered or site conditions (hydrant flow, driveway length) demand them; solar panels are required on a new detached, non-manufactured ADU and may be placed on the main house instead; a smoke alarm interconnected with the main house and a CO detector are required in a JADU |
| Pre-Approved Plans | Yes. The city's ADU Plans Gallery (cityofsanmateo.aduaccelerator.org) listed 17 pre-approved designs in Sept 2026, from 340 to 1,110 sq ft: 2 studios, 4 one-bedroom, 10 two-bedroom and 1 three-bedroom; 16 prefab and 1 site-built. Using one unaltered costs the reduced $1,455.43 permit fee, but the lot must be residential, outside a high-hazard FEMA flood zone and WUI area, on a slope under about 15%, and free of easements and open code cases (Pre-Approved ADU Plans Guide) |
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Construction Costs [12][13][14]
Building an ADU in San Mateo typically costs $300,000-$480,000 ($450-$600 per sq ft); a garage conversion runs $180,000-$280,000.
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| Average Build Cost | $300,000-$480,000 |
| Cost per Sq Ft | $450-$600 |
| Garage Conversion | $180,000-$280,000 |
| Prefab ADU (Installed) | $160,000-$350,000 |
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Rental Income Potential [15][16]
A 2-bedroom ADU in San Mateo rents for around $3,900-$4,400/mo (Zumper Sept 2026 $3,900; RentCafe Aug 2026 $4,448), an estimated 7-9% gross rental yield on a $350,000-$480,000 detached build at $3,100-$3,400/mo (estimate before taxes, vacancy and financing).
| Avg Rent (1 Bedroom) | $3,100-$3,400/mo (Zumper Sept 2026 $3,185; RentCafe Aug 2026 $3,406 apartment averages; ADUs typically rent slightly below) |
| Avg Rent (2 Bedroom) | $3,900-$4,400/mo (Zumper Sept 2026 $3,900; RentCafe Aug 2026 $4,448) |
| Short-Term Rental | No. An ADU or JADU may not be rented for less than 30 consecutive days, and neither may be sold separately from the main home (SMMC 27.19.030) |
| Est. Annual ROI | 7-9% gross rental yield on a $350,000-$480,000 detached build at $3,100-$3,400/mo (estimate before taxes, vacancy and financing) |
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Recent Law Changes
What's Changed Recently
- Ordinance 2026-02 (effective March 4, 2026) amended SMMC Chapter 27.19: no off-street parking for any ADU or JADU, detached ADU plate height raised to 18 ft, no front setback for state-exempt ADUs of 800 sq ft or less, and JADU owner-occupancy only when the JADU shares a bathroom
- FY2026-27 Comprehensive Fee Schedule (July 1, 2026) kept the flat ADU permit fees at $2,831.57 / $2,031.12 / $1,454.02 and lists a $1,455.43 rate for manufactured, prefab and pre-approved-plan ADUs; the ADU wastewater capacity charge is $5.87 per sq ft
- City reissued its Single-Family and Multifamily ADU/JADU handouts (July 24, 2026) and the ADU Development Standards Table (Aug 31, 2026)
- SB 543 (Jan 2026): ADU applications deemed approved if the city fails to act within 60 days; AB 1154 (Jan 2026): JADU owner-occupancy only when the JADU shares a bathroom with the primary dwelling; both now written into the city code
- SB 1211 reflected in SMMC 27.19.070: up to 8 detached ADUs on existing multifamily lots, 2 on proposed multifamily
Tips for Building an ADU in San Mateo
- Stay at or under 800 sq ft with 4-ft side and rear setbacks: that makes the unit state-exempt, so no front setback, no daylight-plane test, no FAR count and no size objection even if the lot's floor area is used up
- Stack units: San Mateo allows a JADU + an attached or conversion ADU + a new detached ADU on one single-family lot
- Skip the parking space; since March 2026 none is required, and you keep the driveway and curb cut if you convert the garage
- A prefab, manufactured or gallery pre-approved design cuts the flat permit fee from $2,831.57 to $1,455.43, but the design cannot be altered and the lot must be flat and outside FEMA high-hazard and WUI areas
- Budget the $5.87/sq ft wastewater capacity charge and possible new water, sewer and power connections for a detached unit; a conversion pays neither
- Check the heritage tree map (removal permit $113) and the FEMA flood map before siting the unit; both are flagged on the city's ADU page
- Cost basis (2026-09-19 re-verification): avg build cost: $300,000-$480,000 for a new detached ADU of 600-800 sq ft at the County ADU Resource Center's $450-$600/sq ft all-in estimate; attached ADUs $220,000-$350,000, garage conversions $180,000-$280,000 and JADUs $50,000-$120,000 per 2026 industry guides | cost per sqft: $450-$600/sq ft including design and fees (San Mateo County ADU Resource Center). The County's City of San Mateo calculator assumes $460/sq ft for new construction over 500 sq ft and $615/sq ft under 500 sq ft | garage conversion: $180,000-$280,000 (industry guide, April 2026); the County calculator assumes $345-$460/sq ft for garage conversions plus about $27,000 per new bathroom | prefab adu: $160,000-$350,000 installed for 500-1,000 sq ft, based on the County calculator's roughly $315-$350/sq ft modular assumption; foundation, utilities and site work are extra. A manufactured or prefab ADU qualifies for the reduced $1,455.43 permit fee
ADU Builders in San Mateo
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Single Family ADU and JADU Standards and Requirements handout (updated 7/24/2026) | City of San Mateo Planning Division Planning Department — Accessed 2026-09-19
- ADU Development Standards Table (revised 8/31/2026) | City of San Mateo Planning Department — Accessed 2026-09-19
- SMMC 27.19.060 Single-Family ADU Development Standards (Ord. 2026-02) | City of San Mateo Law Library Municipal Code — Accessed 2026-09-19
- SMMC 27.19.070 Multifamily ADU Development Standards | City of San Mateo Law Library Municipal Code — Accessed 2026-09-19
- SMMC 27.19.080 JADU Development Standards | City of San Mateo Law Library Municipal Code — Accessed 2026-09-19
- Accessory Dwelling Unit Information and Resources | City of San Mateo Planning Department — Accessed 2026-09-19
- Adopted Comprehensive Fee Schedule FY2026-27 (Building Division p. 2.5; Public Works p. 9.5) | City of San Mateo Government Data — Accessed 2026-09-19
- Pre-Approved ADU Plans Information and Submittal Guide | City of San Mateo Planning Department — Accessed 2026-09-19
- City of San Mateo ADU Plans Gallery Planning Department — Accessed 2026-09-19
- SMMC 27.19.050 General Requirements (impact fees, utility connections) | City of San Mateo Law Library Municipal Code — Accessed 2026-09-19
- New, Addition or Alteration (ADU/JADU/SFD/Duplex) Residential Plan Submittal Guide (rev. 3/10/2025) | City of San Mateo Planning Department — Accessed 2026-09-19
- How much does it cost to build an ADU? | San Mateo County ADU Resource Center (21 Elements) Government Data — Accessed 2026-09-19
- City of San Mateo ADU Calculator cost assumptions (County ADU Resource Center / ADU Accelerator) Government Data — Accessed 2026-09-19
- ADU Cost Guide for San Mateo County in 2026 | Genuine Kitchen & Bath industry — Accessed 2026-09-19
- Average Rent in San Mateo, CA (September 2026) | Zumper Market Data — Accessed 2026-09-19
- Average Rent in San Mateo, CA (August 2026) | RentCafe Market Data — Accessed 2026-09-19
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