Everything homeowners ask about building an ADU in San Mateo — answered with 2026 data.
Yes, ADUs are allowed in 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 in San Mateo, CA. The maximum size for a detached ADU is 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)). Attached ADUs can be up to 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. San Mateo has an ADU-friendliness score of 8/10. Check our full San Mateo ADU rules page for complete zoning details.
ADU construction in San Mateo typically costs $300,000-$480,000. The cost per square foot averages $450-$600. Garage conversions are more affordable at $180,000-$280,000, while prefab ADUs cost $160,000-$350,000. Permit fees add 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 to your budget. See our detailed San Mateo ADU cost guide.
Detached ADUs in San Mateo can be up to 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. Attached ADUs are limited to 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. Junior ADUs (JADUs) have a cap of 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)). Maximum height is 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. You are allowed up to Single-family lot: 1 JADU + 1 attached or conversion ADU + 1 new detached ADU may be combined on a qualifying lot.
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. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
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. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
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). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in San Mateo is typically 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. 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 availability: 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).
Average ADU rental income in San Mateo: 1-bedroom units rent for $3,100-$3,400 (Zumper Sept 2026 $3,185; RentCafe Aug 2026 $3,406 apartment averages; ADUs typically rent slightly below)/month, 2-bedroom units for $3,900-$4,400 (Zumper Sept 2026 $3,900; RentCafe Aug 2026 $4,448)/month. Short-term rentals: 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). Expected 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).
The minimum lot size for an ADU in San Mateo is No minimum lot size. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In San Mateo, a garage conversion typically costs $180,000-$280,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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