Everything homeowners ask about building an ADU in San Mateo — answered with 2026 data.
Yes, ADUs are allowed in All residential zones with an existing or proposed dwelling (single-family and multifamily); check the city's Property Zone Look-Up tool in San Mateo, CA. The maximum size for a detached ADU is Up to the FAR of the underlying zone and no larger than the primary unit; 800 sq ft is always allowed (ADU area up to 800 sq ft is FAR-exempt). Attached ADUs can be up to Up to the FAR of the underlying zone and no larger than the primary unit; first 800 sq ft of ADU area is FAR-exempt. San Mateo has an ADU-friendliness score of 7/10. Check our full San Mateo ADU rules page for complete zoning details.
ADU construction in San Mateo typically costs $250,000-$450,000. The cost per square foot averages $350-$550. Garage conversions are more affordable at $120,000-$200,000, while prefab ADUs cost $160,000-$320,000. Permit fees add Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees to your budget. See our detailed San Mateo ADU cost guide.
Detached ADUs in San Mateo can be up to Up to the FAR of the underlying zone and no larger than the primary unit; 800 sq ft is always allowed (ADU area up to 800 sq ft is FAR-exempt). Attached ADUs are limited to Up to the FAR of the underlying zone and no larger than the primary unit; first 800 sq ft of ADU area is FAR-exempt. Junior ADUs (JADUs) have a cap of 500 sq ft. Maximum height is Detached: 16 ft to top plateline, 24 ft to roof peak (from existing grade). Attached: 24 ft to plateline, 32 ft to peak. 12-ft daylight plane at 45 degrees from side/rear property lines. You are allowed up to Single-family lot: 1 JADU + 1 conversion ADU + 1 new detached ADU may be combined. Multifamily: conversion ADUs up to 25% of existing units, plus 2 detached (proposed MF) or up to 8 detached (existing MF) per SB 1211 on a qualifying lot.
Rear setback: 4 ft; none for conversions of existing space. Side setback: 4 ft; none for conversions of existing space. Front setback: Required setbacks of underlying zoning district apply to JADUs/conversions; new ADUs follow zone front setback. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
1 off-street space (10 ft x 18 ft; garage, carport, uncovered, or tandem OK); waived within 1/2 mile walking distance of transit (see city's Parking Exempt ADU/JADU Map) or in a historic district; none for JADUs. 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. JADUs: deed restriction and owner-occupancy required (under AB 1154, from 2026 owner-occupancy applies only to JADUs sharing a bathroom with the main home — confirm current enforcement with the Planning Division). 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-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026). Plan check: Standard building plan check; Code Consultation Counter open weekday mornings (8:30-11:30am) for pre-submittal questions. Pre-approved plans availability: Yes — the city offers a Pre-Approved ADU Plans program.
Average ADU rental income in San Mateo: 1-bedroom units rent for $2,700-$3,100/month, 2-bedroom units for $3,400-$3,900/month. Short-term rentals: No — short-term rental of ADUs/JADUs prohibited per SMMC 27.19.030. Expected ROI: 5-8% annual return (Peninsula rents are among the strongest in the state).
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 $120,000-$200,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.