The City of San Mateo (not to be confused with San Mateo County) approves ADUs ministerially under Zoning Code Chapter 27.19, with published flat planning fees and a pre-approved plans program. Local standards are more generous than the state floor in places — detached ADUs may go to 24 ft at the roof peak and up to three accessory units can be combined on a single-family lot — but a 12-ft/45-degree daylight plane and a parking space (unless transit-exempt) apply.
Zoning & Size Requirements [1][2]
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 | All residential zones with an existing or proposed dwelling (single-family and multifamily); check the city's Property Zone Look-Up tool |
| Max Size (Detached) | 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) |
| Max Size (Attached) | 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 |
| Max Size (JADU) | 500 sq ft |
| Max Height | 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 |
| Max Units | 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 |
Setback Requirements [1][2]
San Mateo requires a rear setback of about 4 ft for a detached ADU, and one off-street parking space is required. Owner-occupancy is not required — you can rent it out without living on the property.
| 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 |
| Parking Required | 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 |
| Owner Occupancy | 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) |
| Min Lot Size | No minimum lot size |
Permit Process & Fees [3]
An ADU permit in San Mateo typically costs Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees and takes 60-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026), through a ministerial, by-right review.
| Approval Process | Ministerial — building permit and related permits only; no planning application or public notice for code-compliant projects |
| Review Timeline | 60-day state-mandated review; deemed approved if the city misses the deadline (SB 543, Jan 2026) |
| Permit Fees | Flat planning fees: $2,831.57 new ADU, $2,031.12 ADU remodel, $1,454.02 JADU — plus building permit fees |
| Impact Fees | Waived for ADUs under 750 sq ft (state law); confirm larger-unit fees with the Building Division |
| Plan Check | Standard building plan check; Code Consultation Counter open weekday mornings (8:30-11:30am) for pre-submittal questions |
| Pre-Approved Plans | Yes — the city offers a Pre-Approved ADU Plans program |
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Construction Costs [4][5]
Building an ADU in San Mateo typically costs $250,000-$450,000 ($350-$550 per sq ft); a garage conversion runs $120,000-$200,000.
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| Average Build Cost | $250,000-$450,000 |
| Cost per Sq Ft | $350-$550 |
| Garage Conversion | $120,000-$200,000 |
| Prefab ADU (Installed) | $160,000-$320,000 |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [6][7]
A 2-bedroom ADU in San Mateo rents for around $3,400-$3,900/mo, an estimated 5-8% annual return (Peninsula rents are among the strongest in the state).
| Avg Rent (1 Bedroom) | $2,700-$3,100/mo |
| Avg Rent (2 Bedroom) | $3,400-$3,900/mo |
| Short-Term Rental | No — short-term rental of ADUs/JADUs prohibited per SMMC 27.19.030 |
| Est. Annual ROI | 5-8% annual return (Peninsula rents are among the strongest in the state) |
Explore Prefab ADU Options
Prefab ADUs can save 20-40% vs. traditional construction and cut build time in half.
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Recent Law Changes
What's Changed Recently
- SB 543 (Jan 2026): ADU applications deemed approved if the city fails to act within 60 days
- AB 1154 (Jan 2026): JADU owner-occupancy may only be required if the JADU shares a bathroom with the primary dwelling
- SB 1211 reflected in city standards: up to 8 detached ADUs on existing multifamily lots, 2 on proposed multifamily
- City published an updated ADU/JADU Development Standards Table and Parking Exempt ADU/JADU Map
Tips for Building an ADU in San Mateo
- Stack units: San Mateo allows a JADU + conversion ADU + new detached ADU on one single-family lot
- Keep the ADU at or under 800 sq ft to stay FAR-exempt and qualify for state-protected approval
- Check the city's Parking Exempt ADU/JADU Map before designing — a transit-exempt lot saves a 10x18 parking space
- Watch the 12-ft/45-degree daylight plane from side and rear lot lines when designing two-story or tall-plate units
ADU Builders in San Mateo
Looking for an ADU builder in San Mateo? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match San Mateo homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- ADU/JADU Development Standards Table | City of San Mateo Planning Department — Accessed 2026-07-10
- SMMC 27.19.050 General Requirements | City of San Mateo Law Library Municipal Code — Accessed 2026-07-10
- Accessory Dwelling Unit Information and Resources | City of San Mateo Planning Department — Accessed 2026-07-10
- San Mateo ADU Regulations and Building Requirements | Acton ADU industry — Accessed 2026-07-10
- The Complete San Mateo ADU Requirements Guide | Samara industry — Accessed 2026-07-10
- Average Rent in San Mateo, CA | RentCafe Market Data — Accessed 2026-07-10
- Average Rental Price in San Mateo, CA | Zillow Rental Manager Market Data — Accessed 2026-07-10
Frequently Asked Questions
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