California ADU rules · San Mateo County
Can I Build an ADU in San Mateo?
Yes — you can build an ADU in San Mateo. California's statewide ADU law requires every city, including San Mateo, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
Local rule1 attached or conversion ADU + 1 detached ADU + 1 JADU
Allows both 1 attached/conversion and 1 detached ADU plus 1 JADU per single-family lot.
Units on a multifamily lot
Local ruleUp to 2 detached ADUs (proposed multifamily); up to 8 detached ADUs (existing multifamily, not to exceed number of existing units); conversions: at least 1, up to 25% of units
Detached ADU cap for existing multifamily is 8 or number of existing units, whichever is less.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
CA state law1,200 sq ft
Statewide default cap for detached new-construction ADUs in the absence of a compliant local ADU ordinance. The unconditional by-right floor a city must always allow is 800 sq ft (see Guaranteed minimum size); a city with a compliant ordinance may set its own ceiling.
Max attached ADU size
Local ruleNot to exceed size of primary dwelling; up to 800 sq ft exempt from FAR; conversion ADUs may expand up to 150 sq ft for ingress/egress
Attached ADUs capped at primary dwelling size; conversion ADUs have no size limit if within existing structure.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local ruleMust meet underlying zone’s front setback, except state-exempt ADUs (800 sq ft, 4-ft setbacks) which have no front setback requirement
Front setback not required for state-exempt ADUs.
Separation from main house
Local rule4 ft
4 ft minimum separation required for detached ADUs on multifamily lots; no explicit separation for single-family.
Parking
Parking required
Local ruleNone
No off-street parking required for any ADU or JADU.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required when converting/demolishing existing parking for ADU/JADU.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
CA state lawOnly for a JADU
No owner-occupancy requirement for ADUs (the prior sunset was removed). A JADU may require owner-occupancy, principally where it shares sanitation with the main house.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
CA state lawBuilding permit
Only a ministerial building permit — no conditional use permit or discretionary design review for a qualifying ADU.
San Mateo allows both an attached/conversion and a detached ADU plus a JADU on single-family lots, with ADU size capped at the primary dwelling size. No parking is required for any ADU or JADU, and detached ADUs have distinct height limits for plateline and roof peak.
Frequently asked questions
Can I build an ADU in San Mateo?
Yes. California's statewide ADU law requires San Mateo to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in San Mateo?
A detached ADU of at least 1,200 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in San Mateo?
Side and rear setbacks are limited to 4 ft. Must meet underlying zone’s front setback, except state-exempt ADUs (800 sq ft, 4-ft setbacks) which have no front setback requirement.
Is parking required for an ADU in San Mateo?
None. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in San Mateo?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in San Mateo?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- San Mateo Zoning Code Chapter 27.19 (amended 2023)
- § 27.19.060(a)
- § 27.19.070(b)
- § 27.19.060(b), (b)(2)
- § 27.19.060(d)
- § 27.19.070(h)
- § 27.19.060(f), § 27.19.070(f)
- § 27.19.060(f)(1)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66315 (owner-occupancy)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of San Mateo's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with San Mateo Planning before relying on it.
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