California ADU rules · San Mateo County
Can I Build an ADU in San Carlos?
Yes — you can build an ADU in San Carlos. California's statewide ADU law requires every city, including San Carlos, 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
CA state law1 ADU + 1 JADU
At least one ADU and one JADU per single-family lot (a conversion ADU may also be possible).
Units on a multifamily lot
Local ruleconversions (>=1 or 25% of units) + up to 2 detached (proposed multifamily) or up to 8 detached (existing multifamily, not to exceed number of existing units)
Detached ADUs: up to 2 on proposed multifamily, up to 8 on existing multifamily, but not to exceed number of existing units.
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 rule50% of existing primary dwelling, unless Statewide Exemption ADU (then at least 800 sq ft allowed)
Local rule is 50% of primary dwelling; state minimum of 800 sq ft applies if exemption ADU.
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
Local rule4 ft
4 ft side setback for detached and first floor attached ADUs.
Rear setback
Local rule4 ft
4 ft rear setback for detached and first floor attached ADUs.
Front setback
Local ruleper underlying zoning, but may be reduced to allow 800 sq ft Statewide Exemption ADU
Front setback per base zone, but must allow 800 sq ft ADU if needed.
Separation from main house
Local ruleNone required
No minimum separation; must meet building/fire code.
Parking
Parking required
Local ruleUp to 1 space
1 space per ADU, with state-mandated exemptions.
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 for garage/carport conversion.
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
Local ruleOnly for a JADU
No owner-occupancy for ADUs through at least Jan 1, 2025; JADU or main house must be owner-occupied if both present.
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 Carlos-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Very High Fire Hazard Severity Zone (VHFHSZ)
Must comply with building/fire code; no local prohibition on ADUs.
Historic District
Additional review required if property is in a historic district.
Hillside Areas
Must comply with all applicable building and safety codes.
San Carlos limits detached ADUs on multifamily lots to 2 for proposed and up to 8 for existing buildings, with no fixed local size cap but a 50% of primary dwelling rule unless the state minimum applies. Height limits are more restrictive than state for some ADUs, and historic district properties require extra review.
Frequently asked questions
Can I build an ADU in San Carlos?
Yes. California's statewide ADU law requires San Carlos 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 Carlos?
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 Carlos?
Side and rear setbacks are limited to 4 ft. per underlying zoning, but may be reduced to allow 800 sq ft Statewide Exemption ADU.
Is parking required for an ADU in San Carlos?
Up to 1 space. 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 Carlos?
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 Carlos?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- San Carlos Zoning Code § 18.23.210 (amended 2023)
- San Carlos Zoning Code § 18.23.210(D)(2)(a)
- San Carlos Zoning Code § 18.23.210(F)(4)(f)
- San Carlos Zoning Code § 18.23.210(F)(4)(e)
- San Carlos Zoning Code § 18.23.210(F)(4)(e), (F)(4)(c)
- San Carlos Zoning Code Table 18.23.210(E)(4)
- San Carlos Zoning Code § 18.23.210(G)
- San Carlos Zoning Code § 18.23.210(G)(3)
- San Carlos Zoning Code § 18.23.210(E)(2)(d)
- 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 § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- 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 Carlos'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 Carlos Planning before relying on it.
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