California ADU rules · San Mateo County
Can I Build an ADU in Millbrae?
Yes — you can build an ADU in Millbrae. California's statewide ADU law requires every city, including Millbrae, 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
Local cap of 2 detached ADUs per multifamily lot, more restrictive than state default of 8.
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
Local rule1,000 sq ft
Local maximum: 850 sq ft (studio/1-bed), 1,000 sq ft (2+ bedrooms); state floor of 800 sq ft still applies.
Max attached ADU size
Local rule850 (studio/1-bed), 1,000 (2+ bed), and not more than 50% of primary dwelling
Local cap is 850/1,000 sq ft and 50% of primary dwelling, but must allow at least 800 sq ft.
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
Local rule16 ft
16 ft standard for detached; 18 ft if near transit/multistory; attached ADUs up to 25 ft or height of primary dwelling.
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
CA state lawNo front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone
Separation from main house
Local rule5 ft
5 ft minimum separation required between ADU and any other structure.
Parking
Parking required
Local ruleUp to 1 space
1 space per ADU; JADUs require no additional parking.
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
CA state lawNot required
When a garage/carport is demolished or converted to build an ADU, replacement parking cannot be required.
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 until Jan 1, 2025; after that, owner must reside in primary or ADU. JADUs always require owner-occupancy.
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.
Millbrae-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)
No additional fire sprinklers required for ADUs if not required for primary dwelling.
Historic Districts
No ADU parking required in historic districts.
Millbrae restricts detached ADUs to 1,000 sq ft (2+ bedrooms) and allows only 2 detached ADUs on multifamily lots. A 5-foot separation from other structures is required, and owner-occupancy for ADUs begins in 2025.
Frequently asked questions
Can I build an ADU in Millbrae?
Yes. California's statewide ADU law requires Millbrae 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 Millbrae?
A detached ADU of at least 1,000 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 Millbrae?
Side and rear setbacks are limited to 4 ft. No front setback may block the mandatory 800 sq ft / 16-ft / 4-ft-setback ADU; otherwise per base zone.
Is parking required for an ADU in Millbrae?
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 Millbrae?
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 Millbrae?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Millbrae Municipal Code Article XXIV (MMC § 10.05.3210–10.05.3260), Ord. 804 (2023)
- MMC § 10.05.3230(C)(5)-(6)
- MMC § 10.05.3250(E)(3)
- MMC § 10.05.3250(E)(2)
- MMC § 10.05.3250(E)(5)(b)
- MMC § 10.05.3250(E)(8); (D)(6)
- MMC § 10.05.3240(A)(2), (B)(5)
- MMC § 10.05.3250(A)
- MMC § 10.05.3250(E)(8)
- 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 § 66314 (local ordinance standards)
- 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 Millbrae'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 Millbrae Planning before relying on it.
Will an ADU work on your Millbrae lot?
Ask GoCodebook about your address and get the size, setback, parking and permit rules that actually apply to your parcel — with the exact citation.
Check my parcel free