California ADU rules · San Mateo County
Can I Build an ADU in Atherton?
Yes — you can build an ADU in Atherton. California's statewide ADU law requires every city, including Atherton, 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 8 detached (not exceeding number of existing units); up to 2 detached on proposed multifamily lot
Limits detached ADUs on existing multifamily lots to 8 or the number of existing units, whichever is less; only 2 detached ADUs allowed on proposed multifamily lots.
Junior ADU (JADU)
Local ruleAllowed
JADUs allowed on single-family lots.
Size & height
Max detached ADU size
Local rule1,200 sq ft
Local maximum is 1,200 sq ft; must allow at least 800 sq ft per state law.
Max attached ADU size
Local rule50% of primary, >=800 sq ft; absolute max 1,200 sq ft if meeting setback conditions
Attached ADUs capped at 50% of main residence, but must allow at least 800 sq ft; up to 1,200 sq ft if setbacks are met.
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
Local rule500 sq ft
JADU maximum size is 500 sq ft.
Max height
Local rule16 ft
Detached ADU max height is 16 ft; attached ADUs follow main residence height limit (max 2 stories).
Setbacks
Side setback
Local rule4 ft
4 ft side setback for ADUs ≤800 sq ft; larger ADUs must meet accessory building setbacks or may encroach up to 20% if attached.
Rear setback
Local rule4 ft
4 ft rear setback for ADUs ≤800 sq ft; larger ADUs must meet accessory building setbacks or may encroach up to 20% if attached.
Front setback
Local ruleNo front setback for ADUs ≤800 sq ft; otherwise, must comply with zone/building area setbacks
Front setback not required for ADUs ≤800 sq ft.
Separation from main house
Local ruleNone required
No separation required for ADUs ≤800 sq ft; larger ADUs must meet accessory structure separation.
Parking
Parking required
Local ruleUp to 1 space
1 space per ADU unless exempt; no parking required for JADUs.
Transit parking exemption
Local ruleYes — no parking required near transit & for conversions
No parking required if within ½ mile of public transit or other state exemptions.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required for garage/carport conversions.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
Local rule60 days
60 days for standard ADUs/JADUs; 30 days for preapproved plan ADUs.
Owner-occupancy
Local ruleNot required
No owner-occupancy required for ADUs or JADUs permitted after Jan 1, 2020.
Impact-fee waiver
Local ruleWaived under 750 sq ft
No impact fees for ADUs <750 sq ft; proportional fees for larger ADUs.
Permits required
Local ruleBuilding permit, Demolition permit (if garage conversion)
Demolition permit for garage must be issued concurrently with ADU permit.
Atherton-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)
ADUs must comply with all applicable fire regulations; sprinklers not required unless for primary residence.
Atherton allows up to 8 detached ADUs on existing multifamily lots (not exceeding the number of existing units) and requires no owner-occupancy for ADUs or JADUs. Detached ADUs are capped at 1,200 sq ft, and no replacement parking is required for garage conversions.
Frequently asked questions
Can I build an ADU in Atherton?
Yes. California's statewide ADU law requires Atherton 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 Atherton?
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 Atherton?
Side and rear setbacks are limited to 4 ft. No front setback for ADUs ≤800 sq ft; otherwise, must comply with zone/building area setbacks.
Is parking required for an ADU in Atherton?
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 Atherton?
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 Atherton?
Owner-occupancy: not required. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Atherton Municipal Code Title 17, Chapter 17.52 (Ordinance 677, adopted Feb 18, 2026)
- AMC §17.52.030(E)
- AMC §17.52.030(F), §17.52.040(C)(2)
- AMC §17.52.040(C)(3), (D)(2)
- AMC §17.52.040(C)(4), (D)(2)
- AMC §17.52.040(H)
- AMC §17.52.040(E)
- AMC §17.52.040(F)(5)
- AMC §17.52.040(L)
- AMC §17.52.040(L)(4)
- AMC §17.52.050(A), (B)
- AMC §17.52.040(N)
- AMC §17.52.040(R)
- AMC §17.52.040(S)
- AMC §17.52.040(P)
- 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 § 66321 (size minimums & height)
Last updated 2026-06-25. This is an AI-assisted summary of Atherton'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 Atherton Planning before relying on it.
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