California ADU rules · San Mateo County
Can I Build an ADU in Portola Valley?
Yes — you can build an ADU in Portola Valley. California's statewide ADU law requires every city, including Portola Valley, 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
Local ruleConditional
Most ADUs are ministerial, but discretionary review applies for larger/complex ADUs or in sensitive locations.
Units on a single-family lot
Local rule1 ADU + 1 JADU (<3.5 acres); 2 ADUs (1 detached + 1 internal) OR 1 detached + 1 JADU (≥3.5 acres)
Larger lots (≥3.5 acres) may have 2 ADUs or 1 ADU + 1 JADU.
Units on a multifamily lot
Local ruleUp to 2 detached ADUs + conversions (up to 25% of units, min 1)
Detached ADUs capped at 2 per multifamily lot.
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 rule1,200 sq ft
1,200 sqft max if meeting base setbacks on lots <3.5 acres; 1,500 sqft on ≥3.5 acres if only 1 external ADU.
Max attached ADU size
Local ruleSame as detached, but also subject to 50% of main building floor area unless that precludes 850/1,000 sqft minimums
Attached ADUs follow detached size limits and 50% of main building rule.
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 rule24 ft
16 ft for reduced setback ADUs; up to 24 ft ministerial if meeting base setbacks; above 24 ft requires discretionary review.
Setbacks
Side setback
Local rule4 ft
4 ft for external ADUs ≤850/1,000 sqft; larger ADUs must meet base setbacks.
Rear setback
Local rule4 ft
4 ft for external ADUs ≤850/1,000 sqft; larger ADUs must meet base setbacks.
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
CA state lawNone required
No statewide separation requirement between the ADU and the primary dwelling.
Parking
Parking required
Local ruleUp to 1 space
No parking required for JADUs/internal ADUs; 1 space for external ADUs unless exempt.
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 required for main unit when garage/carport/covered parking is converted or demolished for ADU.
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 ruleNot required
No owner-occupancy required for ADUs approved 1/1/2020–12/24/2024; otherwise, owner must occupy ADU or main.
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
Local ruleBuilding permit
Additional reviews required for geological/fire hazard areas.
Portola Valley-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) and substandard access
ADUs prohibited on parcels <1 acre with substandard access (single ingress/egress, <18 ft wide); additional fire safety checklist for ADUs not meeting base setbacks.
Hillside/geologic hazard zones
ADUs prohibited in certain geologic hazard zones; subject to geologic and fire safety restrictions.
Historic zones
ADUs in historic zones require ASCC review.
Portola Valley imposes stricter limits on ADU size, height, and number for large lots and multifamily parcels, and prohibits ADUs in certain fire and geologic hazard areas. Discretionary review may apply for larger or complex ADUs or those in sensitive locations.
Frequently asked questions
Can I build an ADU in Portola Valley?
Yes. California's statewide ADU law requires Portola Valley 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 Portola Valley?
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 Portola Valley?
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 Portola Valley?
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 Portola Valley?
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 Portola Valley?
Owner-occupancy: not required. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Portola Valley Municipal Code §18.36.040 (Accessory Dwelling Units)
- PVMC §18.36.040.B.5, B.6, B.7
- PVMC §18.36.040.B.4.a, B.4.b.iii
- PVMC §18.36.040.B.4.c.i
- PVMC §18.36.040.B.4.c.ii
- PVMC §18.36.040.B.4.e
- PVMC §18.36.040.B.4.j
- PVMC §18.36.040.B.4.f
- PVMC §18.36.040.B.4.f.4
- PVMC §18.36.040.B.5.a, B.7.b
- PVMC §18.36.040.B.4
- PVMC §18.36.040.B.3, B.6.c
- PVMC §18.36.040.B.6.c
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- 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 § 66317 (ministerial approval, 60-day deadline)
- 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 Portola Valley'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 Portola Valley Planning before relying on it.
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