California ADU rules · Contra Costa County
Can I Build an ADU in Walnut Creek?
Yes — you can build an ADU in Walnut Creek. California's statewide ADU law requires every city, including Walnut Creek, 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/conversion ADU + 1 detached ADU + 1 JADU
Allows both an attached/conversion and a detached ADU plus a JADU on a single-family lot.
Units on a multifamily lot
Local ruleup to 2 detached ADUs for proposed multifamily; up to 8 detached ADUs for existing multifamily (not to exceed number of existing units); conversions: up to 25% of existing units
Detached ADU cap is 2 for new/proposed multifamily, 8 for existing multifamily, subject to not exceeding number of existing units.
Junior ADU (JADU)
Local ruleAllowed
Explicitly allows 1 JADU per single-family lot.
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 primary, >=850 (>=1,000 for 2+ bedrooms)
Follows state rule for attached ADU size.
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 minimum for new detached ADUs.
Rear setback
Local rule4 ft
4 ft minimum for new detached ADUs.
Front setback
Local ruleNo minimum front setback for at least one 800 sq ft ADU; otherwise, local standards may apply but cannot preclude an 800 sq ft ADU.
Front setback waived for one 800 sq ft ADU.
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 more than 1 space per ADU or per bedroom (whichever is less); no guest parking required; JADUs require no parking.
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 when garage/carport is converted to 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 ruleOnly for a JADU
No owner-occupancy for ADUs; JADUs may 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.
Walnut Creek-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Fire code compliance
ADUs must comply with building/fire code; sprinklers only if required for primary dwelling.
Coastal zone
ADUs in the coastal zone must comply with the Coastal Act in addition to state/local ADU law.
Historic/HPOZ
Demolition notice/placard required if in architecturally/historically significant district.
Walnut Creek allows both an attached/conversion and a detached ADU plus a JADU on single-family lots, but caps detached ADUs at 800 sq ft unless built before any other ADU. Detached ADU height may reach 18–20 ft in some cases, and up to 8 detached ADUs are allowed on existing multifamily lots.
Frequently asked questions
Can I build an ADU in Walnut Creek?
Yes. California's statewide ADU law requires Walnut Creek 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 Walnut Creek?
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 Walnut Creek?
Side and rear setbacks are limited to 4 ft. No minimum front setback for at least one 800 sq ft ADU; otherwise, local standards may apply but cannot preclude an 800 sq ft ADU..
Is parking required for an ADU in Walnut Creek?
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 Walnut Creek?
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 Walnut Creek?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Walnut Creek Zoning Code, Article 5, §§ 10-2.3.501–10-2.3.506; Ord. 2246, effective 11/15/24
- Walnut Creek Zoning Code § 10-2.3.503(A)
- Walnut Creek Zoning Code § 10-2.3.503(B)
- Walnut Creek Zoning Code § 10-2.3.506(C)
- Walnut Creek Zoning Code § 10-2.3.503(A)(1); Gov. Code § 66321(b)(2), § 66314(d)(4)
- Walnut Creek Zoning Code § 10-2.3.503(A)(2)(a)
- Walnut Creek Zoning Code § 10-2.3.506(E)
- Walnut Creek Zoning Code § 10-2.3.506(E); Gov. Code § 66322(a)
- Gov. Code § 66314(d)(11)
- Walnut Creek Zoning Code § 10-2.3.501; Gov. Code § 66317, § 66315
- Walnut Creek Zoning Code § 10-2.3.501
- 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 § 66321 (size minimums & height)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- 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 Walnut Creek'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 Walnut Creek Planning before relying on it.
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