State law · city plans · utilities as of October 2026

Walnut Creek ADU Rules, Permits and Pre-Approved Plans

California law sets the floor: on a single-family lot, a city must approve a qualifying ADU within 60 days, allow at least 800 sq ft, cap side and rear setbacks at 4 ft and drop impact fees under 750 sq ft. Walnut Creek, Lafayette, Danville and the County each add pre-approved plans and their own standards on top.

State law

California ADU law, by section

Topic	What the law says	Gov. Code
Definitions	ADUs and junior ADUs are defined in the state ADU chapter	§66313
Review time	Approve or deny a complete application within 60 days, or it is deemed approved; review is ministerial (no hearing)	§66317
Setbacks	No more than 4 ft side and rear for a new ADU; none for converting an existing structure	§66314
Attached size	Up to 50% of the existing home's floor area	§66314(d)(4)
Detached size	Up to 1,200 sq ft; a city must allow at least 800 sq ft	§66314 · §66321
Height	16 ft detached; 18 ft near major transit (+2 ft for roof pitch); 25 ft or the zone limit for attached	§66321
Parking	No replacement parking when a garage or carport is converted or demolished for an ADU	§66314(d)(11)
Occupancy and sale	No owner-occupancy requirement for ADUs; rentals may be limited to terms over 30 days; an ADU can't be sold separately	§66314 · §66315
Fees	No impact fees under 750 sq ft; proportional above; no new utility connection or capacity fee for conversions of existing space	§66324
Junior ADU	Up to 500 sq ft within the home; owner occupancy can apply only if sanitation is shared	§66333

Summaries, not legal advice. Read the linked sections for exceptions and conditions.

Envelope

Setbacks and height in practice

The 4 ft limit is a ceiling on what a city can require, not a guarantee every design fits: easements, slopes, trees and fire separation still shape placement. Heights are measured under the state formula: 16 ft is the baseline for a detached unit on a single-family lot.

Walnut Creek publishes its own ADU and JADU standards (City ADU/JADU page); confirm the city's size and design limits for your zoning before drawings start.

City plans

Pre-approved plans by city as of October 2026

Jurisdiction	Program	What it does	Source
Walnut Creek	City pre-approved plans; third-party designer plans	Fixed designs already reviewed by the City; still need a permit for your site	City · Designer plans
Danville	Garden Cottage Program	Detached plans downloadable at no charge; pre-plan-checked, so expedited processing and lower permit fees; changes add charges	Town
Unincorporated County + Clayton, Hercules, Lafayette, Moraga, Orinda	County PADU (pre-approved ADU) vendor plans	Vendor plans approved without a specific site; owners still need planning and building permits, foundation plans and site review	County
Lafayette	All ADUs ministerial; city plan library in progress	Approved without a public hearing; impact fees for new ADUs reduced 50% through 2026	City
Concord, Pleasant Hill	City programs	Check with each city for current plans and standards	—

Rules checkPre-approved does not mean permit-free. Every route still ends in a building permit and inspections. Who handles permits? →

Utilities

Sewer, water and power

Central San (sewer). An ADU attached to or inside the house needs no separate sewer line. A separate ADU can't connect to the house's internal plumbing; it connects to the house's sewer lateral on the property, downstream of the main cleanout. An ADU that is part of an SB 9 lot split needs its own connection to the public main. Fees are collected on these projects and a sewer permit may be required depending on the route (Central San plan review). More on laterals: Central San sewer lateral guide.

Water. EBMUD or Contra Costa Water District serves your address; ask the provider whether the ADU needs a separate meter or capacity charge. State law exempts conversions of existing space from new connection fees.

Power. PG&E service and panel capacity often limit all-electric ADUs; a load calculation early avoids redesign.

Site limits

Fire zones, trees and HOAs

Fire zones: much of the Lamorinda hills is mapped as a Fire Hazard Severity Zone; new construction there must meet Chapter 7A wildfire-resistant standards (CAL FIRE).
Trees: local tree ordinances can protect oaks and other species near the build area.
HOAs: Civil Code §4751 voids HOA rules that prohibit or unreasonably restrict ADUs; reasonable standards still apply (Civil Code §4751).
Easements: utility and drainage easements can't be built over.

Process

Permit process and timeline

Typical site-built detached ADU. The 60-day state clock starts once the application is complete; correction rounds add time.

Does a pre-approved ADU plan skip the permit?

No. Pre-approved plans shorten design and plan review, but you still need a building permit with a site plan, foundation and utility details. The County says PADU users "will still need to secure planning and building permits."

Can my HOA stop me from building an ADU?

Not outright. Civil Code §4751 voids HOA rules that prohibit or unreasonably restrict ADUs and junior ADUs on single-family lots. Reasonable design and architectural standards can still apply, so check your CC&Rs early.

Can I sell the ADU separately?

Generally no. An ADU can be rented separately from the main home but not sold or conveyed separately under state law.

Not sure your lot qualifies?Get a Quote or call (925) 709-5438

Rebates & financing → · ADU cost →

Next step

Get your lot checked

A builder can review setbacks, utilities and plan options before you pay for design.

Request a Quote Call (925) 709-5438