Backyard cottages · 1 and 2 bedroom units
Detached ADUs in Walnut Creek
A detached ADU is a separate backyard home of up to 1,200 sq ft. State law guarantees room for at least an 800 sq ft unit, 16 ft tall, set 4 ft from the side and rear lines. In Contra Costa, new detached ADUs typically cost $250,000–$450,000+ (Mazzamuto Construction).
Size + height
How big and how tall
| Rule | State limit | Source |
|---|---|---|
| Maximum size, detached | up to 1,200 sq ft | §66314 |
| Size a city must allow | at least 800 sq ft, regardless of lot coverage limits | §66321 |
| Height, single-family lot | 16 ft | §66321 |
| Height near major transit | 18 ft, +2 ft to match roof pitch | §66321 |
| Side and rear setback | no more than 4 ft | §66314 |
Rules checkWalnut Creek applies its own standards within these limits; confirm them on the City's ADU page before design. ADU rules & permits →
Placement
Placement, access and utilities
Setbacks and access. With 4 ft side and rear setbacks, the rear corner is the usual spot. Builders also need a path for materials, a trench route for utilities and, on tight lots, room for a small excavator.
Sewer. In Central San's area, a separate ADU can't tie into the house's internal plumbing. It connects to the house lateral on the property, downstream of the main cleanout, or within the structure's footprint. Fees are collected and a sewer permit may be required depending on the route (Central San plan review). If your lateral is old clay pipe, have it scoped first: Central San sewer lateral guide.
Power. A detached ADU usually runs on a subpanel fed from the main panel; older 100-amp services often need an upgrade, especially for an all-electric unit. Background: do you need a panel upgrade?
Water. EBMUD or Contra Costa Water District, depending on your address, decides whether a separate meter or capacity charge applies.
Plans
Pre-approved plans or custom design
Pre-approved plans
Walnut Creek publishes pre-approved ADU plans and third-party designer plans already reviewed by the City (City page). Danville offers Garden Cottage detached plans at no charge, with expedited processing and lower permit fees. The County's PADU vendor plans cover Lafayette, Orinda, Moraga, Clayton and unincorporated areas. A pre-approved plan shortens design and review, but you still need a permit with a site plan, foundation and utilities.
Custom design
Custom drawings fit odd lots, slopes, views and specific room counts, and can match the main house. Expect more design time and a full plan check. On hillside lots, custom engineering is often unavoidable anyway.
Site conditions
Hillsides, trees and fire zones
- Slope: stepped foundations, retaining walls and drainage add cost and engineering.
- Protected trees: Walnut Creek, Lafayette, Orinda and Danville regulate tree removal and work in root zones; site the unit before you design it.
- Fire zones: lots in a mapped Fire Hazard Severity Zone need wildfire-resistant construction under the California Building Code's Chapter 7A (CAL FIRE maps).
- HOAs: California voids HOA rules that prohibit or unreasonably restrict ADUs on single-family lots (Civil Code §4751), but reasonable design standards can still apply.
Time + money
Timeline and cost
Plan on roughly a year from first drawings to move-in for a site-built detached ADU; IMKAT reports 12–16 months for an 800 sq ft unit. Cost: $250,000–$450,000+ in Contra Costa; $450–$700+ per sq ft in the Bay Area. Prefab can shorten construction if the lot has crane access.
FAQ
Detached ADU questions
How big can a detached ADU be in Walnut Creek?
How tall can a detached ADU be?
Does a detached ADU need its own sewer line?
Next step
Find out what fits your backyard
Share your lot and goals. A local ADU builder can review feasibility and give you a quote.