Building an ADU in Walnut Creek, California

What you can build in Walnut Creek

Walnut Creek's ADU rules are in Title 10, Chapter 2, Part III, Article 5 of the Municipal Code. The city updated them with Ordinance 2246 (October 15, 2024) and amended the multifamily rules with Ordinance 2250 (August 5, 2025). ADUs are allowed in all single-family residential, multifamily residential and mixed-use zones. JADUs are allowed in single-family zones and in multifamily zones that allow single-family homes. Look up your zoning on the city's Zoning Map.

On a single-family lot, the city says "you can generally build one conversion (or new attached) ADU, one detached ADU and one JADU." The city sorts ADUs into two groups:

• State-exempt ADUs: a new detached ADU of 800 sq ft or less, 16 ft tall or less (18 ft within a half mile of a BART station, plus 2 ft to match the main roof pitch), with at least 4-ft side and rear setbacks. Conversions of existing structures (with up to 150 sq ft added for getting in and out), ADUs built into a proposed new home, and ADUs in the same footprint as an existing structure can also be exempt. These skip the city's design standards.
• Non-exempt ADUs: anything else, which must meet all of the city's ADU development standards.

The city also allows up to two ADUs on faith-based properties such as churches and synagogues. Multifamily ADUs have their own rules; email the Duty Planner to discuss them. ADUs and JADUs don't need a public review; staff approve them through the building permit if they meet the standards. For the statewide rules, see our California ADU guide. [1][2][3]

Size, height and setbacks

These are the city's standards for non-exempt ADUs, from its ADU Handbook (updated September 2025).

Maximum size depends on net lot area:

• lots of 14,999 sq ft or less: 850 sq ft (studio or one bedroom) or 1,000 sq ft (two or more bedrooms)
• lots of 15,000 to 19,999 sq ft: 900 sq ft or 1,000 sq ft
• lots of 20,000 sq ft or more: 950 sq ft or 1,000 sq ft

An attached ADU over 800 sq ft also can't exceed 50% of the existing home's gross floor area.

Height: detached ADUs 16 ft in most areas, or 18 ft within a half mile of a BART station (plus 2 ft to match the main roof pitch) or on a lot with a multistory multifamily building. An ADU that meets the zone's regular setbacks can use the zone's height limit. Attached ADUs can be 25 ft or the main home's height limit, whichever is lower, and no more than two stories. State law allows the 18-ft height within a half-mile walk of any "major transit stop or a high-quality transit corridor," not only BART, so if your lot is near a frequent bus line, ask the Duty Planner which limit applies.

Setbacks: 4 ft side and rear if the ADU stays under those height limits; otherwise the zone's setbacks apply. The front setback follows the zone but can be reduced to fit an ADU of 800 sq ft or less.

ADUs over 800 sq ft must be "architecturally consistent" with the main building (shape, window type, siding and trim, roof form and materials), meet the zone's lot coverage limit, and be "proposed (submitted) or constructed prior to any other ADUs on the lot." In an August 25, 2025 review of the ordinance, HCD found this timing rule out of line with state law and said the city "must remove" it for state-mandated detached ADUs. In its September 2025 reply, the city said the rule applies only to detached ADUs over 800 sq ft, and that a state-mandated detached ADU of up to 800 sq ft can be built at any time. If timing matters for your project, confirm with the Duty Planner. [1][2][3][4][5]

Parking

"No additional parking spaces are required for new ADU or JADU's," the city says. Replacement parking isn't required either when parking is converted into an ADU or JADU, whether the spaces were covered or uncovered. [1][2]

Fees and utilities

You pay permit fees through the city's online permit portal, and a credit card convenience fee applies. When the permit is approved, a permit technician gives you the list of fees and conditions to clear. The city says school fees are "based on the valuation of the project" and recommends contacting your school district (Acalanes, Lafayette, Walnut Creek, Mount Diablo or San Ramon Valley). Its handbook lists development impact fees among ADU costs.

Impact fees: under state law, no city, special district or water company may charge impact fees on an ADU of 750 sq ft or less of interior livable space, or a JADU of 500 sq ft or less. School districts may charge fees on ADUs over 500 sq ft.

Utilities: the city's handbook says new ADUs and JADUs "are not required to provide a separate new utility line connection," though you may need to upsize an existing connection. ADUs also need approval from the Contra Costa County Fire Protection District and the Central Contra Costa Sanitary District (Central San), and the city recommends contacting Contra Costa Water District for water service. Ask your water and sanitary districts early whether connection fees apply. See our ADU cost guide. [1][2][5][6]

How long it takes

State law requires the city to tell you within 15 business days whether your application is complete, and to approve or deny a complete application within 60 days when a home already exists on the lot.

Walnut Creek doesn't publish a fixed review time. It says the details of your lot, such as slope or access, affect how long permitting and construction take, and that unaddressed plan comments can add review rounds. Meeting a city planner before you apply is optional, but the city says it "often leads to faster approval." See our ADU permit process guide. [1][2][5]

Junior ADUs and garage conversions

A JADU is 500 sq ft or less, entirely inside a single-family home (an attached garage counts). It needs an efficiency kitchen with a counter, storage, a cooking appliance, and access to a sink and refrigerator. It may have its own bathroom or share one with the house. You must record a deed restriction before the building permit is issued.

Owner occupancy: the city says the owner must live in the JADU, the house containing it, or an attached ADU. Since January 1, 2026, state law requires owner occupancy only when the JADU shares a bathroom with the house. If your JADU has its own bathroom, confirm with the Duty Planner whether the city still requires owner occupancy.

Garage conversions are usually state exempt. A converted garage must either keep a working garage door that opens to interior storage separated from the ADU, or replace the door with windows or walls matching the house plus a 3-ft landscape buffer next to the driveway. The converted ADU keeps the existing footprint, height and setbacks, with up to 150 sq ft added for getting in and out of a detached structure. See our junior ADU guide and garage conversion guide. [1][2][5]

Renting it out

Walnut Creek says ADUs "cannot be used for short term rentals, unless the building permit was issued before January 1, 2020. Rentals must be 31 days or longer." The city adds, "This does not apply to JADUs." State law now says JADUs "cannot be used as short-term rentals and, if rented, must be rented for longer than 30 days." If you plan to rent out a JADU, confirm with the Duty Planner which rule applies.

You don't have to live on the property to rent out an ADU. ADUs generally can't be sold separately from the main home unless state law allows it. [1][2][5]

Legalizing an unpermitted ADU

The city says that "As of January 1, 2025 property owners with unpermitted ADUs can apply for retroactive permits provided all building requirements are met." The city's ADU page explains how to apply. [1][2]

Pre-reviewed ADU plans (PRADU)

Walnut Creek offers four free Pre-Reviewed ADU plans for detached ADUs, each in cottage, modern and ranch styles: Plan A studio (408 sq ft), Plan B one-bedroom (615 sq ft), Plan C one-bedroom (672 sq ft) and Plan D two-bedroom (796 sq ft). The plans can't be revised, but they can be mirrored or turned in any direction.

As of October 2026, the city says: "PRADU Plans Currently Unavailable: The City is currently updating its PRADU plans to comply with 2025 Building Code requirements." Lots in a flood zone (other than FEMA Zone X) or with more than 5% slope can't use them. Designers can also submit their own detached ADU plans for city pre-approval. Details are on the PRADU page. [1][2][6]

Permit steps in Walnut Creek

1. Read the handbook and check your zoning

   Review the city's ADU Handbook and look up your lot on the Zoning Map. [1]

2. Check utility requirements

   Contact your water, sanitary and other utility districts to see whether connection fees will apply. [1]

3. Meet with a city planner (optional)

   Book a meeting with a planner or email DutyPlanner@walnutcreekca.gov. The city says this optional review often leads to faster approval. [1]

4. Apply online for a building permit

   Apply through the city's online permit portal; paper and emailed applications are not accepted. For a JADU, record a deed restriction before the permit can be issued. [1][7]

5. Get outside agency approvals

   Your plans also need Contra Costa County Fire Protection District and Central San approval. [2][6]

6. Pay fees and print your permit

   Pay in the permit portal's shopping cart. Once approved, print the permit card from the portal. [1]

7. Build and get your address

   The Planning Division issues the new ADU an address using the main home's address plus the letter "A". Build, pass inspections and get your certificate of occupancy. [6]

Pre-approved and sample plans

• Pre-Reviewed ADU Plans (PRADU)

  Four free city plans for detached ADUs from 408 to 796 sq ft; currently being updated for the 2025 Building Code. [2][6]

Who to call

• Walnut Creek Duty Planner

  ADU zoning questions, PRADU and multifamily ADUs

  925-256-3558

  1666 North Main Street, Walnut Creek, CA 94596

  [1]

• Walnut Creek Building Division

  Building permits and general permit help

  925-943-5834

  [7]

Questions people ask