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Guide

ADU Basics in Contra Costa County

By Karla I. Vasquez, REALTOR® · Keller Williams Realty Danville · Reviewed October 2026

Quick answer

An ADU (accessory dwelling unit) is a second, complete home on a residential lot: a backyard cottage, an addition, or a converted garage. California law makes ADUs much easier to approve than they used to be, with limits on what cities and the County can require. In Contra Costa County, your exact rules depend on whether your home is in a city (like Danville or Concord) or in an unincorporated area (like Alamo), so always start with the right permit office.

What counts as an ADU or JADU?

Contra Costa County describes an ADU as a unit on a permanent foundation with independent exterior access and complete living facilities: a place to live, sleep, eat, cook and use the bathroom.

A Junior ADU (JADU) is smaller (up to 500 square feet) and built within the existing home, including an attached garage. A JADU can share a bathroom with the main house.

What does state law allow?

California's Department of Housing and Community Development (HCD) publishes the ADU Handbook, most recently updated in March 2026 with an addendum on laws effective January 1, 2026. Key points from the handbook:

  • More than one unit: On a single-family lot, local agencies must allow at least a converted ADU, a JADU and a new detached ADU, under the state's streamlined rules.
  • Size: Local rules can't block a detached ADU of at least 800 square feet with 4-foot side and rear setbacks.
  • Height: Local rules must allow detached ADUs of at least 16 feet, with more height near major transit, and attached ADUs of up to 25 feet or the main home's height limit, whichever is lower.
  • Parking: No more than one space per unit or bedroom, and no parking required in many cases, including within a half-mile of public transit. A garage converted into an ADU doesn't have to be replaced.
  • Impact fees: ADUs of 750 square feet or less (and JADUs) are exempt from impact fees.
  • Owner-occupancy: Generally not required for ADUs. JADUs that share a bathroom require the owner to live on site.
  • Rentals: ADUs and JADUs can't be rented for 30 days or less.

How long does an ADU permit take?

For a lot with an existing home, HCD's handbook says the agency must approve or deny a complete ADU application within 60 days, using objective standards. Completeness should be determined within 15 business days. Design, engineering, utility and fire review can add time before your application is complete, so plan ahead.

Who approves my ADU in Contra Costa?

  • Unincorporated areas (Alamo, Kensington and others): Contra Costa County Department of Conservation and Development. The County notes ADU applications may need to follow its All-Electric Building Ordinance, and homes on septic systems need Environmental Health review.
  • Pre-approved plans: The County offers pre-approved ADU plans for unincorporated areas, and provides building review for Clayton, Hercules, Lafayette, Moraga and Orinda. You still need site-specific permits.
  • Danville: The Town's Garden Cottage Program offers free permit-ready plans in studio, one-bedroom and two-bedroom options, with an expedited review.
  • Other cities: Check with your city's planning and building department.

Karla's tips

  • Check the site before the floor plan. As Owner & Principal of KCK Construction & Remodeling, I look first at access, slope, utilities, sewer or septic, trees and drainage. Those often drive cost more than square footage.
  • Pull the existing permits. If the main house has unpermitted work, it can complicate your ADU application.
  • Price it fully. Include design, engineering, utility connections, permit fees and site work, not just construction.
  • Buying for an ADU? Before you make an offer, we can look at the lot together and talk to the permit office about what's realistic.
  • Spanish-speaking families: ADUs are a great fit for multigenerational living. Te explico todo en español: (510) 677-8695.

En español: lo básico

Un ADU es una segunda vivienda completa en un terreno residencial, como una casita en el patio, una ampliación o un garaje convertido. La ley de California facilita su aprobación: en general se permite un ADU de al menos 800 pies cuadrados con retiros de 4 pies, y las agencias deben aprobar o negar una solicitud completa en 60 días. En Contra Costa, si su casa está en un área no incorporada como Alamo, el Condado aprueba el permiso; si está en una ciudad como Danville, la ciudad lo aprueba. Antes de diseñar, revise el terreno, los servicios y los permisos existentes.

Sources

General information only, not legal, tax or financial advice. Rules and fees change; confirm with the County, the Board of Equalization, your tax professional or an attorney. Karla I. Vasquez is a REALTOR® and Registered LDA, not an attorney.