Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are small dwelling units allowed in conjunction with another primary dwelling unit in most residential, mixed-use, or agricultural zones. This webpage features County informational resources and checklists that will assist homeowners through the ADU permitting process.

PJR-149 Accessory Dwelling Unit Checklist (PDF)
Use the Permit Sonoma ADU Checklist to assemble the necessary information for building permit application submittal.

California Housing Finance Agency (CalHFA) Grant Program
The CalHFA operates a grant program to provide qualifying homeowners with reimbursement of costs, including site preparation, designs, permits, fees, and reports.

Napa Sonoma ADU Center
The is an organization that assists homeowners with the ADU planning process within Napa and Sonoma County jurisdictions. The center offers educational webinars, personalized feasibility consultations, a home match program, a gallery of plan sets and more.

Accessory Dwelling Units (ADUs) are known by many names like granny units, in-law units, second units, and backyard cottages. An ADU is a small dwelling unit that may be established in addition to single-family or multifamily dwellings in residential, mixed-use, or agricultural zones. An ADU may be detached, attached to the primary dwelling or other accessory structure, or located within a main dwelling and may involve new construction or conversion of an existing structure (such as a detached garage or other accessory structure, or a portion of a dwelling).

A Junior Accessory Dwelling Unit (JADU) is a specific type of ADU that is built within an existing residence, utilizes an existing bedroom, and has a small efficiency kitchen.
More Information on JADUs

Compare Accessory Dwelling Units and Junior Accessory Dwelling Units

Multiple permits are usually required to establish an ADU. The most common permit types are for sanitation, water, and building. Some considerations for each permit type are described below.

Step 1: Sanitation

Before applying for a building permit, find out whether there are any additional requirements related to wastewater disposal.

• Do you have capacity in your current septic system or will you do a "bedroom swap" to create capacity for the new unit? Contact a consultant to analyze the capacity and quality of your current system, prepare a findings report, or prepare an application for a bedroom swap.
• Will you need to build a new septic system? Contact a consultant for professional help to design and apply for a permit for a new system.
• Do you have sewer service? Make sure your sewer provider will provide service to the new unit and contact a professional for help preparing an application for a sewer permit if one is required.

If sewer or septic permits are needed, these permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU.

Step 2: Water

• Before applying for a building permit, find out whether there are any additional requirements related to water supply. Water requirements vary based on the type of water service and location.
• Will the ADU be served by a well in a water-scarce area (see above)? Contact a professional to conduct a dry weather well test ;(conducted between July 15 and October 1) demonstrating that the well meets minimum water yield requirements according to the County’s Dry Weather Well Testing policies. Find your parcel’s Groundwater Availability Area classification in the parcel report.
• If the well is located in a Class 4 Groundwater Availability Area or a Critical Habitat Area, the applicant must demonstrate compliance with Permit Sonoma Policy and Procedure 8-2-2 Guidelines for Net Zero Groundwater Use.
• need a new well? Contact a professional for help preparing an application for a new well.
• Do you have public water service? Check with your provider to make sure they will provide service to the ADU.

Well permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU. Well yield studies should be completed and submitted to the Well & Septic Division prior to applying for a building permit. 

Step 3: Building Permit

At minimum, newly constructed ADUs require a building permit, but depending on the scope of construction, a site review, grading permit, and/or encroachment permit may also be required. 

Apply for a building permit online

Most permit applications are submitted and processed electronically. Visit our online permitting website to start and submit a building permit or other development permit application, pay fees, schedule an inspection, and more.

Please review and ensure you have the minimum submittal requirements for residential building permits.

All property owners must sign and submit the Accessory Dwelling Unit Rules and Performance Standards with the building permit application.

Building an ADU will have a variety of fees associated with the process. 

Contact Permit Sonoma divisions directly with questions.

Find Division Contact Information