Accessory Dwelling Units (ADUs)

Accessory Dwelling Units (ADUs) are known by many names like granny units, in-law units, second units, backyard cottages and Junior Accessory Dwelling Units (JADUs). 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).  

Are you interested in learning more about building an ADU?

Napa Sonoma ADU ( is a non-profit agency assisting homeowners with navigating the process of building an ADU on their property. They provide a full range of services, from initial planning and feasibility consults to supporting you during your design process, including cost calculation, design plans, and connecting you with subject matter professionals such as architects, civil engineers, and building contractors, when needed.

Napa County Affordable ADU Funding Programs

Napa County's Housing and Homeless Services offers funding assistance programs for eligible ADU projects that will charge below market rate rent to qualified low or very low-income tenants. To learn about available programs, please contact Housing and Homeless Services

Approximate Building Permit Fees for ADUs in Unincorporated Napa County

For ADUs in the Unincorporated County, estimated costs are provided for two ADU examples on the provided ADU Estimation (PDF). To understand the requirements for a building permit, please review the information on our Building Division site.

Do you have questions regarding California Senate Bill 9 Urban Lot Splits?

Napa County is preparing an implementing ordinance to address California Senate Bill 9 (SB 9) that became effective January 1, 2022, and which allows within certain limited areas two-unit developments on single family residential properties or a subdivision of a single-family residential lot into two lots. Please refer to Napa County information regarding SB 9 Urban Lot Splits (PDF).