Accessory Dwelling Units (ADUs)

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).  Read more about Second Dwelling Units.

Steps to Completing an ADU

Developing an ADU involves three basic steps: 1) Design your project; 2) Submit and obtain a building permit; and 3) Build your project. Learn more about the development process here: Your Pathway to a Completed ADU and Building Plan Review Process web page.

Are you a Do-It-Yourself Builder/Homeowner?

If so, it is highly recommended you reach out to the friendly folks at Napa Sonoma ADU (www.NapaSonomaADU.org). The mission of Napa Sonoma ADU is to help homeowners build ADU’s.  They provide a full range of support to do-it-yourself ADU developers, from initial planning and feasibility consults to supporting you during your design and building process, including connecting you with subject matter professionals such as architects, civil engineers, and building contractors when needed.

Need Help from County Staff?

If you have questions about zoning requirements for ADU’s, or are submitting an ADU building permit and need assistance processing your application, the following members of the Planning Division are available:

[email protected] or 707-299-1334
[email protected] or 707-299-1331
[email protected] or 707-299-1354

Planning Division staff also assist applicants in resolving any issues that occur during the building permit processing to expedite issuance of your building permit.

Expedited Permits for Guaranteed Low or Very Low Income Affordable ADU’s

Napa County Division of Housing and Intergovernmental Affairs offers loan assistance for ADU projects that will charge below market rate rent to qualified low or very low income tenants.  In turn for the loan assistance, a deed restriction is recorded on the property that requires the below market rate rent.  Processing of building permit applications for guaranteed affordable ADU projects will be expedited through the County’s Quick Permit building review process.

For more information, please contact John McDowell and Housing and Intergovernmental Affairs

Approximate Building Fees for ADU Units

Review the Building Check List of the preparation that will be required and then complete our Residential Permit Application.

Try the Sonoma/Napa Accessory Dwelling Unit Calculator  (Note: Calculator works in Firefox and Chrome)

The valuation tables below are only estimated cost for the various types of accessory dwelling units in Napa County.  You can also print the ADU Estimation tables below from the PDF.

Rebuild based on $200,000 valuation - 500 Sq. Ft. Fees
Inspection – Fire Standard Fee 453.64
Plan Review – Fire Fee 294.87
Plan Review – Environmental Health Fee 154.00
Plan Review – Engineering Fee 493.00
Inspection – Engineering 187.00
Residential – New Dwelling – Planning Fee 1724.00
Plan Review – Public Works 93.00
Plan Review -Energy 256.61
Plan Review -Standard 1796.29
Inspection -New Dwelling -Plumbing 318.20
Inspection -New Dwelling -Mechanical 228.39
Inspection -New Dwelling -Electrical 207.86
Imaging Plan Retention - Residential 15.00
State Strong Motion Residential 26.00
State Building Standards Surcharge 8.00
Permit Issuance ($32.70) 71.00
Building Inspection Fee - Standard 2566.13
Certified Access Specialist Program: Fund 4.00
General Plan Surcharge 292.35
TOTAL

9,189.34

*Fees are estimated only, actual fees may change  
Rebuild based on $500,000 valuation - 1200 Sq. Ft. * Fees
Inspection – Fire Standard Fee 942.04
Plan Review – Fire Fee 612.33
Plan Review – Environmental Health Fee 154.00
Plan Review – Engineering Fee 493.00
Inspection – Engineering 187.00
Residential – New Dwelling – Planning Fee 1,724.00
Plan Review – Public Works 93.00
Plan Review -Energy 347.81
Plan Review -Standard 2,434.69
Inspection -New Dwelling -Plumbing 431.29
Inspection -New Dwelling -Mechanical 309.55
Inspection -New Dwelling -Electrical 281.73
Imaging Plan Retention - Residential 15.00
State Strong Motion Residential 65.00
State Building Standards Surcharge 20.00
Permit Issuance 71.00
Building Inspection Fee - Standard 3,478.13
Certified Access Specialist Program: Fund 4.00
General Plan Surcharge 381.96
TOTAL

12,045.53

* May be subject to Housing Impact fees  
*Fees are estimated only, actual fees may change