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