Accessory Dwelling Units (ADUs)

1. ADUs are permitted in all single-family and multi-family zoning districts.

2. ADUs must have permanent and independent provisions for living, sleeping, eating, cooking and sanitation.

3. ADUs must be placed on a solid foundation, such as a concrete pad.

4. ADUs can be made from pre-fabricated materials or created from a manufactured home. See ACMC Section 19.10.130 for detail.

5. Along with the primary dwelling unit, a property owner can have one attached ADU, one detached ADU, and one Jr. ADU.

The four main types of ADUs are (1) attached, (2) detached, (3) conversion and (4) Junior.

1. Attached ADUs create a separate living unity by extending the exterior walls of the primary home on at least one side.  In addition to the expanded primary home footprint, an attached ADU may also include converting a portion of the existing floor area to the new ADU.
2. Detached ADUs are a separate structure on the same parcel. 
3. ADU conversions convert existing space in the home or garage to a separate living unit larger than 750 square feet. This definition also includes converting an existing detached structure of any size on the property.
4. Junior ADUs convert existing space in the primary dwelling into a separate living unit no larger than 750 square feet.

1. If the primary dwelling unit has fire sprinklers, the ADU is required to have fire sprinklers.

2. If the primary dwelling unit does not have fire sprinklers, the ADU is not required to have fire sprinklers.

3. There may be an unusual situation where fire sprinklers will be required when the primary dwelling unit does not have fire sprinklers.  Please contact George Apple, CSG Contractor for the American Canyon Fire Protection Agency at (916) 416-0332.  

There are two types of tiny homes: a tiny house on wheels, classified by the State of California as a recreational vehicle (RV) and a tiny house on a foundation, which may be considered an ADU. While tiny homes are an innovative way to add living space to your home, keep in mind the following standards:

1.    Tiny Homes are classified as an RV because they may not be built to standards that meet the building code for a living unit. Building Code examples that characterize a living unit include: structural, plumbing, electrical, energy, mechanical, and fire protection standards, as well as requirements for light, ventilation, heating, minimum room sizes, ceiling heights, sanitation, toilet, bath and shower spaces, emergency escape and rescue openings, means of egress, smoke alarms and carbon monoxide alarms.

2.    The Municipal Code does not permit people to live in an RV on a residential lot, so a tiny house built on wheels cannot serve as an ADU until it is attached permanently on site.

3.    Some tiny homes are as small as 80 square feet. An ADU must have a minimum living area of 150 square feet.

4.    All ADUs are required to meet the Building Code for a living unit (issues listed above), including a connection the primary dwelling or aseparate connection to City water and waste-water system. Use of a septic tank is not permitted.

1. Detached ADUs must be separated from the primary dwelling by at least 6 feet.
2. Detached ADUs must be a minimum of  150 square feet and a maximum of 1,200 square feet unless the detached ADU converts an existing detached structure that is greater than 1,200 square feet.  In that case, the maximum square footage equals size of the existing detached structure.  
3. Detached ADUs may not be more than 18 feet in height, with an additional 2 feet in height to accommodate a roof pitch.
4. Attached ADUs may not be more than 25 feet in height and may not exceed two stories.
5. Detached ADUs must be placed at least 4 feet from the side and rear property lines.  
6. Attached ADUs not exceeding 800 square feet must be placed at least 4 feet from any property line.
7. Development standards shall be waived to permit a detached ADU that is no greater than 800 square feet, 16 feet in height, and has 4-foot setbacks.  
8. Accessory structures converted to ADUs are subject to existing accessory structure setback requirements.

1. ADUs cannot be sold separately from the primary dwelling unit.

2. ADUs cannot be used for short-term rentals of less than 30 days (such as Air BNB or VRBO).

1. ADUs are subject to building permit fees, JADUs have a reduced plan-check fee rate.

2. ADUs are exempt from water and wastewater (sewer) capacity fees.

3. ADUs that are 750 square feet or less are exempt from all City impact fees.

4. ADUs that are over 750 square feet are subject to impact fees proportional to their size.

5. ADUs that are over 750 square feet are subject to Napa Valley Unified School District (NVUSD) facility fees. The fees are paid directly to NVUSD, and not through the City. ADUs that are less than 750 square feet will require proof of exemption from NVUSD. For more information, visit the following NVUSD page.

1. The dwelling must have its own exterior entrance.

2. The kitchen must include a sink, cooking appliance, counter surface and storage cabinets that meet minimum CBC standards.

3. The bathroom may be included in the unit or shared with the primary residence.

4. JADUs are exempt from the building code wall separation requirements with the primary residence.

The size of the ADU is measured by the livable space (or conditioned space). Porches, overhangs or decks for the ADU do not count towards the ADU size. For example, you can have a 1,200 square foot ADU with a 50 square foot porch with the appropriate building permit(s).

Solar panels are not required on a new dwelling, however, the California Energy code requires that all new dwelling be “solar ready”. This all depends on how the building is designed and whether the energy analysis is done prescriptive methods or per the performance measures.

Developing an ADU can be an exciting project, but becoming a landlord means starting a business with many new responsibilities. Along with setting the rent price and finding a tenant, the landlord will need to know how to get insurance, set up utilities and handle finances.

The Napa Sonoma ADU Center can help.  They have resources to assist new landlords  understand tenant  rules and regulations.

Another resource is the California Department of Consumer Affairs' California Tenants guide.  This guide provides information for tenants and landlords rights and responsibilities.

1. The Public Works Department requires an Erosion and Sediment Control Plan (ESCP) for all projects moving over 50 cubic yards of earth. Detached ADUs will most likely require an ESCP. For ESCP information, please contact the Public Works Department at: (707) 647-4458.

2. The Public Works Department requires an Encroachment Permit for any construction in the public right-of-way. If the ADU requires an altered or new driveway, the applicant will need to obtain an encroachment permit. Click here to apply for an Encroachment Permit online. For any other questions, please contact pwencroachment@americancanyon.gov. 

• Standard Project Information on Building Permit Application

  • Include a complete description of work, sheet index, designer's contact information and project location map. Include the square footage of any existing buildings on the property that will either remain or be demolished. Include the lot coverage calculation and the area of the new ADU.

• Site Plan

  • Provide dimensions from the main building to the proposed structure and to the property lines.

• Construction Details

  • Provide details as necessary to describe the method of construction for all building elements.

• Structural Calculations (If needed per design)

  • Structural calculations will be required for ADUs. If you plan on installing a pre-fabricated unit, provide the manufacturer's spec-sheets.

Visit the Building Permit Application checklist page to access the application and more information.

Property-owners may build an ADU themselves.  However, because an ADU is a new living unit, the ADU plans must comply with the Building Code.  The Building Code is a complicated State Regulation so unless you are familiar with the Building Code, it is better to hire a professional who has this knowledge.

The Owner-Builder building permit application form is available online HERE.

ADUs may increase your property tax, but the City does not determine the amount. For more information, check with the Napa County Assessor’s Office.

Yes, a new address is required, through the Address Assignment Request Form.

The property owner can install separate water, and wastewater, utilities, but it is not required. Additional utility lines will require new meters, which require additional fees. For more information on water and wastewater utilities, please contact Edison Bisnar, Senior Civil Engineer, by email at ebisnar@americancanyon.gov or by phone at 707-647-4588.

Please contact PG&E regarding their current rules for electric utilities for ADUs.  PG&E has an information brochure for new and expanded electric utility service.  Click HERE.

Contact the Community Development Department at (707) 647-4348 if you have any questions.