Accessory Dwelling Units (ADU)

"Accessory dwelling unit" or "ADU" means an attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence. An accessory dwelling unit also includes the following:

  • An efficiency unit, as defined by Section 17958.1 of the California Health and Safety Code
  • A manufactured home, as defined by Section 18007 of the California Health and Safety Code

"Junior accessory dwelling unit" or "JADU" means a residential unit that:

  • Is no more than five hundred (500) square feet in size
  • Is contained entirely within an existing or proposed single-family structure
  • Includes its own separate sanitation facilities or shares sanitation facilities with the existing or proposed single-family structure
  • Includes an efficiency kitchen, as defined in subsection (C)(5) of this section
  • Includes a separate entrance from the main entrance to the structure

Additional Resources

Napa Sonoma ADU Standard Plans Program

The City of St. Helena is excited to partner with Napa Sonoma ADU and other jurisdictions around Napa and Sonoma counties to support residents in planning for and building ADUs. The Napa Sonoma ADU Standard Plans Program provides property owners with easy access to dozens of "off the shelf" ADU plans that can be purchased at low cost, saving thousands of dollars and months of time over a custom design. Plan options include site-built and prefabricated designs of various sizes, including "pre-reviewed" plans.

Pre-Reviewed Plans

Pre-reviewed plans include prefabricated ADUs that have already been reviewed by state or federal code reviewers as well as site-built ADU plans that have been reviewed by a third-party reviewer, BPR Consulting Group, for compliance with applicable building codes. Visit the program's FAQs page for more information.

Pre-Reviewed ADU Plans in the City of St. Helena

The staff has not reviewed any of the plans for code compliance. We will accept plans that have been reviewed and approved by BPR (or other approved third-party plan review). When using the Napa Sonoma ADU Standard Plans gallery you can select our jurisdiction to see the plans that we consider to be pre-reviewed.

Remember that even pre-reviewed plans must be submitted along with additional information to receive a building permit. Please visit our ADU permits page for more information.

Why Use a Pre-Reviewed Plan

Because these plans have already been reviewed for code compliance, the Building plans will not need to be reviewed again; however, the project will still need to be reviewed and approved by the Building Department for location on the lot, utility connections (including lift-station approval if needed), fire setbacks and fire sprinklers (if required), Planning Department and Public Works approval. The typical plan review for most projects is 28 days. Pre-reviewed ADU plans will be considered a Rapid Permit which will be a 14-day review time period.

Due to site- or project-specific issues or staffing, these turn-around times may vary, and additional costs may apply if non-standard issues arise that require additional information, time, and attention.

First Steps

Regardless of whether you choose a pre-reviewed plan, another standard plan, or develop a custom plan, it is essential that you determine in advance the size of ADU you can build on your property and other factors that might limit or define necessary features of your ADU. You can do this by contacting the folks at Napa Sonoma ADU and/or by making an appointment with our Building Permit Technician.

Webinar of Real-World ADU Stories From Your Neighbors

Presented by the Napa Sonoma ADU Center, a nonprofit organization fiscally sponsored by Napa Valley Community Foundation with support from Community Foundation Sonoma County.