Accessory Dwelling Units

An accessory dwelling unit (ADU) is an independent living unit located on a lot where there is already an existing primary residence, or where a new primary residence is proposed along with the ADU. 

The City of Mill Valley encourages the construction of accessory dwelling units. ADUs are an important strategy to increase the diversity of housing options available in Mill Valley. ADUs are generally smaller than primary residences and offer studio, one- and two-bedroom housing options. These small units can function as rental housing or accommodate multi-generational families on one property. 

Mill Valley Planning & Building staff is here to help and support you through ADU design and construction. 

ADU Regulations

ADUs are subject to different zoning regulations compared to other types of buildings. These ADU regulations are provided in Mill Valley Municipal Code Chapter 20.90 as well as California Government Code sections 65852.2 and 65852.22

An ADU can be attached to, converted from part of, or detached from the primary residence. An ADU must include a kitchen, bathroom, and bedroom area, and must have its own exterior entrance. 

There are three types of ADUs (not including JADUs): 

  1. Detached ADU – A new construction ADU that does not share any walls with the primary dwelling and is separated by at least 6 feet. Detached ADU height is limited to 16 feet unless it is above a garage, in which case the maximum height is 25 feet. There is a minimum 4-foot setback from the side and rear property lines and a minimum 15-foot setback from the front property lines.
  2. Attached ADU – A new construction ADU that shares wall(s) with the primary dwelling. Attached ADUs can be up to 25 feet tall. Setback distances are the same as for detached ADUs.
  3. Conversion ADU – An ADU converted from existing space in a primary dwelling or accessory building. 

Maximum ADU size is dependent on the type of ADU, the number of bedrooms proposed, and the maximum allowed building area on the property. However, regardless of existing site conditions, an 800-square-foot ADU is allowed. 

A Junior ADU (JADU) is a small ADU (maximum 500 square feet) converted from existing space in a primary dwelling. Compared to ADUs, JADUs are less independent, with efficiency kitchens rather than full kitchens, and the ability to have internal connection and share bathrooms with the primary dwelling.  

Mill Valley allows one ADU or JADU per single-family property, with one ADU plus one JADU allowed under certain conditions.  On properties with multifamily housing types such as apartment buildings, multiple ADUs are allowed subject to California Government Code section 65852.2.

See Mill Valley Municipal Code Chapter 20.90 for a full listing of ADU regulations. Exceptions to standards can be permitted through the Design Review process. 

ADU Approval Process 

All ADU construction projects require a building permit, which is processed online:

Some ADU projects also require special approval from the Planning Division as a first step before the building permit application. These types of ADUs include: 

  • New construction ADUs larger than 800 square feet
  • ADUs that do not meet one or more objective development standards
  • ADUs that require tree removal triggering a tree removal permit
  • ADUs that are proposed as part of a larger project that requires Planning approval

ADU Forms 

Other Resources

More resources available at ADUMarin for inspiration, design, construction, and operation of ADUs: