About Accessory Dwelling Units

Across the United States, but especially in California, communities are experiencing challenges in housing affordability. One potential solution is increasing the production of accessory dwelling units (ADUs), which are separate small dwellings embedded within single-family residential properties. ADUs are a low-cost and readily implementable approach to infill development, particularly in high-cost cities characterized by little to no vacant land and an abundance of single-family lots.

However, barriers such as lot regulations and fees can make ADU construction physically impossible. Thus, California has launched a concerted effort to push its jurisdictions to design ADU ordinances that facilitate production. Starting in 2016, state legislators began enacting a new set of reforms that addressed parking requirements, setbacks, fees, and other barriers. This website introduces an ADU scorecard that aims to evaluate the robustness of ADU ordinances in over 200 CA jurisdictions based on an A to F grading system. 

The scorecard accounts for 15 different ADU requirements, assessing their compliance with state law and user-friendliness for homeowners. The scorecard can help cities shape ADU requirements by highlighting ADU ordinance practices in ‘good-grade’ jurisdictions. Note that these are interim grades that will be updated on an ongoing basis as jurisdictions improve their ordinances to comply with new state legislation. The current grades do not reflect the municipalities’ compliance with the 2019 ADU legislation that became effective on January 1, 2020.

Key Findings

  • The mean and median ADU ordinance grade in California is C+. Not surprisingly, grades varied across the state.
  • California’s rural region did relatively well, with an average B- score among its jurisdictions.
  • There is a positive correlation between a jurisdiction’s grade and how many ADUs it permitted in 2018. Many of the bigger municipalities that have already amped up their ADU production got scores in the A- or B range. For example Los Angeles County, with an A-, permitted 706 ADUs in 2018, and San Francisco, with an A-, permitted 364.
  • Across California, jurisdictions tend to score points for allowing generous ADU sizes, as well as including detailed language such as about junior ADUs (JADUs).
  • Jurisdictions lost points for adding additional layers of review or entitlements, or charging extra fees ranging from impact fees for parks, to fees for utility connections, to permitting fees. In general, Northern Californian jurisdictions perform worst in terms of these onerous requirements.
  • Jurisdictions in the southern half of the state adopt a variety of other measures that may slow ADU production.
    • Inland Empire jurisdictions stand out for high minimum lot sizes, low height limits, challenging setback regulations, and arduous parking requirements.
    • Height limits are also problematic in Los Angeles County jurisdictions, and high minimum lot sizes are a barrier in neighboring Orange County jurisdictions as well.
    • Jurisdictions in the Capitol and San Diego regions do not do well in terms of height limits and parking requirements.
    • Many jurisdictions in the Central Valley and rural parts of the state have ordinances which are extremely vague, and others have overly strict setback regulations.
    • Almost all jurisdictions in the state have owner occupancy requirements of some kind.

Case Study of Sebastopol:​

Sebastopol is a small city located in Sonoma County in Northern California. It is just 1.86 square miles in size, and in 2017 Sebastopol’s population was estimated at 7,666 residents.

Sebastopol Sonoma Case Study

Sebastopol runs a model ADU program, as exemplified by its overall permissiveness in terms of ADU construction and active promotion of ADUs. Its high-scoring ADU ordinance was first adopted in 2017 and later amended in 2018. It does not impose any parking requirements on new ADUs, exempts ADUs from the city’s residential lot coverage requirements, and allows two story ADUs to be constructed up to 25 feet tall. In addition to allowing ADUs in all residential zoning districts, Sebastopol also allows an ADU to be constructed on any parcel already containing an existing single-family home, regardless of whether or not that parcel is currently zoned for residential uses. The ordinance allows for the construction of junior ADUs.  Sebastopol does not require owner occupancy for either the primary unit or the standard accessory dwelling unit. Sebastopol’s 2018 amendments increased the maximum permissible unit size for ADUs proposed on lots greater than 10,000 square feet from 840 square feet to 1,000 square feet. Additionally, Sebastopol is already in the process of updating its ordinance to comply with the new 2019 state ADU legislation.

The City developed supporting documents making the ADU requirements more user-friendly and easier to understand for property owners. Notably, these documents contain detailed information about the types of fees, e.g., sample fee amounts for an 840 square foot unit, that a property owner should expect to pay upon Building Permit issuance. In addition to developing online resources, Sebastopol collaborated with Sonoma County in 2018 to host a “Raise the Roof” housing fair and expo. This event helped spread awareness about ADUs and JADUs to 150 attendees, and demonstrates Sebastopol’s commitment to promoting ADUs as a viable tool to address the housing crisis. Since Sebastopol’s ADU program began, the City lowered the fees associated with both standard ADUs and Junior ADUs to make them more financially feasible to construct. The City’s ordinance also promotes manufactured and mobile home ADUs as a viable construction typology. According to Sebastopol’s Annual Housing Element Report, 16 ADUs were constructed in 2018.