ADUs, or Accessory Dwelling Units, are housing units that are built in addition to a main house. They provide individual living facilities like a bathroom, bedroom, and kitchen (CA Department of Housing and Urban Development). ADUs are sometimes also referred to as in-law suites, granny flats, or casitas. These units are a vital component of California’s strategy to meet the ever increasing demand for housing.

That’s okay! The state of California uses these model prototype buildings to represent the theoretical average for this building type. Individual buildings won’t match the prototype exactly.

The Cost-Effectiveness Explorer team estimates the number of new ADUs that will be built during the code cycle for each local jurisdiction in California. These estimates start from our estimates for new Single Family Units (learn how we projected single family and multifamily units here). Then we took the following steps:

1. We turned to the California Department of Housing and Development’s publicly available Housing Housing Element submissions (Housing Element Implementation and APR Dashboard) to determine the ratio of single family units to ADUs for each jurisdiction. We divided the number of proposed ADUs by the total number of proposed ADUs and Single Family Units. We calculated this ratio using data from the past four years, to ensure the most up to date trends are represented. If a jurisdiction’s information was not available, a statewide average ratio was applied.

2. Multiplying the number of projected single family units by the ratio of single family homes to new ADUs in step 1 to determine the number of projected ADUs.

You can adjust these forecasts by:

You may notice that we do not have a separate category for existing ADUs, and that instead they are included in the overall number of existing single family units. This is because ADUs are considered a separate category only for new construction, during which time they may experience special treatment such as priority permit processing, permit discounts, etc. Once an ADU has been constructed and becomes an “existing building”, it then loses this special status and will be treated as a standard single family unit.

While ADUs come in many different forms, the 2022 Single Family New Construction Study looked solely at detached ADUs. The prototype ADU used in this study has 625 sqft, one story, one bedroom, and a 20% window-to-floor area ratio.