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.

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.

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 from 2023 through 2025 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. Initially, this ratio was calculated using only the most recent year’s data (2021) to best represent the increasing popularity of ADUs, but found that many jurisdictions then had no projected ADUs. To correct for this, we instead calculated this ratio using data from all four years available (2018, 2019, 2020, and 2021). If a jurisdiction’s information was not available, a statewide average ratio of 0.37 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.

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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.

We predict new construction volume for the years 2023 to 2025, the time period for which the 2022 Code Cycle will be in place. The cost-effectiveness results presented in the current round of state-wide new construction studies are only applicable for the 2022 Code Cycle, as these studies use the 2022 code as the baseline against which to compare other packages. Also, new construction reach codes that amend the energy code automatically expire after 2025. While other local policy types will remain in effect beyond 2025, policy impact projections past 2025 would have a much higher degree of uncertainty. Policy impact modeling requires one to assume a baseline or business-as-usual case, which is hard to do without knowing what future building code cycles will specify.