An ADU is a secondary dwelling unit with complete, independent living facilities for one or more persons and generally takes three forms:

•   Detached: The unit is separated from the primary structure

•   Attached: The unit is attached to the primary structure

•   Repurposed Existing Space: Space (e.g., master bedroom) within the primary residence is converted into an independent living unit

•   Junior Accessory Dwelling Units: Similar to repurposed space with various streamlining measures

ADUs offer benefits that address common development barriers such as affordability and environmental quality. ADUs are an affordable type of home to construct in California because they do not require paying for land, major new infrastructure, structured parking, or elevators. ADUs are built with cost-effective one or two story wood frame construction, which is significantly less costly than homes in new multifamily infill buildings. ADUs can provide as much living space as contemporary condos being built in new infill buildings, and serve very well for all types of households.

ADUs are a different form of housing that can help California meet its diverse housing needs. Young professionals and students desire to live in areas close to jobs, amenities, and schools. The problem with high-opportunity areas is that space is limited. There is a shortage of affordable units, and the ones that are available are out of reach for many people. To address the needs of individuals or small families seeking living quarters in high opportunity areas, homeowners can construct an ADU on their lot or convert an underutilized part of their home (like a garage) into a junior ADU. This flexibility benefits not only people renting the space, but the homeowner as well, who can receive an extra monthly rent income. ADUs give homeowners the flexibility to share independent living areas with loved ones; they allow seniors to age peacefully with room for extra care and help bring extended family members together while maintaining privacy.

Relaxed regulations and the cost to build an ADU make it an easy, affordable housing option. A UC Berkeley study noted that one unit of affordable housing in the Bay Area costs about $500,000 to develop, whereas the highest price for an ADU goes up to approximately $200,000.

ADUs are a critical form of infill-development that are affordable and offer important housing choices within existing neighborhoods. ADUs are a powerful type of housing unit because they allow for different uses and serve different populations: ranging from students and young professionals to young families, people with disabilities, and senior citizens. By design, ADUs are more affordable and can provide additional income to homeowners. By encouraging the development of ADUs, local governments can improve access to jobs, education and services for many Californians.