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CA ADU Overview

M. Corbin Jones
April 23, 2023

What is an ADU?

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

California’s housing production is not keeping pace with demand. In the last decade, less than half of the needed housing was built. This lack of housing is impacting affordability with average housing costs in California exceeding the rest of the nation. As affordability becomes more problematic, people drive longer distances between the regions that are affordable and where they work or double up to share space. Both scenarios reduce the quality of life and produce adverse environmental impacts.

Beyond traditional market-rate construction and government subsidized production and preservation, there are alternative housing models and emerging trends that can contribute to addressing home supply and affordability in California. One such example gaining popularity are Accessory Dwelling Units (ADUs) (also referred to as second units, in-law units, or granny flats).

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, significant 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 the new apartments and condominiums being built in new infill buildings and serve very well for couples, small families, friends, young people, and seniors.

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 regions is that space is limited. There is a shortage of affordable units, and the units that are available can be 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 just people renting the space, but the homeowner as well, who can receive an extra monthly rental income.

ADUs give homeowners the flexibility to share independent living areas with family members and others, allowing seniors to age in place as they require more care and helping extended families to be near one another while maintaining privacy.

Relaxed regulations and the cost to build an ADU make it a very feasible 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 an ADU can range anywhere up to $200,000 on the expensive end in high housing cost areas.

ADUs are a critical form of infill-development that can be affordable and offer essential housing choices within existing neighborhoods. ADUs are a dominant type of housing unit because they allow for different uses, and serve diverse 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. Local governments can encourage the development of ADUs and improve access to jobs, education, and services for many Californians.


Understanding Accessory Dwelling Units and Their Importance.