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ADUs (Accessory Dwelling Unit)

What is an ADU?

An ADU is an accessory dwelling unit that is built (or converted) with independent living facilities for one or more occupants. ADU's are classified in the following ways:

  • Attached - the unit is attached to the primary residence

  • Detached - the unit is separated from the primary residence

  • Converted Existing Space - an existing space (e.g. accessory structure on lot, master bedroom, attached garage, detached garage, storage area) that has been converted to an independent living unit.

  • Junior Accessory Dwelling Unit (JADU) - conversion of an existing space which is contained entirely within an existing or proposed residence

Rise and Importance of ADUs


The production of housing in California has been unable to keep up with rising population and demand. By 2018, the state of California ranked 49th in housing units per resident. This lack of housing impacts affordability and raises housing costs, most notably for renters.

ADUs are typically significantly cheaper to build and provide benefits which help address common housing production barriers such as affordability and environmental impact. Considering the rising cost of housing in California they also provide those searching for housing a more affordable option.

ADU Frequently Asked Questions

When building an ADU on top of an existing detached garage, is this an addition?

Yes. Regardless of whether the shared floor/ceiling of an existing structure is conditioned this would be considered an addition.

Is it still an addition if the new ADU is built side-by-side with the existing garage?

Yes. An ADU sharing a common wall with an existing garage is considered an addition.

When a newly built ADU is attached to the existing home by a breezeway, or covered walkway, is this an addition?

No. Because it does not share a common wall or common (or adjacent) ceiling/floor this is considered new construction.

When converting existing conditioned space, like a conditioned basement, into an ADU or junior ADU, is this an addition?

No. This would be considered an alteration.

(Information excerpted from California Energy Commission (CEC) Blueprint and CA Housing and Community Development ADU Handbook)


Building or converting an ADU?

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