What is an ADU?

ADU means accessory dwelling unit: extra living quarters on the same lot as a house, independent of the main home. HUD’s June 2008 case study distinguishes attached and detached units. For your project, identify whether you want a separate building or an addition before asking a contractor to describe the scope.

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By InstaBids editorial team with AI research agents. Updated 28 October 2026

What this means for your home project

Decide whether your proposed living space would be a separate building; ask the contractor to label it detached in the project description. Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28.
If the space would be an addition to your house, ask the contractor to describe it as an attached unit rather than a separate building. Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28.
In California, ask which site plan and building plans the local agency will inspect for your application. Source: California Department of Housing and Community Development, read 2026-10-28.

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What does ADU mean?

ADU stands for accessory dwelling unit: a second, self-contained home on the same lot as a house, known by several everyday names. “Accessory dwelling units (ADUs) — also referred to as accessory apartments, second units, or granny flats — are additional living quarters on single-family lots that are independent of the primary dwelling unit.” Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28. “ADUs have been known by many names: granny flats, in-law units, backyard cottages, secondary units and more.” Source: California Department of Housing and Community Development, read 2026-10-28.

Detached ADU

A detached ADU stands as its own building on the lot; the other kinds are attached to the house or converted from space that already exists. “Detached ADUs are structurally separate from the primary dwelling.” Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28. “They can be constructed over existing accessory structures, such as a detached garage, or they can be built as units that are separate from accessory and residential structures.” Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28. “Attached ADUs are living spaces that are added on to the primary dwelling.” Source: U.S. Department of Housing and Urban Development (HUD USER), read 2026-10-28. In California, “Converted ADUs are dwelling units that are created from existing space within proposed or existing primary dwelling units or existing accessory structures.” Source: California Department of Housing and Community Development, read 2026-10-28.

Building an ADU

In California, building an ADU starts with a permit application that the local agency reviews against fixed, written standards rather than case-by-case judgment. “A ministerial review ensures that the permit application meets all the applicable objective standards effective at the time of the review and uses no discretionary judgment, opinion, or subjective standards.” Source: California Department of Housing and Community Development, read 2026-10-28. “Agency staff inspect the submitted application, site plan, and building plans for compliance with applicable standards.” Source: California Department of Housing and Community Development, read 2026-10-28.

ADU size

In California, a city or county may cap ADU size, but the cap has to allow at least 850 square feet, or 1,000 square feet with more than one bedroom. “A local government may, by ordinance, establish minimum and maximum unit size requirements for both attached and detached ADUs; however, maximum unit size requirements must allow an ADU of at least 850 square feet of interior livable space, or 1,000 square feet of interior livable space for ADUs with more than one bedroom.” Source: California Department of Housing and Community Development, read 2026-10-28. “An efficiency unit may be as small as 150 square feet in floor area (HSC, § 17958.1).” Source: California Department of Housing and Community Development, read 2026-10-28.

ADU requirements

In California, the legal definition requires an ADU to offer complete, independent living facilities on a lot that has, or will have, a primary home. “An attached or a detached residential dwelling unit that provides complete, independent living facilities for one or more persons, and is located on a lot with a proposed or existing primary residence.” Source: California Department of Housing and Community Development, read 2026-10-28. “It shall include permanent provisions for living, sleeping, eating, cooking, and sanitation on the same parcel as the single-family or multifamily dwelling is, or will be, situated.” Source: California Department of Housing and Community Development, read 2026-10-28.

JADU vs ADU

In California, a junior ADU is capped at 500 square feet and must sit inside a single-family home, while an ADU can be attached or detached. “A “junior accessory dwelling unit” or JADU is a unit that is no more than 500 square feet of interior livable space in size and contained entirely within a single-family residence.” Source: California Department of Housing and Community Development, read 2026-10-28. “A JADU may include separate sanitation facilities or may share sanitation facilities with the existing structure.” Source: California Department of Housing and Community Development, read 2026-10-28. “No, JADUs are not allowed in accessory structures.” Source: California Department of Housing and Community Development, read 2026-10-28.

How long does it take to build an ADU?

In California, state law sets a 60-day clock for the permitting agency once it has a completed application; the sources read here give no construction time. “Obligates a permitting agency, when it denies an ADU or JADU application, to “return in writing a full set of comments to the applicant with a list of items that are defective or deficient, and a description of how the application can be remedied by the applicant” within 60 days from when a completed application is received.” Source: California Department of Housing and Community Development, read 2026-10-28.

Sources

Accessory Dwelling Units: Case Study — U.S. Department of Housing and Urban Development (HUD USER); published June 2008; read 2026-10-28.
Accessory Dwelling Units — California Department of Housing and Community Development; read 2026-10-28.
Accessory Dwelling Unit Handbook — California Department of Housing and Community Development; published March 2026; read 2026-10-28.