An accessory dwelling unit (ADU) is a secondary, independent living space located on the same property as a primary residence or residential development. Depending on local rules, an ADU may be attached to the main home, constructed as a detached backyard unit, or created by converting existing space such as a garage or part of a house. Some jurisdictions also recognize smaller categories such as junior ADUs.
ADU permit requirements vary substantially by city and state. Local zoning rules commonly determine where an ADU can be built, how many units are allowed, and standards such as setbacks, height, lot coverage, and placement. For example, Austin ties ADU eligibility and size to zoning and property characteristics, while Seattle distinguishes attached ADUs from detached ADUs and applies its own location and development standards. State law can also shape local requirements: California regulates many aspects of ADU approval statewide while allowing cities to administer compliant local permitting rules.
A building or construction permit is commonly required to create a legal ADU. Permit review may address structural design, fire and life safety, energy requirements, electrical and mechanical systems, plumbing, utilities, drainage, and access. Converting an existing garage or interior space can follow a different review path from constructing a new detached unit. Separate utility connections, addressing, sewer requirements, or electrical-service changes may also be required depending on the jurisdiction and project design.
Parking requirements, owner-occupancy rules, size limits, and development standards are especially jurisdiction-specific and can change with state legislation. Location can introduce additional reviews as well. San Diego, for example, applies separate coastal requirements to some ADUs within its Coastal Overlay Zone. Historic districts, environmentally sensitive areas, flood zones, and other overlays can similarly affect projects elsewhere.
Select a city-specific ADU permit guide to confirm the zoning rules, permit process, required plans, inspections, and additional approvals that apply to a particular property.