Accessory Dwelling Units — ADUs — are compact, fully equipped secondary homes built on the same lot as a primary house. Often called “granny flats,” carriage houses or backyard cottages, ADUs typically include a bedroom, kitchen and bathroom and can be attached to the main home or stand alone. Because they reuse existing lots and infrastructure, ADUs can add rental or owner-occupied units much faster and more cheaply than new subdivisions or apartment towers.
In recent years ADUs have served an urgent need in places where housing is scarce or costly. After wildfires and other disasters displaced people in parts of the West, homeowners and communities turned to ADUs to provide nearby, familiar shelter for relatives and neighbors. They also offer long-term benefits: increased rental supply, extra income for homeowners, multigenerational living options and incremental density that better matches neighborhood character.
But turning lots into livable ADUs usually runs into practical and legal barriers. Common hurdles include zoning rules and minimum lot sizes, restrictive parking requirements, complex permitting and inspection timelines, utility hookups and storm- and fire-safety standards. Costs — design, permitting fees, site work and construction — plus insurance and financing constraints can put ADUs out of reach for many owners. Neighborhood resistance and uneven local policies create additional friction.
Cities and states that want more ADUs are experimenting with solutions: streamlined permitting, pre-approved plans, lower fees, tax incentives, and clearer rules around occupancy and fire safety. Those steps can lower costs and speed approvals while protecting neighbors and emergency access. The result: ADUs can become a practical, flexible piece of the broader strategy to expand housing in established neighborhoods.