An accessory dwelling unit (ADU) is a smaller, self-contained dwelling on the same lot as a primary residence — typically a backyard cottage, converted garage, basement apartment, or "in-law suite." Historically common in older American neighborhoods, ADUs were widely banned by mid-20th-century zoning codes that emphasized single-family detached housing.
California began aggressively liberalizing ADU rules in 2016, with subsequent legislation removing barriers around parking minimums, owner-occupancy requirements, and approval discretion. Other states — Washington, Oregon, Vermont — have followed with statewide ADU-permitting laws. Permits and construction have surged in California in particular.
Proponents argue ADUs are the least disruptive form of new housing: they add supply, generate rental income for homeowners, provide housing options for aging parents and adult children, and require no demolition or rezoning. Permit data shows ADUs are produced by ordinary homeowners, not large developers.
Opponents argue ADUs strain parking, infrastructure, and neighborhood character, and that owner-occupancy requirements should remain to prevent investor concentration. Some emphasize design and lot-coverage standards rather than outright bans. Empirically, ADU production has been modest in most cities even after liberalization, raising questions about whether they meaningfully address affordability at scale.