UC Irvine

Problem, research strategy, and findings: Multigenerational living arrangements have become more common in the United States as housing costs rise and family structures evolve. Accessory dwelling units (ADUs) could support this shift by allowing relatives to live nearby while maintaining privacy and independence, and advocates have invoked this promise to justify laws intended to facilitate ADU development. In the past decade, at least eight U.S. states have adopted such laws. We present the first large-scale, parcel-level test of this claim, linking permit records from 14 California counties, covering approximately two-thirds of the state’s population, to geocoded voter registration and consumer reference data, identifying multigenerational households from shared surnames and age differences across single-family parcels. Parcels with ADU permits are, on average, slightly less likely to be multigenerational than comparable parcels without permits, by roughly 2 to 3 percentage points after controlling for parcel and neighborhood characteristics. This negative association persists among owner-occupied parcels. This finding is sensible in light of prior research showing household characteristics associated with multigenerational living, such as lower incomes, are also associated with informal (i.e., unpermitted) ADU development. Takeaway for practice: Broad ADU legalization alone does not appear to have increased multigenerational living in California. Planners seeking to promote multigenerational housing should pair by-right ADU authorization with complementary measures such as preapproved plans and alternative financing methods.