California has issued 82,000 building permits for accessory dwelling units under a state law that allows homeowners to add small secondary residences on single-family lots, according to Dana Cuff, professor of architecture and urban design at the University of California, Los Angeles. The structures, commonly known as ADUs, are fully-functional secondary homes located on the same property as a main home, usually in the backyard. State law allows at least three modest-sized ADUs on a single-family lot, permits only long-term tenants, and prohibits their use as short-term rentals.

"It's had more impact than any other housing law in the last 10, 12 years," Cuff said. She lives in an ADU constructed in her backyard and rents out her original home to mostly young tenants.

Cuff described the policy as part of a shift in how California approaches residential development. "Sprawl has hit the wall now. You can't keep going out. So then, I mean, the beauty of that, from an environmental and a housing and an urban perspective, is that then you start building back in," she said.

Among the residents using ADUs as long-term housing are Kaetriauna Bowser-Smith, Jared Weber, and their nine-month-old daughter, who have lived in a 400-square-foot ADU for nearly three years. "We've tried to look at other places, just to even see what there is still on the market, and there's nothing comparable to what we have," Bowser-Smith said.

In another California household, Mona Field converted her garage into a two-bedroom ADU for herself, while a family with two small children lives in the main house. "I knew I did not want to stay aging in a big house by myself. I don't feel cramped at all. I feel like I have a lot of space for one old lady!" Field said.