Even so, there are many barriers to the large-scale development of ADUs—not least of which is legislative in nature. Dana Cuff has studied accessory units for more than 10 years as the cofounder and director of cityLAB, a think tank within the Department of Architecture and Urban Design at UCLA. She found that secondary dwellings were being built informally across Los Angeles—on as many as 60 percent of properties, in some communities. “But because of a couple of really arcane laws, it was almost impossible to do legally,” she says.

In 2016, armed with a decade of research, Cuff and cityLAB fellow Jane Blumenfeld, working in concert with colleagues at UC Berkeley, coauthored a bill that removed a city’s ability to enforce regulations (such as the required addition of parking spaces, and high fees to connect to utilities) that were inhibiting the development of ADUs in California. The bill went into effect in January 2017, opening the floodgates for the construction of new units and permitting of existing ones in the state, making it one of the friendliest in the country to this type of housing.

Beyond legal hurdles, which still cripple the widespread use of secondary dwellings in many municipalities and states, the building type presents an interesting design challenge to architects. Many have pursued modular or prefabricated solutions, but the unique context of each project typically necessitates some degree of customization. Existing infrastructure—such as the primary house, neighboring buildings, driveways, and city utility hookups—means “each one of those things interrupts scaling up production,” says Cuff.

So how far can prefabrication extend? “This is the line of research we’ve been conducting for a number of years,” says Danny Samuels, Rice University architecture professor, who co-directs the school’s design-build studio, Rice Architecture Construct, with assistant professor Andrew Colopy. “The idea we eventually hit on is that you prefabricate a core,” says Samuels, consolidating the kitchen, bathroom, and air-conditioning, heating, and electrical systems into one compact unit.

This vision came to fruition in +House, an ADU with a 3,000-pound prefabricated core, designed and built over the last four semesters by Construct students. Situated in the backyard of a house in Houston’s Third Ward, the 360-square-foot dwelling will accommodate live-in counselors who work for Agape Development, which provides supportive housing (including the primary structure on the +House site) to at-risk young adults.

Video courtesy Rice University

The prefabricated-core model preserves the ability to customize a structure but offers the efficiencies of prefabrication—a crucial factor in making ADUs an attractive, viable option for homeowners. “There are already economic forces pushing people toward this solution,” says Colopy, “so we want to find more ways to encourage individual property owners to make this decision.”

Architect Derek Leavitt, who cofounded the Los Angeles–based design-build firm Modative with contractor Christian Návar in 2006, is exploring a developer model to quickly deliver units. “We’re trying to get costs down by eliminating the custom-design portion of architecture”—which, he admits, “sounds very counterintuitive to say as an architect.” This approach led Leavitt and Návar to partner in May with venture capitalist Steven Dietz, whose new company, United Dwelling, is one of several in the country aiming to flip the grassroots model of ADU development on its head.

The startup will handle all aspects and costs of converting a detached garage into a living space, then split the rental income with homeowners for a set number of years under a land-lease agreement. Modative designed a standardized unit that works within that specific context. “By coming into each project knowing that we’re doing the exact same thing every time, we can get the cost way down,” says the architect. United Dwelling expects to sign their first lease this fall.

Elsewhere in Los Angeles, LA-Más is pioneering another model: the urban-design studio recently developed a program that leverages a novel loan product and the federal Housing Choice Vouchers program (also known as Section 8, which helps very low income families and individuals afford private rental housing) to bring ADUs to people who are “asset-rich but cash-poor,” says architect Elizabeth Timme. She founded the firm in 2013 and serves as co-executive director with policy wonk and planner Helen Leung.

Working with collaborators in the finance, community-development, and nonprofit sectors, LA-Más launched the Backyard Homes Project in July, which recruits homeowners to become Section 8 landlords, offering pro bono project management, discounted design and construction services, and flexible financing (a loan, rather than a second mortgage) in exchange for their five-year commitment to rent to Section 8 tenants. Construction is slated to begin mid-2019 on the first cohort of 10 homeowners’ accessory dwellings.

“It’s really meaningful to apply our formal agenda to these alternative living spaces, to use history and precedent to make them something more,” says Timme. “What we really care about doing is supporting the most vulnerable populations in any community. Here, that’s renters, and the ADU was a clear mechanism for maintaining neighborhood stability.”