“I believe that affordable housing is the single biggest problem that California faces, and it’s entirely man-made,” said Mr. Dietz, the chief executive of United Dwelling, a company that won a million-dollar grant last year from Los Angeles County to help bring his vision of garage conversions to life.

Garage conversions, granny flats, backyard cottages, in-law apartments, guesthouses, crash pads: In California as of 2017, they’re all “accessory dwelling units,” or ADUs, and state laws regulating their construction have been relaxed. Governor Gavin Newsom signed a law this month that further encourages their construction.

Larger, multiroom ADUs built from the ground up are also a part of efforts to contend with the housing crisis, but Mr. Dietz, a longtime venture capitalist who invested in Costco and Starbucks and has taught a class on entrepreneurship at the University of Southern California’s Marshall School of Business, took inspiration from the new laws to focus on a garage-based solution — with research help from his students at U.S.C.

His idea is simple: United Dwelling enters a partnership with a homeowner, pays for the garage conversion, manages the rental of the apartment to a qualified applicant and splits the rent with the homeowner.

Since most of the detached garages in Los Angeles aren’t used for cars — 91 percent of the 2,100 homeowners surveyed by Mr. Dietz’s students use their garages for something else, mostly storage — this can provide rental income and affordable housing in many neighborhoods. And it does it by using existing structures.