Since around 2010, the city has been encouraging the development of such accessory dwelling units, or A.D.U.s — modest living spaces (800 square feet, tops) that share property with a main house. A.D.U.s, whether stand-alone or connected to the main house, are being embraced by a growing number of cities and states in response to their lack of affordable housing. The California Legislature, for one, has gone all in on them, passing a series of bills to encourage development statewide.

More particularly, A.D.U.s are seen by experts on aging as a promising tool for addressing a critical shortage of housing for older Americans. AARP, the research and advocacy behemoth, has begun aggressively promoting the spread of these units, often referred to as casitas or, more cheekily, granny flats.

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Housing experts often talk about the missing middle — a lack of investment in buildings that have more units than a single-family home but are less dense than midrise apartments. Townhouses, garden apartments, duplexes and triplexes fell out of favor after World War II, in part because of restrictive zoning laws imposed by local governments, many of which were looking to keep out poor people and minorities.

Read more in The New York Times.