Via @mslynross, here's the Washington Post's Dion Haynes on zoning conflicts created by the pressure to carve rentable apartments out of aging homeowners' big houses. There's a link within the Haynes column to this further recent coverage.
In the DC area I guess they call those "accessory dwellings." Here in San Francisco it's "in-law apartments" and the need for them is probably similar. Probably everywhere. Especially given the "aging in place" trend and the attractiveness for empty-nester seniors of having a tenant who helps around the house for something off the rent.
All of which brings up the fine old line from the late Phil Frank's "Farley" comic: "If in-laws are outlawed, only outlaws will have in-laws!"