Reply #29: There was a real estate story in the NYTimes last year...

[Cassandra]

about a couple with a child who bought a brownstone with a friend. The couple took the gound floor with the backyard, the friend took the top floor with the roof and they rented out the middle floor to a couple. Seems to be working out fine and of course, in Manhattan, multiple dwellings are no problem. This sort of arrangement, with perhaps a single owner, has been going on since the Civil War.
Out in Queens and Westchester, as well as in Chinatown, there are problems with single family housing being subdivided into dormitories, which can grow to be a health hazard for those involved.