That being said: while the number of students certainly contributes to the crunch, exclusionary zoning in the suburbs is the primary driver. In many (or most) of the second ring suburbs (and even some of the first ring), multi-family housing of any kind is prohibited or severly restricted. This includes so-called in-law apartments, accessory apartments and garage apartments. All along Route 128 (circumferential highway ten miles from downtown Boston) there are towns with nothing but 1+ acre single family residential zoning (Weston, Lincoln, Dover, etc.). There are many more as you move farther from the city (Carlisle, Boxford, Harvard, Sherborn, etc.).