A cross between apartments and townhouses, the "stacked townhouse" is becoming a popular house type among DC-area homebuilders and buyers. While they're great for urban neighborhoods, a quirk in zoning means they're most common in far-flung suburbs.
Also called a two-over-two or maisonette, the stacked townhouse is basically a rowhouse divided into two two-story units, one over the other. Both units have doors on the street, usually in a little alcove, making it look like it's one big house. The garages are tucked in back, on an alley.
This house type is what some architects call the "missing middle," not quite a house, not quite an apartment, but a good alternative housing choice in places where the only options are a detached house or a high-rise.
Historically, lots of cities have rowhouses divided into multiple apartments: Boston's triple-deckers, Chicago's two- and three-flats, Montreal's plexes. In those cases, each building generally has a single owner who rents out the other unit. They don't seem to have been common in DC.