Building with two separate housing units. In most of the US, probably similar to what you call a "semi-detached" or Germans call "Doppelhaushälfte" (units are next to each other, sharing a wall in the middle), but especially in the Northeast, sometimes a larger old house split by floors (ground floor and basement as one unit, second floor and attic as another unit, with some sort of separate entrance to reduce disturbance of first unit residents)