I never have understood the distinctions in English real estate, despite hearing terms like "detached", "semi-detached" and "walk-up" in imported British television. But I was heavily involved in real estate in the US for years before doing the writing thing full-time, so maybe someone could translate US terms into Brit-ese (or other EU countries) :
"Apartment" = a building (generally multi-story) with side-by-side dwellings having common walls--e.g., the west wall of one dwelling is the east wall of the next.
"House" = a freestanding dwelling with or without a garage.
"Duplex" = a freestanding building separated into TWO dwellings, having one common wall and often a common garage.
"Townhouse" = a freestanding building separated into up to EIGHT dwellings, all on one level, with common walls, and no garages (but sometimes with a common underground or connected garage structure or parking lot.) More than eight, at least in Colorado and Texas, turns a townhouse into an apartment building.
So, how do those compare across the pond?