[luuckyy]

From my own experience, I usually modelize a building/house in a single mesh.
This way I'm sure to have a proper model base, ie. with no extra and unneeded surfaces.
Then I "cut away object" some surfaces (roof, windows, doors, balconies, chimney, ...) for texturing purposes (except if I 'm using a big texture map for the entire model).
The polycount stays the same, only the amount of vertices increases ... but who cares (?).