The rules given here assume that your building is of a fairly small size (less than 6" x 6") and composed of a fairly homogenous structure (i.e. is made out of the same kinds of materials). None of this is to say that you can't use bigger or more complicated buildings in your games, but you'll probably want to break them down into several sections, each to be treated as its own building, which can be occupied and assaulted separately.