[-=[XTC]=-Body Bagger]

You can do it by making your buildings seperatley then saving them as prefabs, however make your terrain fisrt, them add a base for each building, select a base then save it as prefab eg building 1. open your editor, then select load prefab and build away.
Making a base first will help you so you don't have to try and postion your built house afterwards as it can get messed up.
Also when making your prefab building, make it facing the way it will be in the map.
Craig