Each of your houses must be built in order: Floor, then Walls, then Roof.
You may work on multiple houses at a time, but you may not start a new house if you have an unfinished house of the same type.
Once you have finished a house, you are allowed to begin a new house of the same type.
There are enough house sections to build five houses of each type. Once these run out, you may not build that house section.
You may only start a new house if you have an empty Build Site on your Player board; each Player Board has 5 Build Sites.