You can also do it with foundations. Turn constrainfloorelevation off and turn on moveobjects. Build your foundation for the outside shell of the house. Place a single story column next to it, then level the top off. You'll still need to place the inside walls, but it's a heck of a saving
Especially for starter homes. If you're building from edit town mode, then you should place the walls first to add the roof, then remove them and replace them with foundation, Otherwise you can't place a roof.