This is how you make your house float :

Step 1 : make the bottom floor of your house

Step 2 : Choose any type of stairs you want and put them where ever you want

Step 3 : Make another floor on top of the first one

Step 4 : Hold down the CTRL botton and delete the bottom floor

And thats how you kake your house float !

( P.S if you don't know how to delete the walls on your house there are instructoins on the game !)