This Tutorial isn't difficult at all. Only you have to be precise in making it to work.
You only need the baseline for this to work.
Now, let's get started.
1 Build the foundation for your house.
2 Build the first four levels. Add a roof, if you like.
Now we are going to raise the foundation to the exact height of a wall.
3 Press Ctrl+shift+C to open op the cheat screen and type in: constrainfloorelevation false.
4 Build one closed square of four walls on the ground near you foundation.
5 In Build Mode, drag a floor from the second level of the square walls over the foundation. The activated cheat will now raise the foundation to the height of the wall.
6 Turn off the cheat by pressing Ctrl+shift+C and type in: constrainfloorelevation true.
7 Now remove the foundation and replace it with a normal wal.
And that's it. Easy as that. The reason for this to work is because a foundation is for the game just another level. It doesn't matter how high it is to be calculate as.