How do you delete foundations in the Sims 2?

Answer 1

you got to read the manual. or if there is a question mark on the bottom left hand corner of the blue bar, than click on that.

Answer 2

while in buildmode, go into the foundation part and hold down control (I think, maybe shift, I don't remember which one makes a room and which one is the bulldozer) Anyway, you will see the blue arrow have a bulldozer on the top of it, just highlight the parts you want to delete, as if you were building and they will go away.

If you already have a whole house and just don't want a foundation you will have to completely tear down and rebuild the house.