Invisible Foundation

@Pamlady I think I'm going to need some screenshots of what you're talking about, please.

In the mean time, I hope some of this information will answer some of your questions/issues? I experimented a bit with things I thought may be what you have in mind and decided if I was off-target it would not be useful, so I will wait for your screenshots.

I am adding a few pictures here of an area where the center room has a spiral double staircase and open area and has no foundation and the outer room has a raised foundation. There is also a screenshot of the plumbob in the center of the room that appears when you click the wall. You drag the plumbob up or down to raise or lower the foundation. 

This may be confusing but I found I could make an inner room without a foundation (by sledgehammering the floor), but if I add an inner room with a foundation the foundation will not adjust separately like it will outside the home walls. (As a separate building on the lot separated by at least one tile.) If this is confusing just ignore that part and notice the plumbob is what you drag to adjust the foundation:

I'm not sure if either of these stairs questions I answered in the past would be helpful at all in what you're trying to create, but I'm adding the links to videos just in case:

• Open stairway to different floors (Harry Potter style): https://youtu.be/f1KrGsx2ako
• Open spiral stairwell: https://youtu.be/fq_gn4oIiGw