Another related question is why it's not possible to move levels up and down in the levels setting. I started by creating the second floor first, and afterwards it was not easy to create a lower level. Apparently it's much easier to create an upper floor than a lower floor once you've started. I'm still blocked from exchanging positions for my floors, they're always locked in place, clicking on the arrows does nothing. That's why, to create a lower floor, I had to fiddle with the elevations and height manually, and this is what resulted in not having enough space between the floors.