Hi again, I hate to bring this old thread back, but I'm having the same problem again. To answer Seabody, no I've never used CFE on the previous house or on the house that I'm having the same problem again, nor can I put any column under it and my sims cannot walk on it. I tried putting a wall around it, but that didn't work. When I tried to put a floor tile on it, the tile appeared in random spots on the lower level directly below it. When I tried putting a wall around it on the same level, a wall appeared just to the side on the lower level. I'm going to try to reset the household and see if that works to clear it up.
Edit: Resetting the household did not work. Inerestingly, when I looked through the hole, trying to place the floor tile, it appeared on the lower floor. I may just have to move the family out and into a new house.