There is definately something going on with that particular lot. I've tried it in a fewgames and it's always the same, build a room or house and you get the foundation top. I tried importing houses from the gallery that had no foundation, but it is still evident even on these houses. To add to the glitch, more often than not, if you try to bulldoze the newly imported house, an error comes up saying unable to compute costs and the house remains. I even tried using the sledgehammer to demolish it the slow way, but it kept saying there was nothing to select. However after this error, if you go to manage world and re-enter the lot, the house will be gone!
I had thought that it didn't happen if you buy one of the styled rooms, but the only time it doesn't happen is if you buy one of the new buisiness type styled rooms... it even happens if you buy one of the outdoor rooms!