Before building another lot completely. See what happens if you move the sim family out through edit town, save the lot to the library and try replacing. Then move the sims back in and see if it works. I have had luck doing that.
I have also experienced where sometimes the game keeps telling me that when I am trying to put the stairs against one of the basement walls, but if I put them in the middle of the basement and not against the walls, it works okay.