I saved a zombie villager and when he got turned he only stayed at the entrance to the house (the one I was using for a base) and I had carpet in the doorway that would lead up to the second level. Once I removed the carpet he finally got fully into the house and went straight to the grinding stone (since he is a weapon smith)   so it appears villagers will not move onto carpet that is near stairs.   He's upstairs now, but this is the set-up for the house I had and the villager only stood in the doorway to the house