Any villager who moves in can plot anywhere they wish. They have a chance of plotting where the old villager used to live, or they may choose somewhere else entirely. I think that people have said that it seems like new villagers moving in tend to like plotting where old villagers used to live, so if you don't want anyone else moving in that exact area, I'd set down a PWP, even temporarily, to prevent it.
Thanks. I am so used to a house being there, I don't mind if they plot there. I was just curious.