It also helps to put back the original lot, use the "flatten terrain" tool (not sure how it's called, I mean the one that flattens the ground with one single click instead of dragging) on it and bulldoze it afterwards. Only if the game *lets* you put it back, of course
.
I've taken to build most lots in a "building only" subhood with completely flat terrain and have never had problems to place them since, though I like to move them around a lot. Only lots where I want to keep the characteristics of the terrain (e.g. for houses on steep slopes) are built in the respective "playing hood" itself.