Maybe there is an even more simple way to do it, but I use the "bulldoze lot", I think it's called. So when you are on the lot, you have a bar on the top of your screen (The bar where the sledge hammer is, for example), and one of the icons is house-shaped (It's called "Lot Info"), and on it should be an option to "bulldoze lot". Use that and then you have an empty lot to place or build anything on. Sorry if I'm not clear enough, I don't have my game open right now.