I'm new to model railroading and starting an N gauge layout. I've picked up a few books on planning the layout, scenery, etc, and from everything I've read, they recommend that you plan the placement of structures while planning the track. What I plan to do is a relatively dense city -- I have a book by John Pyke (spelling) that does a good job of explaining how to go about. I will most likely use many commercial kits with a few kit-bashed or scratched. The problem is, at this point I don't really know what the footprints of those buildings are. So, how do the rest of you plan for structures that are not yet constructed? Or do you just get a general idea of how you want it to layout and choose structures based on the rough idea?