Haha, yeah that happens, always since I'm a kid I would have dreams where I'd end up at a house I lived when I was a kid, and it always has a basement and blacony that never had in real life, but in my dream world they're always there. I think it has some how to be tied up with when you are a child and get to know your enviorment and prob get a little creative with it in dreams subcounsiously (sp?) diferent from other things, and it just keeps going on forever on that particular aspect of the dream. Like for example, maybe once as a kid you watched some cartoon with a secret room or something while you where at your gramps or something and the thought manifested in the dream , as you grow up and have other dreams, diferent things manifest like that.
I've had dreams recently that happen to much on the highway, with me not involved on the actual highway(like having sex, turning up and noticing you are on a car thats moving ), I've been traveling lately so. Hope that makes sence, I'm a little drunk of right now.

By the way, whats the nightmare part on this?