12ft by 9ft sounds like a blessing! If I were lucky enough to have this sort of space I would have a large station on one of the 12 ft sides and a storage yard opposite, disguised by a 12ft length of open country track. Although it sounds like a big space, it isn't really. But it's enough space to have a decent 'watching the trains go by' type of layout, with a bit of shunting and a locomotive depot. My advice would be to not over complicate things. Keep it all on one level and as simple as possible, but not so simple that you don't enjoy playing....sorry, operating it.
I don't know how I've got the nerve to post really, I can't get a decent 'N' gauge layout onto a door!
Good luck!