if you build a room with a door against a solid wall what happens to the door space?

it becomes part of the solid room. you should always plan your house to make sure the doors match up on both rooms.

Indeed, it's been a while since I've trained construction, but I do believe it becomes a part of the Wall. So the door won't exist. (Don't take my word, It's been 6 months + since I've built anything in my house). :P