Thank you for your reply.
I have managed to fix the problem and am greatly thankful. However I've had to delete all my outer walls which contained windows and doors, and will have to start over!
I also noticed that whilst I was creating the walls I didn't receive the 'External' and 'Internal' writing that you did. Although I guess that might be because I may not have the latest version.
However, I think that it would be a good idea if the program allowed users to reverse the garage door similarly to the other normal doors. It would make the situation a lot easier. Just a suggestion of course. I had no idea that walls had to be drawn clockwise.
Thank you once again for your help!