I edited one of the existing interiors (Bards College) by making it a bit bigger but I'm having graphical problems in game. I cannot see the new interior from the vanilla interior, and I cannot see the vanilla interior from the new interior (screenshots: http://imgur.com/a/vQpIg)! Any ideas? I'm new to level design in Skyrim so hopefully this is easy to solve.

This is caused by room markers / portals

Watch the Bathesda Tutorial - https://www.youtube.com/watch?v=acfuZiQh83Y

Problem solved, thanks. I knew it had to be something simple.