I extended Vlindrel Hall and put a forge and some mannequins in a new room, but when i go in on the game, i look through the open door and the only thing that shows up is the Mannequins i placed. The floor, forge, chests, walls, ceiling etc are gone. you'd think you'd fall into void when you step in, but instead, as soon as you go in the room, it loads up perfectly. Howeve when you turn to go back into the main house, thats gone now and it does near enough the same thing. Though again if you step in, it loads and the room a made dissapears again.

Any ideas?

sounds like there is a roombound. you will need to add a new roombound and link it to the existing one via a portal, or if the new room is small enough, you can probably just extend the existing room bound or attach a new one onto it
http://www.creationkit.com/Bethesda_Tutorial_Optimization