I made a back yard to the house in Hjerim. I have two problems though. The rear of the house has no collision so I can walk through it, same goes for the back wall adjacent to the mountains.

Secondly there's a line just before the blacksmith furnace that blocks the player.

I'm sure I can figure out how to create an invisible boundary to prevent falling into the house and back wall but how do I remove the invisible wall? There doesn't seem to be anything there to remove.

In the image below the red line indicates where the invisible wall is. The blue lines indicate where invisible walls need to go.

Naturally doing anything back here wasn't such a good idea considering the player wouldn't normally have access to the area but there was space so I thought why not.

http://s33.photobucket.com/user/soopytwist/media/Skyrimmodproblem_zps63cd39f8.png.html

EDIT

Added some collision planes but I can't remove the invisible wall that's already there, there are no collision planes there.