Going with the word 'Ruin' in place of 'building'...
rodt777 wrote:Ran into this problem earlier today in a game, we just said that it can't go in the buiding. But whats the real rule?
I don't think there is one, beyond the normal rules on model placement. Although the 'wobbly model' rule would allow you to potentially put it as far as possible into the ruin and call it a good'un.
I would go with not being able to enter, for simplicity's sake.
Also can a model go on a building floor they can't fit on?
Nope. That one's covered in the Ruins rules. They can only move onto upper floors if they physically fit.