To do this cheat friendly fire must be on and the player that will do the glitch will need the Hammerburst.
At the right spawn near the door to the mansion there will be a rock on the right, the player with the hammerburst must hop over this rock and be downed on top of it by his own player.
Then you walk into the wall, and while still in the wall walk towards the mansion.
You will then slip through the wall and be able to freeroam the outside of mansion.