VIDEO: https://www.youtube.com/watch?v=JupdHrNDJ1o
It seems that not all houses are accessible, only the ones with only one balcony in the back. If you can stick your head a little bit into the wall, you can run through (it helps when looking down, so you are running head-first)
Movement is not totally free inside the house, since there are couple of invisible walls, but they are not that large (and you can run through most of them).
This could be major abuse (even if you couldn't shoot from there), because you can stay hidden and still see other players. When they drop their guard it is easy to pop out of the walls and kill them. (screen: http://cloud-4.steamusercontent.com/ugc/710779919464610455/3DEACCDF6BC7A3FE0852BA66BF187AD5913DFF82/)