Board Thread:Roleplay/@comment-24947676-20160423103318/@comment-24947676-20160426095851

(Also, for reference, here's a map of Waterfall.)

But of course, since the door behind the house requires a key from Sans's room, which you can only access if you have the key to Sans's room (which you can only obtain near the end of the game on a pacifist route), there's no way to get through the door.