It's not actually a house. It's just for decoration.
How Do You Open The Little House Thing In Celestic Town In Platinum?
I heard somebody say that if you shine your badges to the max and play a special tune with them outside the building the building will start to shake and Darkrai will come out to battle you but its probably a Myth?...
I think you have to play a tune when all of the gym badges are polished and then Celibi will appear.
I heard that you have to polish ALL the gym badges once you get them all, and then go to celestic town. Then go in front of the house and then play a tune with the POLISHED gym badges. And some legendary pokemon will attack.