This is the most complex of all Ghost Houses within the game, with a key that will lead to Larry Koopa's Castle. If you want to get a better score, however, do this with out the key.
This house has three rooms:
Room 1 has a supermushroom/fireflower block, ghostly-green death orbs, and a door to Room 2.
Room 2 has a P-block that'll make a star drop and some platforms appear, ghosts everywhere, a door on the far-left that'll take you to Room 1, and a hallway on the far-right with five doors. For the special key, you'll need Door 5, which means you gotta be fast (cape helps). The Hallway -
Door 1 goes to room 3, Right-Side Door 2 goes to room 1 Door 3 goes to the default exit Door 4 goes to the default exit Door 5 goes to room 3, Left-Side
Room 3 is divided in the middle by a barrier which can only be traversed from the left side (Door 5). The left side has a P-block and a dragon coin. Don't hit the P-block just yet; take it with you to the right side of the room pass the barrier (Unless you really need that last dragon coin, of coarse). On the room's right side is a remote-control coin block, which you'll use with the P-Block to make stairs to the key in the upper-right corner. You'll need to be small to fit.