Unlock Hidden Mansions
Your mansion will gain ranks based on the amount of money you have at the end of the game. For each rank, you will receive a portrait of a new mansion. You can view the portraits in the gallery of Professor E. Gadd. The mansion ranks range from A to H.
Rank A: Over 100,000,000G
Rank B: Between 70,000,000G and 100,000,000G
Rank C: Between 60,000,000G and 70,000,000G
Rank D: Between 50,000,000G and 60,000,000G
Rank E: Between 40,000,000G and 50,000,000G
Rank F: Between 20,000,000G and 40,000,000G
Rank G: Between 5,000,000G and 20,000,000G
Rank H: Less than 5,000,000G
Secret Rooms
There are two hidden treasure rooms in "Luigi's Mansion." For the first, you will need to attach your Game Boy Advance in order to use the in-game "Game Boy Horror." Go to the southwest corner of the mansion. Examine the room that has the butler with your Game Boy Horror. Press "A" to enter the mousehole that leads to the treasure room.
For the second room, take the elevator to the roof of the mansion. Climb up the chimney and jump in it to find another treasure room.
Ura-Yashiki Mode
"Ura-Yashiki" means "Hidden Mansion" in Japanese, and it is a secret playable mode. Finish the game once and save. The game will give you an option to restart in Ura-Yashiki mode. In Ura-Yashiki, the mansion layout is slightly different.
Mirrors
Use your Game Boy Horror to examine any mirror in the mansion. The mirrors will allow you to warp back to the first floor of the foyer. The warp does not work in the Mirror Room, however.