There are currently ten rooms that can be accessed and unlocked after playing the Main Story. Two further rooms can be unlocked on Forgotten Hourglass playthroughs, making it twelve rooms in total.
The Piano Room is the central room. Deemo and the little girl reside in this room, close to a tree in the center of the room. Upon a Tree Height of 20 m onwards, Deemo no longer appears here; only the little girl and the Masked Lady may be present.
The Attic can be accessed on the right door of the Piano Room after reaching 10m Tree Height.
The Downstairs Room is unlocked after first accessing the Basement. It connects the Attic and the Basement.
The Second Floor can be accessed after reaching 33m Tree Height
The Upstairs Room can be accessed after reaching 35m Tree Height. It connects the Second Floor and the Top Floor.
The Top Floor can be accessed upon reaching 50m Tree Height and clearing songs from this point stops increasing Tree Height; rather, they increase the Staircase Height leading to the exit.
Alice's Room can be accessed at the end of a playthrough, after viewing the ending. The Forgotten Hourglass is located here on the piano and Moments can be viewed here.
The Mask Room can be accessed in Forgotten Hourglass playthroughs from the Basement. Moments can be viewed here. It is unlocked after clearing the piano song in the Basement.
The Sakura Forest can be accessed in Forgotten Hourglass playthroughs by selecting the telescope at the Balcony. It is unlocked together with the balcony.