You will be required to visit every area in the Treehouse as you progress through the game. However you can knock this out before you start your first investigation as you will have full access to the Treehouse from the start of the game. The rooms to visit are:
Bedroom
Observatory
Main Room (Upper)
Secondary Room (Upper, need to cross the rope bridge)
Main Room (Lower)
Bathroom
Secondary Room (Lower, the armory)
Loot Room
Outside (Go out the door in the Loot Room)