Once you awaken in the house, check the mirror, and choose to check the stairs. Grab the key pin from the drawer, halfway up the stairs, and then during the event,choose to enter the door on the left.(door 1, I believe)
Once you're in the room, step out on the balcony to get the achievement.
This will end the chapter, so you may want to make sure you first check the room, and attached bathroom, as there are several achievements available just by interacting with objects in both rooms.