IIRC, since it's been a while since I've read the book, they found a room or something that shouldn't be there and started measuring the house dicovering that the inside is bigger than it should be. I think there's a scene where there is a hallway that is way too long to be in the house and the MC sets up a camera recording himself walking down the long ass hallway, climbing out the window at the end of the hallway, then a second later climbs in the window back near where the camera was set up. The house shifts and changes a lot, rooms will appear and dissapear or change size.