The Glass House

Because Mr. Greene is an architect, the Greene family lives in a big glass house that people are always slowing down to look at. From the outside it appears as though you can see everything, but that's just an illusion:

From the outside, you could see our entire downstairs: the living room, split by the huge stone fireplace, the kitchen beyond, and past that the pool in the backyard. You could also see the stairs and part of the second floor—the doorways to my room and to Whitney's, and the landing between them, split by the chimney. The rest was tucked away behind, out of sight. So while it seemed like you were seeing everything, you really weren't. Just bits and pieces that looked like a whole. (3.46)

The house represents the deceptive nature of appearances. From the outside the Greene family looks like the picture perfect American family, all sitting down to dinner together. However on the inside, they have fights and inner demons that they're majorly battling with, and they don't know if they can fully keep it together.

Similarly, the characters in the novel aren't all that they appear to be. When Owen first meets Annabel he tells her that he always thought she was too perfect and entirely unapproachable, even though she's far from perfect and is hiding a huge and painful secret. And Owen may seem like a big violent brute at first, but he's a kind and compassionate guy who helps Annabel to come out of her shell—unlike Will, who looks perfect but is rotten inside.

Yup—appearances are pretty consistently illusions in this book, masking trickier truths below the surface. The glass house makes sure we pay attention to this.