The floorplan is completely different than in the movie. It's easy to see even without looking at the movie apartment's floorplan.

Hitchcock presents the floorplan.

Deviantart recreation of floorplan (assumed bedrooms and bathrooms).

Brandon and Phillip share one bedroom. The chest in which David is hidden does not appear.

Excerpt:

Page 123

Something seemed to be slowly tearing in Phillip's mind, destroying the fabric of his slim residue of control.