A lady buys her new flat, but feels she is being watched. A guy (quite hunky and has longish dark hair, but bad/weird) is stalking her and lives in the duct space that he has created between her flat and walls. He has built a system of access point through her flat and can see her every move and get in when he wants to. She gets help from a chap who measures her rooms and finds that her floor space should be much more, and that the walls have been moved in. I can't remember the actor/actress and it was a good film.

Through the Eyes of a Killer (http://www.imdb.com/title/tt0105583/)