The home of movie actress Chris MacNeil (Ellen Burstyn) has gone on to become one of the world's most famous film locations. Supposedly making a student protest movie at Georgetown University (the alma mater of author Blatty), she's staying nearby, at 3600 Prospect Avenue at 36th Street NW, close to the Potomac River in Georgetown, southwest DC.
Don't recognise it? Well, apart from a high black fence, presumably erected to discourage rubberneckers, the wing containing Regan's bedroom (from which the shaft of light streams in the famous poster image) seems to be missing. It was a false section built specially for the movie. The famous flight of steps, down which Father Karras (Jason Miller) ultimately hurtles, can be seen alongside the house, leading down to M Street.
The Exorcist film location web page