3600 Prospect in 'Movie Makeup'
Compare the above screenshot (appears at time code 28:49 in the "Special Edition" version of The Exorcist) of the "McNeil House" with the actual structure as it appears today (below), minus the built-on mansard roof and side wing created for the film. While the screenshot is a bit vertically compressed, the core of the house and portions of the front gate are easily recognizable in both images.
If you've seen the actual house in Georgetown, you may wonder about the film's logistics regarding characters Burke Dennings and Damien Karras being flung from an upstairs bedroom window down the "Hitchcock Steps" - in reality, the windows are are a good 20 feet away from the fence surrounding the house! In addition, an early scene in the film is set in a full-height attic, which the actual building doesn't appear to be able to accomodate above its second floor.
That's where the made-for-the-movie side wing addition comes in. By extending the house closer towards the 'Steps,' and building a full attic height mansard roof, the film's creators built a visual setting faithful to the novel. [Top image © Warner Brothers Corp., lower image on Flickr under Creative Commons 2.5.]