Mason's home, in the Mason Neck section of Fairfax County.   It is quite well preserved.

And by the way -- Leavs on the Current (my wife) did a paper on Mason when she was in high school because she became fascinated afer reading about his role drafting the Virginia Declaration of Rights.  She still refuses to call the Island on the Virginia side of the Potomac that is a nature preserve by the common name of "Roosevelt Island" because it had in fact been the home of John Mason, son of George, for a good number of years.  She is wont to call it by its traditional name of Mason's Island.