That's the former Northern Virginia Training Center. It was a residential facility for the seriously mentally handicapped that closed last year, after having been open for a touch over 40 years. (It opened in 1973 as the Northern Virginia Training Center for the Mentally Retarded, but lost the latter part of that appellation in the early 90s, IIRC.)
The commonwealth sold the 78-acre site to Erickson Living last month, and they're going to turn it into a retirement facility, à la Greenspring down in Springfield.