Here is some information from "Guide to Loudoun" by Eugene Scheel:
South on Fannie Wilson Hill Rd, Rt 681, to Corby Hall at .5 mi on W. Four wings, earliest of stone at W, show development of home. Fine wrap-around porch on hip-roofed brick wing at E. Marker to S of house notes earlier wing as stopping place for George Washington, June 1-2, 1788, after he inspected canal works at Seneca Falls (Tour 1) en route to mouth of Shenandoah. Vistas to SE of Catoctin Valley and Waterford from Fannie Wilson Hill .6 mi to S. Turn E (1) into Waterford.
More information is given in "Loudoun Discovered, Volume Five."