Belvoir mansion was a two story brick mansion in Fairfax County, overlooking the Potomac, built by William Fairfax between 1736-1741. Mount Vernon was located about three miles distant. In 1757 Fairfax's son, George William Fairfax (1724-1787), who was a close friend of George Washington, inherited the house. Belvoir burned in 1783.
“029-0041 Belvoir (Belvoir Mansion Ruins and the Fairfax Grave).” Accessed September 6, 2022. https://www.dhr.virginia.gov/historic-registers/029-0041/.
Snowden, William H. [from old catalog]. Some Old Historic Landmarks of Virginia and Maryland, Described in a Hand-Book for the Tourist over the Washington, Alexandria and Mount Vernon Electric Railway. Alexandria, Va.: Printed by G. H. Ramey & sons, 1904. https://catalog.hathitrust.org/Record/009561180.