The Dey Mansion, in Passaic County, New Jersey, was constructed circa 1772 and served as the military field headquarters for General George Washington, who occupied the building during July, October, and November of 1780. This high style Georgian building, sitting on a 2.15-acre site, was restored in the 1930’s by the Civil Works and Emergency Relief Administrations. Because 75 years had passed since that restoration, the need for significant improvements to the property became evident to address deferred maintenance, correct inappropriate repairs that compromised the building’s historical integrity and upgrade public accessibility. Prior to developing a scope for the project, our firm led an investigation phase that included review of historic documents, assessment of existing architectural and structural conditions, analysis of finishes, dendrochronology and archaeology. Major work categories included structural repairs and stabilization of roof and floor framing and a leaning masonry gable end wall; roof replacement; masonry repointing; window restoration; flooring restoration; plaster repairs; and painting. Mechanical and electrical systems were replaced. Site improvements included the introduction of French drains and seepage pits to handle rainwater run-off, thereby avoiding introduction of gutters and downspouts for which there was no historic evidence. Grade was raised along the north-side of the building to provide accessible entrances at two different floor levels. This was achieved by dry-laying mortarless retaining walls constructed of stones salvaged from the site. A separate Visitors’ Center is planned for a second phase of the project, which will include accessible interpretive and toilet facilities.