The Heintzman House (circa 1817), also known as Sunnyside Manor Farm, is one of the oldest buildings in Thornhill- Markham, and has one of the most interesting histories of any building in the area. The earliest mudhouse on record in the province, the house is constructed of adobe brick, fired brick and frame construction. The central five bays are the oldest portion of the house, which includes the adobe brick construction.
The Yonge Street Crown Grant property (Lot 32, Concession 1, Markham) was awarded to United Empire Loyalist, Anthony Hollingshead in July of 1798. Hollingshead, originally from New Jersey, was a veteran of the American Revolutionary War, serving as an officer with the 3rd New Jersey Volunteers under the command of General Courtland Skinner. Prior to receiving the patent (final property deed), Crown Grant recipients had to complete settlement duties which included clearing land for cultivation, building a home no less than 16 feet by 20 feet in size, and clearing the land facing Yonge Street for use as a public road. Settlers were given two years to complete their duties, extensions were often granted. Anthony, his wife Elizabeth, son Anthony Jr., including a daughter, her husband and child all made their home in a modest two- room, one-half storey adobe brick farmhouse. Portions of the adobe farmhouse are believed to have been incorporated into the existing house, which has seen several additions and changes over the past many years.