Heintzman House, Heritage house in Markham, Canada.

Heintzman House is a 13-room mansion on the crest of Bay Thorn Drive in Markham, Ontario, also known as Sunnyside Manor Farm. Its central five bays are believed to incorporate the Hollingshead mudhouse, and the black walnut paneling and moulding throughout were milled from trees harvested on the estate. The Crookshank additions give it a mix of Classic Revival and Georgian look, while the Heintzmans' pillars, portico and conservatory add Georgian and Regency elements.

Anthony Hollingshead, a New Jersey-born Loyalist officer, received the 190-acre Yonge Street crown grant in 1798 and built a two-room adobe farmhouse; he obtained the patent in 1802. Around 1817 the Honourable George Crookshank bought the property and replaced it with a 13-room mansion, using black walnut milled on the estate. In 1930 Charles Heintzman of the piano family bought it and added the portico, pillars and conservatory.

The house is named after Charles Theodore Heintzman, grandson of the founder of the Heintzman Piano Company, who bought Sunnyside Manor in 1930 and kept a herd of Jersey cattle there. In May 2000 the United Empire Loyalists' Association of Canada presented the First Union Flag at the house in recognition of Anthony Hollingshead, followed by a Hollingshead clan reunion that July.

The house stands on the crest of Bay Thorn Drive in Thornhill-Markham, north of Toronto. It is owned by the City of Markham and has served as a community centre and recreation offices; the grounds and exterior can be seen from the street, while interior access depends on the city's bookings. Parking is on site.

The house's central five bays are believed to incorporate the original Hollingshead mudhouse, built of adobe brick, fired brick and frame. In May 2000 the United Empire Loyalists' Association of Canada presented the First Union Flag here in recognition of Anthony Hollingshead, and a Hollingshead clan reunion followed in July 2000.