The William Foster House is a historic house in Andover, Massachusetts. The core of this house is a three-bay two-story late First Period structure built c. 1720. It was added onto about 1750, adding a two-story one-room ell to the southwest corner of the house. A new chimney core was added on the north side late in the 18th century, and a second two-story wing was added on the northwest. Although the house is traditionally dated to c. 1660 (with ownership of the land traced to 1635), the stylistic construction techniques indicate a later construction date.