 In 1889, Duncan designed a five-story limestone mansion for Philip Lehman at 7 West 54th Street, which is now known as the "Philip Lehman Masion" and designated as a New York landmark in 1981.He also designed Walhall, a "great estate" in Greenwich, Connecticut.Although the main house no longer exists, an outbuilding intended eventually for the superintendent still exists and is a private residence today.