In December 1897, before the original Singer Building was completed, Bourne bought three five-story structures at 85–89 Liberty Street, on a plot measuring . Flagg was retained to design the fourteen-story Bourne Building on the site, and when he submitted building plans in 1898, the annex was estimated to cost $450,000. Bourne did not take title to the Bourne Building's site until September 1899, and the Bourne Building was completed the same year. By 1900, the original Singer and Bourne buildings were both fully occupied. The tenants included the law office of Augustus Van Wyck, and the Trust Company of America. Boiler manufacturers Babcock & Wilcox were long-term tenants, occupying Singer Building for more than forty years from the beginning of the 20th century.