Encouraged by the early success of the building, Shepherd Brooks purchased the 68-by-200-foot (21 by 61 m) lot adjoining to the south in 1893 for $360,000 ($ in dollars). Aldis recommended the firm of Holabird & Roche, who had designed the Pontiac Building for Peter Brooks in 1891, to extend the Monadnock south to Van Buren. William Holabird and Martin Roche had trained together in the office of William LeBaron Jenney, and in 1881 formed their own firm, which would become one of the most prolific in the city and the acknowledged leader of the Chicago school of architecture. The north half had struggled with cost overruns and Holabird & Roche presented a far more cost-effective design. The design, for two buildings called the Katahdin and Wachusett (also named for New England mountains), connected them to the north half as a single structure at an estimated cost of $800,000 ($ in dollars). Construction began in 1892, under the supervision of Corydon T. Purdy who would later earn accolades as the structural engineer for many famous Chicago and New York skyscrapers.