The main building (also known as the McKim Building), constructed between 1902 and 1906 as the original structure in the complex, was designed in the Classical Revival style by Charles Follen McKim of McKim, Mead & White. The original building occupies a lot of , and was intended to be similarly scaled to New York Public Library branches of the era. The 1928 annex to the building, designed by Benjamin Wistar Morris to harmonize with McKim's original, contains architectural detail differing from that of the original structure. The annex measures , with a later addition.