The present Treasury Building was built over a period of 33 years between 1836 and 1869. The east and center wings, designed by Robert Mills, architect of the Washington Monument and the Patent Office Building, comprise the first part of the building constructed between 1836 to 1842. The most architecturally impressive feature of the Mills design is the east colonnade running the length of the building. Each of the 30 columns is 36 feet tall and is carved out of a single piece of granite. The interior design of the east and center wings is classically austere, in keeping with the Greek Revival style.
Later additions were made to the original wings, beginning with the construction of the south wing from 1855 to 1860 and the west wing from 1855- 1864. The preliminary design of the wings was provided by Thomas Ustick Walter, architect of the Capitol dome, but architects Ammi B. Young and Isaiah Rogers refined the plans, designed the interior details, and supervised construction. While the exterior of the building was executed along the lines of the original Mills wings, the interiors of the later wings reflect changes in both building technology and aesthetic tastes. Iron columns and beams reinforced the building's brick vaults; the architectural detailing became much more ornate, following mid-nineteenth century fashion. The final addition to the Treasury Building was the north wing, built from 1867 to 1869. Its architect was Alfred B. Mullett. Similar in construction and decor to the south and west wings, the north wing is unique as the site of the Cash Room -- a two-story marble hall in which the daily financial business of the U.S. Government was transacted. The room opened in 1869 as the site of President Grant's inaugural reception.http://www.nps.gov/nr/travel/wash/dc35.htm