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.
The Treasury Building is the oldest departmental building in Washington and has had a great impact on the design of other governmental buildings. At the time of its completion, it was one of the largest office buildings in the world. It served as a barracks for soldiers during the Civil War and as the temporary White House for President Andrew Johnson following the assassination of President Lincoln in 1865. The Treasury Building is unquestionably a monument of continuing architectural and historical significance. In acknowledgment of the building's significance, Treasury was declared a National Historic Landmark in 1972.
Guided tours of the building are available free of charge. The tour features restored spaces such as the 1864 Burglar-Proof Vault and the marble Cash Room. Also on the tour is the restored office of Salmon P. Chase, Secretary of the Treasury during the Civil War and the temporary office used by President Andrew Johnson following Abraham Lincoln's assassination, which has been restored to its 1860s appearance.
The Treasury Building is located at 15th and H Sts., NW. Tours of the building have been suspended, for security reasons.