The building was expanded dramatically in the 1850s. The original timber-framed dome of 1818 would no longer be appropriately scaled. Thomas U. Walter was responsible for the wing extensions and the "wedding cake" cast-iron dome, three times the height of the original dome and 30 m in diameter, which had to be supported on the existing masonry piers. Like Mansart's dome at Les Invalides (which he had visited in 1838), Walter's dome is double, with a large oculus in the inner dome, through which one views the Apotheosis of Washington painted on a shell suspended from the supporting ribs, which also support the visible exterior structure and the tholos that supports the Freedom, a colossal statue that was added to the top of the dome in 1863. The weight of the cast-iron for the dome has been published as 8,909,200 lb of iron (4,041,100 kg). For construction details, see links.

 
The Capitol when first occupied by Congress, 1800.When the dome of the Capitol was finally completed, but to a significantly enlarged design than had initially been planned, its massive visual weight overpowered the proportions of the columns of the East Portico, built in 1828. An extended addition to the Capitol was constructed in 1958, but the historic Corinthian columns were rendered homeless, until landcape designer Russell Page created a suitable setting for them in a large meadow at the National Arboretum, where they are combined with a reflecting pool in an ensemble that reminds some visitors disconcertingly of Persepolis.