Which part of the Capitol received a new room in the 1851 extension?

The new room added to the Capitol in the 1851 extension was designated for the House of Representatives. This extension was a significant part of the building's expansion to accommodate the growing needs of Congress as the country evolved. At the time, the House was experiencing an increase in membership due to the westward expansion of the United States and a rising population, necessitating more space for representatives and their operations.

The other parts of the Capitol did not receive new rooms during this specific expansion. The Senate had already been accommodated in its own space, and while the Supreme Court's facilities were also considered later on, they were not part of this particular expansion. The Executive Office is not housed within the Capitol, as it is located in the White House. Thus, the extension clearly catered to the pressing needs of the House, reflecting the legislative body's growth.