When Rome was annexed to the Kingdom of Italy (1870) and proclaimed capital of the new State (1871), Palazzo Madama was chosen to be the seat of the new Senate.
This made further conversion work necessary. In the space occupied by the courtyard of the Papal State's postal administration, architect Luigi Gabet constructed the sitting chamber of the Senate. The Senate of the Kingdom met here for the first time on 28 November 1871. In 1888 a new structure, designed by Gaetano Koch, was built inside Palazzo Madama to accommodate the library.