State government grew steadily for many decades while the State House remained unaltered. Then, in 1845, a major addition was constructed under the direction of John Notman, a well-known Philadelphia architect. He created a one, two and three-story stepped office wing on the north side of the original building, facing what is now State Street. The new entrance had a two-story porch and six fluted Doric columns. A grand rotunda with a stairhall connected the old and new wings. This area was capped by a spherical dome and cupola. A two-story portico with pairs of Corinthian columns and a classical pediment was added to the river-side facade.

In 1865, the river-side portico was extended 68 feet. Another major building campaign began in 1871, when Samuel Sloan, also a Philadelphia architect, was commissioned to modify the northern State Street wing and design new wings for both legislative houses. These two wings flanked the 1865 southern extension. While little detail is known for certain about the final structure, it is believed that the new wings both contained a two-and-one-half-story chamber surrounded by a gallery, offices and caucus rooms. The old Senate chamber was modified to accommodate the Governor’s office, while additional offices were created in the former Assembly chamber.

Early in the morning of March 21, 1885, a fire broke out and raced through the empty building, totally destroying the State Street wing. Lewis Broome of Jersey City was selected to plan the reconstruction. He designed the building in a simplified Second Empire style with three stories and limestone facing. He also added a new rotunda and dome that were more proportional to the scale of the building.

Twenty years after Sloan’s new Assembly wing was erected, it was replaced by a larger wing of late Victorian style. James Moylan, an Assemblyman, was the architect. Due to space limitations, Moylan decided to rotate the wing so that it paralleled the building’s center wing. Accompanying this work was an addition to the west end of the original 1792 structure, which created private offices for the Governor and judges. A third floor was also added to the south end of the center wing. In 1900, the wing was extended 95 feet, ending at the edge of a water power canal, known as the Sanhican Creek. The addition was designed by the architectural firm of Karr, Poole and Lum. George Poole, one of the firm’s principals, was also an Assemblyman.

In 1903, under the direction of Merchantville architect Arnold Moses, the Senate wing was reconstructed in American Renaissance style to mirror the Assembly quarters. The wing was enlarged using classical forms and rich materials, particularly in the decorative interior and exterior treatments.

The original 1792 east wing was replaced with a four-story office section in 1906. The front area was extended on the east side in 1911. Similar work was done on the front west side the next year. In the decades following, no major structural changes occurred, aside from the modernization of the main corridor in the late 1950’s.

The effort of all these years was nearly lost in the 1960’s when a master plan called for the demolition of almost the entire building. Luckily, the plan was never executed. With today’s new respect for historic buildings, the focus has turned to preserving and restoring the structure. This is evidenced by the on-going efforts to restore the building to the grandeur of its former years.

A long renovation project began in 1987, which addressed structural, mechanical and electrical deficiencies in the State House, restored the legislative portion of the building and added legislative office space, known as the South Addition. Next, the State House Annex was renovated and a pedestrian tunnel and multi-level parking garage constructed. A public-private partnership allowed for the golden dome and interior rotunda space to be refurbished. A Welcome Center, cafeteria and landscaped "Plaza" are the most recent improvements.