The "new" annex to the State House was built between 1902-1906 to replace two 19th century annexes that were poorly built and inadequate in size. The Baltimore architects Baldwin & Pennington designed and supervised the construction of the new annex, which houses both the Senate and the House of Delegates.

Both chambers feature unusual black and gold marble in tribute to the colors of the Maryland flag, as well as skylights by the studio of Louis Comfort Tiffany. The doors of this annex that lead out to State Circle are actually the back doors of the State House but are now the main entrance for visitors. The doors are of cast bronze and the Maryland seal is featured in the roundels.

Another feature of the annex is the grand staircase to the second floor. On the landing is the monumental painting of Washington Resigning His Commission painted in 1859 by Edwin White.

The walls in the Annex are lined with Italian marble. The black and white alternating tile floors in the new annex were selected to match the 1881 tiles in the original portion. The fossils in the black marble date back 450 million years.

Noteworthy plaques and memorials in the State House Annex include:
o The State House Building Commission
o The American's Creed by William Tyler Page, December 23, 1919