The Capitol Extension, located on the north side of the Capitol, is a four-level underground structure which was completed in 1993. It was built to provide the Capitol with much-needed additional space without detracting from its appearance and historical importance. It is connected to the Capitol by three pedestrian tunnels.

Carved out of solid rock, the 65-foot-deep site provides the Capitol with four floors and approximately 667,000 gross square feet of new space. The Extension includes state government/legislative office space, conference rooms, 16 Committee Rooms, an Auditorium, a large Cafeteria and a Gift Shop.

Additional features include: a Seal Court with its large terrazzo rendition of the reverse side of the Texas Seal; skylights which provide a large amount of natural light in the building and allow spectacular views of the Capitol dome; five additional tunnels connecting the Extension with other buildings on the Capitol Complex; an Open-Air Rotunda patterned after the historic Capitol Rotunda; and many architectural elements and motifs complementing those in the Capitol.