Rather than adopt the neo-classical style of the first two office buildings, the architect gave the Hart Building a more distinctly contemporary appearance, although with a marble façade in keeping with its surrounding. The architects sought to design a flexible, energy-efficient building that would accommodate both the expanded staff and the new technology of the modern Senate. The building's design also deliberately spared the adjacent Sewall-Belmont house, a historic structure that serves as headquarters for the National Woman’s Party and a museum for the woman suffrage movement. As construction proceeded, however, rapid inflation in the 1970s multiplied costs and caused several modifications of the original plan, most notably the elimination of a rooftop restaurant and a gymnasium.
The nine-story structure provides offices for fifty senators, as well as for three committees and several subcommittees. Two-story duplex suites allow a senator’s entire office staff to work in connecting rooms. Where solid walls limited the arrangement of office space in the two older buildings, movable partitions permit reconfiguration of offices in the Hart Building to meet changing needs. Designed for modern telecommunications, removable floor panels permit the laying of telephone lines and computer cables, further aiding the rearrangement of offices as computers rapidly alter staff functions. On the building's roof, microwave satellite dishes expand senators’ communication links with the news media in their home states.
The large Central Hearing Facility on the second floor of the Hart Building was designed for high-interest events attracting crowds that could not be accommodated in the regular hearing rooms. The facility offered more seating, better acoustics, and movable side panes where television cameras could operate without distracting the participants. Behind the dais where committee members sit, the Senate seal is affixed to a white and gray marble wall, which contrasts with the wood-paneled side walls. The room has become familiar to television viewers as the site of numerous Senate investigations and confirmation hearings.
Situated the farthest from the Capitol, the Hart Building was connected underground to an extension of the existing Capitol Subway to the Dirksen Senate Office Building. In 1994 a new train loop was installed that provided more cars and speedier service to handle the increased traffic between the buildings. With wider doors and trains at platform-level, the new system is also fully accessible to the handicapped. In addition, the Hart Building provides three floors of underground parking.
The centerpiece of the atrium is Alexander Calder's mobile-stabile, Mountains and Clouds. The monumental piece combines black aluminum clouds suspended above black steel mountains, with the tallest peak being high. It was one of Calder's last works. The sculptor came to Washington on November 10, 1976 to make the final adjustments to his model, and died later that evening after returning to New York. Budget cuts delayed construction of the sculpture until 1986, when former New Jersey Senator Nicholas F. Brady raised private funds to underwrite the installation.