Portcullis House was designed and built by the architects Michael Hopkins and Partners on a simple, rectangular, courtyard plan. Seven storeys high, including two attic storeys, its external profile is similar to that of the neighbouring Norman Shaw Building. At ground level, an open arcade extends along the two street frontages, sheltering the entrance to the tube station, a row of shops on Bridge Street, and the main public entrance on Victoria Embankment.
The courtyard is covered by a glass roof at second floor level and surrounded by a two storey cloister. The upper level of the cloister serves the committee rooms and is accessible to the public via a glazed staircase projecting into the courtyard from the main entrance hall. The floors above this cloister provide the necessary additional office space for almost one third of all MPs.
Two restaurants, a coffee shop and an e-Library open onto the courtyard itself, which, though clearly visible from the entrance hall, is mainly for the use of MPs and serves as the social focus for the whole parliamentary campus north of Bridge Street. A secure underpass links the courtyard to the Palace of Westminster.
The building was designed in conjunction with the seven storey high underground chamber of the new tube station. A continuous substructure was available round the perimeter of the site to support the outer walls, but the inner walls round the courtyard had to be supported on only six columns, arranged like the spots on a domino. Shallow, asymmetrical concrete arches braced by steel tension members span between these columns at the level of the first floor cloister, forming what is effectively an above ground transfer structure.
A flat glass roof around the perimeter is propped off the columns by raking timber struts. This forms a level base for a domed diagrid of straight timber members with stainless steel joints. The frameless glass skin of the roof is supported on a delicate secondary web of steel struts and tension members which is laced through a framework of timbers.