In plan the building is a 60 x 60m trapezoid; the structure is a steel frame with concrete-encased columns at the perimeter and perforated cell beams supporting floors of profiled steel decks with structural cast in situ concrete topping.
The east and west facades are five and six storeys high respectively, with additional floors set back behind them.They are clad with a robust framework of Portland stone, which was designed, in the architect's words, 'to maximise light yet retain a sense of gravitas'.
The primary elements of the framework are a series of bays formed by the columns, set at 6m centres, which rise to meet a continuous parapet lintel. Both columns and parapet are constructed with Portland stone masonry.The facade at ground and first-floor level is glazed, with a beadblasted aluminium bressummer beam enclosing the first floor. Each upper-floor bay is glazed with double-glazed windows in polyester powder-coated frames, set behind a frame of a 500 x 250mm Portland stone mullion and a 500 x 250mm precast concrete lintel. Window heads and mullions are concealed behind the masonry frame, and the precast lintel is fitted with an aluminium sill canted to direct water away from the leading edge.
The Portland stone masonry, a Grove Whitbed taken from the Albion Quarry, is a more rugged variety, with a higher shell content than that used for St Paul's.The masonry blocks,150mm thick and 670mm deep, are self-supporting; they were handfixed with stainless steel restraint cramps to stainless steel channels cast into the concrete columns and with stainless steel restraint cramps at corners.The masons worked from a gantry suspended from the roof, which rose to each stone course as required.