One King William Street involves the extension and reinvention of two neighbouring buildings into high-specification office space within the heart of the Bank Conservation Area. The project’s aims are threefold: to restore the listed neoclassical corner building by reinvigorating its original features; to reclad and extend vertically its southern neighbour; and to internally unify the buildings such that, from within, the two are unified. To the south, the composition of a new masonry shell is a direct response to the heritage of the listed corner building. The principal massing on the vertiginous St Swithin’s Lane is formed of a tripartite system of stepping planes and varied window reveals, which progressively reduces in scale towards the top of the building. The double order of the lower four levels terminates at a cornice line that aligns with that of the neo-classical building, while the restrained material palette of hand-thrown bricks and bronze-toned metals confidently complements the existing Portland Stone. Internally, the single floorplate (across the two buildings) is reorganised and a new core inserted to clarify spatial sequences. A new terrace, located on the roof of the listed building, capitalises on views toward the Bank of England and maximises external amenity.