Custom-made Cantilevered Steel Staircase to Westminster Atrium

3 St James’s Square is currently undergoing a full high‑end refurbishment for Realty Income, including new Portland Stone interiors, upgraded core areas, premium office fit‑out works, and the installation of Assesmont’s sculptural feature staircase within the atrium.

Assesmont Group Ltd is installing a bespoke cantilevered feature staircase at 3 St James’s Square, London. This staircase will form the centrepiece of the building’s atrium and is a key part of the ongoing high‑end refurbishment works. Because of its complex geometry and premium finishes, the project demands precise engineering and careful coordination. The structure is built around a custom steel carcass made from rolled 30 mm plate stringers. These heavy curved sections create the strength needed for the stair’s cantilevered design. As a result, the staircase appears to float while still transferring load safely back into the building’s new steel frame. This approach supports the architect’s vision for a clean, sculptural form within the atrium.

Once the steelwork is complete, the staircase will be clad in carved Portland Stone (Jordans Whitbed). This stone finish will give the stair a monolithic appearance and create a seamless connection with the building’s wider interior palette. In addition, the stone cladding is designed to sit tightly against the steel carcass, producing crisp shadow gaps and smooth transitions. Our work includes design development, structural calculations, 3D modelling, fabrication detailing and site surveys. These steps ensure that every part of the staircase is engineered accurately before installation begins. Furthermore, Assesmont is manufacturing all architectural metalwork associated with the stair. This includes bronze‑finished balustrades, leather‑wrapped handrails mounted on custom bronze spigots, and additional balustrade elements around the landing and atrium void.