Client Investec currently occupies two floors of the building at 30 Gresham Street, and the new fit-out will see it move into the fourth, fifth and sixth floors, as well as the lower ground.
The project involves the full strip-out and fit-out of the existing floors, which includes installing an atrium staircase from levels four to eight.
ISG used 3D digital modelling to identify the logistics required to install the feature staircase.
Matt Blowers, managing director of UK Fit Out, ISG, said: “This latest win reflects the strength of ISG in the London office fit out market.
Demand for premium office space remains at a high level across London as organisations seek high-specification facilities in optimum locations. ”
The project is expected to be finished in May 2018.