It is one of the largest commercial office refurbishment schemes to be awarded this year.
The building, now owned by Japanese company Kato Kagaku, will be renamed the Aldwych Quarter on completion.
The four buildings and central courtyard that make up the Aldwych Quarter will be stripped back to shell and core and completely overhauled. ISG is providing more than 300,000 sq. ft. of lettable Cat A open-plan office space with new mechanical and electrical services.
The contractor will re-roof all four buildings, clean the façades, carry out masonry repairs and replace a number of windows as part of an external envelope package of works. Numerous architecturally listed elements will also be cleaned, repaired and preserved, including the Bush House portico, staircases and lobby areas.
Three of the buildings are targeting a BREEAM Excellent performance rating and ISG will achieve significant sustainable enhancements through improved insulation and the highly efficient services installation.
ISG Fit Out managing director Paul Cossell said: “A key factor in our appointment to the Aldwych Quarter development was our innovative and flexible delivery programme, providing a significant reduction in project duration. Our phased handover strategy allows earlier than anticipated occupation of key areas of the development, providing an attractive potential revenue boost for our client.
“The Aldwych Quarter scheme represents an excellent win for the business and reinforces our high quality delivery credentials as London’s leading commercial office fit out specialist. Our expertise delivering complex, large-scale projects within architecturally sensitive environments will undoubtedly prove critical in the successful completion of this landmark London office development.”
ISG chief executive David Lawther added: "We are witnessing growing improvement in activity across the London Fit Out market and this latest win reinforces our position as the sector's lead contractor."