The radical transformation and upgrade of Walsingham House will see the firm demolish the interior building structure while retaining the period Portland stone façade.
New piled foundations will allow construction of a steel frame structure, adding two extra floors to the existing eight-storey building and creating nearly 50% more internal office space.
Externally, the retained façade will be cleaned and restored, and to achieve a targeted BREEAM Excellent rating, ISG will upgrade glazing with high performance units sympathetic to the original art deco window designs.
Located in a conservation area, Walsingham House lies opposite the Grade I listed St Olave’s Church and adjoins a four star hotel.
The area is steeped in history and the project kicks off with enabling works and an archaeological investigation to uncover and record remains, which are expected to include sections of a Roman road and structural elements from the 13th, 16th and 17th centuries.
Matt Blowers, managing director of ISG’s UK Fit Out and UK Construction South business, said: “Current schemes at nearby Dixon House, Alderman’s House and 20 Old Bailey, continue to demonstrate our capability in these highly technically façade retention projects.
As prime office locations continue to attract high rental yields, property owners are increasingly looking at all options to increase capacity and provide modern layouts to maximise portfolio yields.”