ISG is main contractor for Japanese owner Kato Kagaku Co. Ltd who are revamping the former home of the BBC.
Mountfield won a £1m contract at the site in June.
The firm has now confirmed its work at the site has extended both in terms of scope and value and is now worth £2.1m and is “expected to increase beyond that figure over the coming months.”
Sister company Connaught Access Flooring Limited has also won a deal on the job worth £400,000.
Group CEO, Graham Read said: “We are very encouraged by the continued growth in scope and value of our works at this site and also by the way in which our divisions are working together to ensure that the Group gains a substantial amount of work from the refurbishment and fit-out on this prestigious project.”