The firm is to develop the new BBC Wales Broadcasting House and the Two Central Square office building adjacent in Cardiff.
The scheme, for developer Rightacres, was designed by architect Foster + Partners.
The 150,000 sq ft broadcasting house is located opposite Cardiff Central Railway Station and will be built adjacent to Two Central Square.
Set over four floors, it will include space for studios and production, while also providing office space for over 1,200 BBC Cymru Wales staff.
ISG will form a 75,000 sq ft basement structure by installing secant piles and excavating approximately 50,000 cubic metres of material, which will be removed and re-used off site.
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Both the BBC building and Two Central Square use a hybrid structure, with a reinforced concrete frame to ground floor level, concrete cores and a steel frame superstructure for the upper floors.
Both buildings will be delivered simultaneously, with Two Central Square providing approximately 120,000 sq ft of office space across seven floors, with ground floor retail accommodation.
The BBC's design team includes Capita and Sweets, and part of the ISG contract will include delivering the first phase of the BBC's fit out, which has been designed by IDSR and AECOM.
Phil Brown, ISG's UK Construction MD, said: "Constructing the new headquarters for BBC Wales is a major win for the business and brings all of our office expertise to bear on one of the largest ever private developer-led projects to be built in Wales.
"This is also a highly-specified building, with demanding acoustic and vibration tolerances and advanced mechanical and electrical services, which plays to our significant strengths delivering complex technical services infrastructure in the data center, R&D and healthcare sectors."
(LM/CD)
