The firm is to refurbish the 2002-era building at 400 South Oak Way in Green Park.
Green Park is owned and managed by Oxford Properties.
As well as mechanical and electrical services upgrades, ISG is to update five lift lobbbies and modernise the building's atrium.
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Externally, photovoltaic panels are to be installed, while extensive hard and soft landscaping works will be undertaken.
Other features include new cycle paths and cycle storage, additional parking and improved external lighting.
Graeme White, ISG's divisional director, said: "As the Thames Valley continues to carve out a reputation as the business destination of choice for major domestic and international blue chip organisations, property investors are responding by upgrading existing assets to extremely high standards."
(LM/CD)
