The firm is working on the project, which has just gone in for planning, under a pre-construction services agreement.
The £50m revamp of 100 New Bridge Street, between City Thameslink and Blackfriars railway stations, has been designed by architect Gensler.
Developer Helical, bought the 9-storey office at the end of 2021 for £160m, and now plans to upgrade and reclad the building to improve its energy performance as well as adding another storey.
Gensler is targeting BREEAM Outstanding, a NABERS 5-star rating with the aim of the building achieving net zero carbon in operation.
Under the design, the base of the building will be opened up for shopfronts, while much of the roof will be a large landscaped terrace.
Work is expected to start by the Autumn of next year for completion the following summer.