Co-owner British Land has confirmed the start date for work at 1 Finsbury Avenue.
Sir Robert Mcalpine will carry out the work under its framework agreement for the Broadgate complex.
The revamp of 1 Finsbury Square will add new shops, a cinema and roof terrace to the building.
British Land added that it is in exclusive discussions regarding 78,000 sq ft of improved office space – representing 27% of the total available – with technology company Mimecast.