The £25m revamp of a 1980s office at 24 King William Street for Beltane Asset Management will add two floors, a limestone and glazed façade and a new interior.
Rooftop plant will be relocated to make space for offices and a viewing terrace. Mecserve is acting as building services consultant on the project which was designed by Ben Adams Architects.
The 9-storey building near monument station has an estimated gross development value of over £100m and is due to complete in the second quarter of 2017.
Beltane co-founder Duncan Roe said: “Site works have just begun and we are confident of this building’s success given the shortage of quality space coming to the market in the City.”