The scheme for clients Wrenbridge and Legal & General JV is on Tufton Street in the heart of Westminster.
Refurbishment work will start on site this month and is due for completion in April 2015.
The work comprises demolition of the existing mansard roof structure and southwest corner, together with internal structural modifications on all floors.
Other elements include rebuilding of the southwest corner with an enlarged floor-plate, a new steel-framed two-storey mansard roof, replacement windows and full internal fit-out.
The mansard roof will accommodate three spacious duplex penthouses with bespoke feature staircases.
Wrenbridge Managing Director Peter Jarman said: “We are very excited to be starting on site with this high quality development and looking forward to delivering a successful project with McLaren as our construction partner.”
Phil Pringle, Managing Director, McLaren Construction added: “We are extremely pleased to have secured this project which increases our profile in the high end residential sector in London Zone 1.
“We look forward to working closely with both Wrenbridge and Legal & General to deliver this scheme in line with their expectations and to reinforce the confidence they have shown by awarding McLaren the contract.”