The former Ministry of Defence building was bought for £350m in 2015 by Indian conglomerate Hinduja Group and Spanish construction group Obrascón Huarte Lain Desarrollos.
They are converting it into a 125-room Raffles hotel, and 85 apartments range in size from studios up to five-bedroom.
The works will encompass the addition of three storeys and extension of its basement down to five levels to include a ballroom and deluxe spa facilities.
Works will also include the demolition, relocation and reconstruction of the facades of the building's Triangular Courtyard to incorporate new external extensions.
Gardiner & Theobald, quantity surveyor on the job, puts the overall project cost at £1bn.
Ardmore managing director Cormac Byrne said: “We are delighted to have been selected from amongst other London contractors to undertake the restoration and renovation of The Old War Office. We share the vision of the building’s owners to breathe new life into it and create a very special legacy for generations to come. The redevelopment will bring back into use a much-loved building in the heart of London and represents a huge vote of trust in the professionalism and skill of all our colleagues at Ardmore. Ardmore continues to go from strength to strength and is looking forward to working on more major developments across the capital.”
Recent Ardmore jobs in the capital include Southwark Fire Station, Greenwich’s Design District, Hornsey Town Hall and Queen’s Square.