The Belfast-based firm plans to completely overhaul and extend the Modernist structure, known as the War Memorial Building, to create a 63-room hotel including conference and leisure facilities.
Located in the city's Cathedral Quarter, the War Memorial Building was built on the site of the Queen Anne Hotel, which was obliterated during the Belfast Blitz in 1941.
Constructed between 1959 and 1961 to the designs of English architect J Michael Bowley, the replacement building initially provided offices for charities and organisations linked to the Armed Forces.
McGonigle McGrath – a finalist for the 2015 RIBA House of the Year award – plans to restore the original building, including the stone framework, bricks and slate panelling.
It also proposes to revive a copper-clad water, described by the firm as "distinctly Corbusian", and will add several new details including bronze-framed windows and a simple concrete canopy.
A new building will be constructed alongside the restored memorial block, featuring grey brick piers and exposed concrete floor plates that mimic the gridded facade if its neighbour.
"The new building is derived from the original, emphasising its rhythm and proportion," said studio co-founder Kieran McGonigle.
"The use of brick piers as the expression of the vertical structure at once implies a subservience to the original building but also acknowledges the tradition of structural brick warehouses in Belfast, and references the spirit of the modern movement, handsomely retained in the existing building," he added.
The two building will frame a new courtyard garden with a tree at its centre, enclosed behind an additional brick wall.
The site is located close to the city's historic centre and its northern border is flanked by Sugarhouse Entry, a historic passageway once home to a number of inns, which is named after a sugar refinery built there in the 17th century.
McGonigle McGrath plans to reestablish this route.
The project marks the latest in a series of new hotels planned in Belfast's Cathedral Quarter, including one in a former warehouse on Donegall Street, and one in a former office on Victoria Street.
Visualisations are by Forbes Massie.