Renovations cost $1.3 million CDN and were supervised by the architect Septimus Warwick, who moved the main entrance from Trafalgar Square to Cockspur Street.

The building was officially opened on 29 June 1925 by King George V.[3] When he declared Canada House open, King George said: "Canada is a great country: alike in the literal sense of vast extent from 'sea to sea' and great in achievement and in promise: and it is right and necessary that its official representatives here should be housed in a manner worthy of the Dominion and adequate to the discharge of their ever-growing and important duties.

"[4] During the London Blitz, a bomb fell near the building, only 20 yards (18 m) away from future Canadian prime minister Lester B. Pearson, who was the secretary to the High Commissioner at the time.

To commemorate the reopening, a detachment from the 3rd Battalion, Princess Patricia's Canadian Light Infantry, came to London and mounted the Queen's Guard at Buckingham Palace.

The main work of the High Commission, including consular, public affairs, political, trade and administrative functions was then carried out from Macdonald House in Mayfair.

[9] The building at 2-4 Cockspur Street was constructed between 1926 and 1929 for Sun Life Assurance Company of Canada and had served as the Canadian Army's overseas headquarters in London during World War II.