I'm not at all sure that it can be replaced with the same quality of construction, and the current building torn down for less money.
http://www.historicplaces.ca/en/rep-reg/place-lieu.aspx?id=4298
The Prime Minister’s Residence was designated a Classified Federal Heritage Building because of its direct association with six prime ministers of Canada, its status as a nationally known landmark, and because of the impact of the house and its grounds on the character of the area.
During its first 75 years, 24 Sussex Drive was associated with three lumber barons of the area. By 1943, it was the last remaining private residence on Sussex Drive, in an area given over increasingly to foreign embassies and parkland. In 1949, it was expropriated by the Government of Canada and became the official residence of the prime minister.
Sources: Jacqueline Adell, Prime Minister’s Residence, Ottawa, Ontario, Federal Heritage Building Review Office Building Report 85-067; Prime Minister’s Residence, Ottawa, Ontario, Heritage Character Statement, 85-067.
Historic building. Would they tear down the White House?