Rideau Hall
Built in 1839 for Thomas McKay, Rideau Hall was originally an elegant stone villa set in a picturesque landscaped park. In 1865 it was leased as a temporary residence for the Governor General until a suitable house could be erected elsewhere. The planned new residence was never constructed. Instead the old building was expanded and eventually almost engulfed by a series of additions between 1865 and 1914. Today Rideau Hall is the official Government House. As the residence of the Crown's representative in Canada it has been a focus of political and social life in the capital.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.