Commissariat Building 1827
This structure, the oldest existing stone building in Ottawa, was used as a storehouse, office and treasury during the construction of the Rideau Canal (1826-32) under the direction of Lieutenant-Colonel John By, R.E. Its superb masonry and solid construction are typical of the stonework done by Scottish masons along the Rideau Canal and, at a later date, on private homes in eastern Ontario. In 1854, the building was turned over to the Canadian government and, until 1951, was used successively by various departments concerned with the maintenance of the canal.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.