What era was the well known buliding on the border of United States and Canada declared a heritage building by both?
Line house:  The most well known such building on the boundary between the United States and Canada is the Haskell Free Library and Opera House in Stanstead, Quebec and Derby Line, Vermont. Haskell Free Library and Opera House:  It was declared a heritage building by both countries in the 1970s.
It was declared a heritage building by both countries in the 1970s.