The council of any city, including any city of this state operating under a home rule charter adopted pursuant to the Constitution of the state of Minnesota, article 4, section 36, article XI, section 4, or article XII, section 5, may establish along any street or highway within such city a building line upon the land adjoining such street or highway, or any portion thereof, and distant not more than 50 feet from the margin of such street or highway, and may, in behalf of the city, acquire an easement in the land between such line and exterior street line, such that no buildings or structures shall be erected or maintained upon this land. Such easement shall be known as a building line easement. The governing body may, at the time they designate the easement to be acquired and define the line by which it is bounded, provide in the resolution designating such easement that buildings or structures or any portions of buildings or structures existing within the boundaries of the easement at that time may remain thereon for stated periods of time or remain thereon during the life of such buildings or structures or portions thereof, but no alteration of any such buildings or structures or portions thereof upon such easement shall be permitted after the designation of such easements, and when such buildings are removed no other buildings or structures shall be erected thereon. Such permission to maintain existing structures upon such easement shall be clearly defined as to time in such resolution and shall confer the right upon the owner of such buildings or structures or portions thereof to maintain the same as defined in such resolution.