Whenever and wherever a building line shall be established as aforesaid, all structures extending within such building lines shall be required to conform to the new line within a period of not more than twenty-five years from the time of establishing said lines, such time to be provided in the ordinance providing for the establishment of such line. At the expiration of the time limit in which all structures are so required to conform to the new building line, the proper municipal authorities shall proceed in the manner then provided by law relating to condemnation proceedings by such cities to determine the additional damages sustained by the removal of such structure then within the building line and may in the same proceeding acquire the land within the building lines or any part thereof as a street.