If, at or prior to final confirmation of any special assessment, more than fifty per cent of the number of owners of privately owned real property to be assessed for any improvement, or in case of paving or similar improvements more than fifty per cent of the number of owners of frontage to be assessed for any such improvement, shall object in writing to the proposed improvement, the improvement shall not be made by proceedings authorized by this charter without a five-seventh vote of the members of the council, provided that this section shall not apply to sidewalk construction.