The governing body of the city shall, by ordinance, determine the plans, specifications (including widths and location in the street), for the construction, repair, and reconstruction of all sidewalks: Provided, That plans and specifications may be adopted by reference, in which case a copy or copies of the plans and specifications as adopted shall be kept on file in the city clerk’s office and, if the city has a city engineer, in the city engineer’s office, and shall not be changed or amended or added to without a readoption by ordinance.