For the purposes of promoting the public health, safety, morals, comfort, and general welfare, conserving the values of property throughout the city, and lessening or avoiding congestion in the public streets and highways, the city council may, from time to time, in the manner set forth in §§ 153.209 through 153.215, amend the regulations imposed and the districts created by this chapter; provided, that in all amendatory ordinances adopted under the authority of this section due allowance shall be made for existing conditions, the conservation of property values, the directions of building development to the best advantage of the entire city, and the uses to which property is devoted at the time of the adoption of any amendatory ordinance.
('69 Code, App. A, § 10.7-1)