In the R-3 zone each lot shall have a rear yard of not less than twenty feet (20'), except that the rear yard of an apartment building may be reduced to the same dimension as the required side yard if a common open space which is equal to or greater in area than the reduction in the required rear yard is provided. Such common open space shall be provided in addition to the required space between buildings. (Ord. 1000 § 20.20(H), 1955)