A side yard of not less than 20 feet shall be provided for on each side of every building, except that in the case of lots existing as of May 6, 1958, and less than 100 feet in width measured at the narrowest place adjacent to the location of the dwelling, the side yard may be reduced by 1/4 of the difference between said width and 100 feet; provided, however, that the side yard shall in no event be less than 15 feet.