The distance that any principal building is required by the other provisions of this chapter to set back from a side lot line shall be reduced three inches for each foot that the width of the lot, if it is located in a Residence AA-1 or A-1 District, is less than 100 feet and two inches (or four inches if such side lot line is a street line) for each foot that the width of the lot, if it is located in a Residence A-2 or A-2a District, is less than 90 feet, but in no event in any district to less than 10 feet or to less than 15 feet if such lot line is a street line.