Notwithstanding anything in the above provisions to the contrary, in addition to the requirements set forth therein, the front line setback of a dwelling, to be erected on a lot in the RA-3 Single-Family Residence Zones which within 100 feet on either side of such lot upon which said proposed dwelling is to be erected there are existing dwellings, shall in no case be less than the mean of the setback lines of the nearest lot on each side on which dwellings are erected.