Where a lot exists of less than required depth and is pocketed by developed lots and the lot was in separate ownership on or before the date of adoption of this chapter, the required minimum total side depth of both side yards may be reduced by one-half foot for each one (1) foot interval of lot width less than required width except that in no case shall only one (1) side yard be reduced to less than three (3) feet and the total side yard less than ten (10) feet and further, in no case shall the residential structure be less than eighteen (18) feet in width.