Where a district boundary line divides a preexisting lot in single or joint ownership of record at the time such line is adopted, the regulations applying to the major portion of the lot shall apply to the whole lot, and, in the case where a lot is divided exactly in half by the district line, the regulations of the more restrictive district shall apply to the whole lot. However, if the back of such a lot fronts on a residential street, and the effect is to extend a commercial or industrial use onto the residential street, the above provisions shall not apply, and each portion of the lot shall conform to its respective district.