In any residential zone, only a one-family dwelling may be erected on a nonconforming zone lot of official record at the effective date of this chapter, irrespective of its area or width; provided, however, that no adjacent or adjoining vacant land exists or existed at the time of the effective date of this chapter which would create a conforming lot if all or part of the vacant land were combined with subject lot. No lot or lots in single ownership hereafter shall be reduced so as to create one or more nonconforming lots.