No parcel of land shall be less than one (1) acre (43,560 square feet) in the A-3 zone, or one-half (1/2) acre (21,780 square feet) in the A-4 zone, nor shall any parcel of land existing in single ownership at the time of passage of this Code henceforth be divided for sale in units of less than one (1) acre (43,560 square feet) in the A-3 zone, or one-half (1/2) acre (21,780) square feet) in the A-4 zone.