Any parcel of land with an area or width less than that prescribed for a lot in the zone in which such lot is located, which was under one ownership and in existence at the time of the adoption of this chapter, and where the owner thereof owns no adjoining land, may be used as a lot for any purposes permitted in the zone, provided that the minimum lot size requirements for such lot shall be 7,500 square feet of lot area and 60 feet of lot width, and further provided that all other regulations prescribed for the zone by this chapter are complied with.