Lot Area. There shall be a lot area of not less than five thousand (5,000) square feet, and all lots shall have a frontage on a public street of not less than fifty (50) feet; provided, however, that any lot of record as of the effective date of this chapter which shall have a lot area of not less than three thousand three hundred (3,300) square feet and frontage on a public street of not less than thirty-three (33) feet may be utilized for construction of and occupied by a single-family detached dwelling. Nothing herein shall be construed as to permit the division of an existing lot for the construction of a building unless all requirements with respect to area and yards are complied with. Further, any structure in existence as of the effective date of this chapter may be converted to any use permitted in this district, as long as no alteration to the exterior shall increase any existing legal nonconformance as to front, side or rear yard or otherwise violate front, side or rear yard regulations contained herein.