Should any structure be damaged or destroyed by any means to an extent of less than 50% of its replacement cost at the time of destruction, it may be reconstructed or restored, provided that its degree of nonconformity is not increased beyond that which existed just prior to such damage or destruction. Should any structure be damaged or destroyed by any means to an extent greater than 50% of the replacement cost at the time of destruction, it shall not be reconstructed or restored except in conformance with the provisions of the district regulations in which the structure is located, provided that the Board of Appeals, after a public hearing, may approve reconstruction or restoration not in conformance with the applicable district regulations as a variance and subject to the provisions of Article VI. In granting such variance, the Board shall ensure that the district regulations are followed as closely as possible. Any dwelling unit lawfully existing at the time of adoption of this chapter which shall be destroyed to any extent by any means may be repaired or reconstructed at its prior location without Board action.