Nothing in this code or the rules and regulations pursuant thereto shall affect any building permit lawfully issued, or construction on any building or structure lawfully begun in conformance with the permit, before July 1, 1977, in any city or town. Provided, that work under the permit is commenced within six (6) months after its issuance, and that the work, whether under the permit or otherwise lawfully begun, proceeds in good faith continuously to completion so far as is reasonably practicable under the circumstances.