No new state building code, rule or regulation or amendments thereof shall affect any building permit lawfully issued, or any building or structure lawfully begun in conformance with such permit, before the effective date of the state building code in a city or town, provided, that construction work under such a permit is commenced within six months after its issue, and that such work, whether under such permit or otherwise lawfully begun, proceeds in good faith continuously to completion so far as is reasonably practicable under the circumstances.
Massachusetts Legal Code