Any building, structure or use for which a valid building permit has been issued and actual construction was lawfully begun prior to the effective date of this title, or amendments to this title, may be completed and used in accordance with the plans, specifications and permit on which the building permit was granted. "Actual construction" is defined to be the actual placing of construction materials in their permanent position, fastened in a permanent manner, actual work in excavating a basement or the demolition or removal of an existing structure begun preparatory to rebuilding; provided, that in all cases, actual construction work shall be diligently carried on until the completion of the building or structure. (PZSC § 03-05-007)