Nothing herein contained shall require any change in the plans or construction of a building or a structure actually under construction on the Effective Date of this Ordinance, if a building permit shall already have been obtained for the construction of such building or structure and provided further that construction of the entire building or structure, if a residence, shall be completed within a period of one (1) year from date of passage of this Ordinance in accordance with the provisions of such prior ordinance and the plans and specifications upon which the building permit was issued and construction of commercial structures shall be completed within a period of eighteen (18) months from Effective Date of this Ordinance in accordance with the provisions of such prior Ordinance and the plans and specifications upon which the building permit was issued.