Except as provided in this section, nothing herein shall require any change in the plans, construction or designated use of a building or structure for which a building permit has been lawfully issued prior to or, under the circumstances noted below, within 60 days after the effective date of this chapter, provided that the proposed use complies with the use requirements of this chapter, and further provided that construction shall begin within six months of the date of such permit, the foundation or 15% of the construction valuation shall be completed within one year of the date of such permit and the entire building or structure shall be completed within two years of the date of such permit.