Nothing in this chapter shall be construed as applying to a structure existing prior to April twenty-first (21st), 1975, or to a structure in existence and in full compliance with this chapter on the effective date of an amendment to this chapter, that causes the structure to be out of compliance with this chapter unless the structure is to be substantially improved after that date. Nothing in an amendment to this chapter shall be construed as applying to any structure for which fifty (50) percent or more of the date of construction, prior to the effective date of the amendment. Any structure that will be substantially improved is subject to the provisions of this chapter in the same manner as new construction and the entire structure shall meet the requirements of this chapter. [Ord. 10-15 § 2.]