As to impervious cover only, notwithstanding the requirements of § 115-96B above, a single-family detached dwelling located in the R-1 or R-2 District on any single lot of record in existence on March 21, 1988, which dwelling does not meet the impervious coverage requirements for the district in which it is located, shall be permitted to surpass the impervious coverage requirement in the district in which it is located by no more than 15% of the existing cover or 15% of the maximum otherwise applicable in the district in which it is located, whichever is greater. The amount surpassed may be applied to either building coverage or impervious coverage, provided that in no case shall impervious cover exceed 35%. Any modification, alteration, repair, reconstruction or improvement of any kind to an extent or amount of 50% or more of its impervious cover shall be undertaken only in full compliance with the impervious coverage standards for the district in which it is located.