For lots of less than forty-three thousand five hundred sixty (43,560) square feet (one (1) acre), but of greater than or equal to twenty-one thousand seven hundred eighty (21,780) square feet (one-half (.5) acre), i.e. 21,780 SF < lot size < 43,560 SF, which lots were existing at the time of annexation, the minimum greenspace percentage shall be sixty percent (60%) to a maximum impervious surface area of ten thousand eight hundred ninety (10,890).