(a) In the SD1 and SD2 Districts, machine shop facilities with a minimum of ten percent (10%) of the gross floor area being used as a retail store for the sale of the goods produced may be permitted as a Conditional use.| (b) In the SD1 District, machine shops shall not exceed four thousand (4,000) square feet in gross floor area, as provided in Sec. 61-11-173 of this Code; this prohibition may not be waived by the Board of Zoning Appeals; in the SD2 District, machine shops shall not exceed five thousand (5,000) square feet in gross floor area, as provided in Sec. 61- 11-193 of this Code; this regulation may not be waived by the Board of Zoning Appeals.