(2) An owner who converts a housing accommodation containing more than 3 and fewer than 11 units, each with a separate certificate of occupancy, including vacant buildings but excluding a building that the Mayor has determined to be a blighted vacant building, as that term is defined in § 42-3131.05(1) , and thereby reduces the number of units in the housing accommodation, shall pay the Mayor an additional conversion fee of 5% of the appraised value of the most valuable unit in the building multiplied by the net decrease in units. When calculating the net decrease in units pursuant to this paragraph, the Mayor shall subtract one unit for each unit that qualifies for the fee exemption under subsection (b) of this section; except, that the result shall not be less than zero.