(1) The tax shall be imposed at the rate of one dollar ($1.00) per one hundred dollars ($100) of construction or improvement costs. Construction or improvement costs shall be calculated in the same manner as required for permits under Section A-902.3; provided that the Department may by regulation establish an alternative methodology for determining costs, including, at the Department's discretion, by using estimated costs. For a structure used or intended for use in significant part for non-residential purposes, the tax shall be charged only with respect to the costs associated with improvements to the portion of the structure used or intended for use for residential occupancy, including common space related to such purposes; provided that the Department may by regulation establish a method for estimating such costs.