Eligible basis reduced where disproportionate standards for units
(A) In general Except as provided in subparagraph (B), the eligible basis of any building shall be reduced by an amount equal to the portion of the adjusted basis of the building which is attributable to residential rental units in the building which are not low-income units and which are above the average quality standard of the low-income units in the building.
(B) Exception where taxpayer elects to exclude excess costs (i) In general Subparagraph (A) shall not apply with respect to a residential rental unit in a building which is not a low-income unit if— (I) the excess described in clause (ii) with respect to such unit is not greater than 15 percent of the cost described in clause (ii)(II), and (II) the taxpayer elects to exclude from the eligible basis of such building the excess described in clause (ii) with respect to such unit. (ii) Excess The excess described in this clause with respect to any unit is the excess of— (I) the cost of such unit, over (II) the amount which would be the cost of such unit if the average cost per square foot of low-income units in the building were substituted for the cost per square foot of such unit. The Secretary may by regulation provide for the determination of the excess under this clause on a basis other than square foot costs.