a separate new building. (f) Definition and special rules relating to credit period (1) Credit period defined For purposes of this section, the term credit period means, with respect to any building, the period of 15 taxable years beginning with— (A) the taxable year in which the building is placed in service, or (B) at the election of the taxpayer, the succeeding taxable year, but only if the building is a qualified middle-income building as of the close of the 1st year of such period. The election under subparagraph (B), once made, shall be irrevocable. (2) Special rule for 1st year of credit period (A) In general The credit allowable under subsection (a) with respect to any building for the 1st taxable year of the credit period shall be determined by substituting for the applicable fraction under subsection (c)(1) the fraction— (i) the numerator of which is the sum of the applicable fractions determined under subsection (c)(1) as of the close of each full month of such year during which such building was in service, and (ii) the denominator of which is 12. (B) Disallowed 1st-year credit allowed in 16th year Any reduction by reason of subparagraph (A) in the credit allowable (without regard to subparagraph (A)) for the 1st taxable year of the credit period shall be allowable under subsection (a) for the 1st taxable year following the credit period. (3) Determination of applicable percentage with respect to increases in qualified basis after 1st year of credit period (A) In general In the case of any building which was a qualified middle-income building as of the close of the 1st year of the credit period, if— (i) as of the close of any taxable year in the credit period (after the 1st year of such period) the qualified basis of such building exceeds (ii) the qualified basis of such building as of the close of the 1st year of the credit period, the applicable percentage which shall apply under subsection (a) for the taxable year to such excess shall be the percentage equal to 2/3 of the applicable percentage