has not been required to be recaptured under section 50(a). (c) Definitions (1) Qualified converted building (A) In general The term qualified converted building means any building (and its structural components) if— (i) prior to conversion, such building was nonresidential real property (as defined in section 168) which was leased, or available for lease, to office tenants, (ii) such building has been substantially converted from an office use to a residential, retail, or other commercial use, (iii) in the case of conversion to a residential use, such converted building meets the requirements of subparagraph (D), (iv) such building was initially placed in service at least 25 years before the beginning of the conversion, and (v) depreciation (or amortization in lieu of depreciation) is allowable with respect to such building. (B) Substantially converted defined (i) In general For purposes of paragraph (1)(A)(ii), a building shall be treated as having been substantially converted only if the qualified conversion expenditures during the 24-month period selected by the taxpayer (at the time and in the manner prescribed by regulation) and ending with or within the taxable year exceed the greater of— (I) the adjusted basis of such building (and its structural components), or (II) $15,000. The adjusted basis of the building (and its structural components) shall be determined as of the beginning of the 1st day of such 24-month period, or of the holding period of the building, whichever is later. For purposes of the preceding sentence, the determination of the beginning of the holding period shall be made without regard to any reconstruction by the taxpayer in connection with the conversion. (ii) Special rule for phased conversion In the case of any conversion which may reasonably be expected to be completed in phases set forth in architectural plans and specifications completed before the conversion begins, clause (i) shall be applied by substituting 60-month period for 24-month period . (iii) Lessees The Secretary