at which it becomes irrevocably devoted to use in the building, and (ii) as if (at the time referred to in clause (i)) the taxpayer had expended an amount equal to that portion of the cost to the taxpayer of such component or other property which, for purposes of this subpart, is properly chargeable (during such taxable year) to capital account with respect to such building. (B) Certain borrowing disregarded Any amount borrowed directly or indirectly by the taxpayer from the person converting the property for him shall not be treated as an amount expended for such conversion. (C) Limitation for buildings which are not self-converted (i) In general In the case of a building which is not self-converted, the amount taken into account under paragraph (1)(B) for any taxable year shall not exceed the amount which represents the portion of the overall cost to the taxpayer of the conversion which is properly attributable to the portion of the conversion which is completed during such taxable year. (ii) Carryover of certain amounts In the case of a building which is not a self-converted building, if for the taxable year— (I) the amount which (but for clause (i)) would have been taken into account under paragraph (1)(B) exceeds the limitation of clause (i), then the amount of such excess shall be taken into account under paragraph (1)(B) for the succeeding taxable year, or (II) the limitation of clause (i) exceeds the amount taken into account under paragraph (1)(B), then the amount of such excess shall increase the limitation of clause (i) for the succeeding taxable year. (D) Determination of percentage of completion The determination under subparagraph (C)(i) of the portion of the overall cost to the taxpayer of the conversion which is properly attributable to conversion completed during any taxable year shall be made, under regulations prescribed by the Secretary, on the basis of engineering or architectural estimates or on the basis of cost accounting records. Unless the taxpayer establishes otherwise by clear and