buildings having 4 or fewer units eligible for credit where development plan (i) In general Subparagraph (C) shall not apply to the acquisition or rehabilitation of a building pursuant to a development plan of action sponsored by a State or local government or a qualified nonprofit organization. (ii) Limitation on credit In the case of a building to which clause (i) applies, the applicable fraction shall not exceed 80 percent of the unit fraction. (iii) Certain unrented units treated as owner-occupied In the case of a building to which clause (i) applies, any unit which is not rented for 90 days or more shall be treated as occupied by the owner of the building as of the 1st day it is not rented. (2) New building The term new building means a building the original use of which begins with the taxpayer. (3) Existing building The term existing building means any building which is not a new building. (4) Application to estates and trusts In the case of an estate or trust, the amount of the credit determined under subsection (a) shall be apportioned between the estate or trust and the beneficiaries on the basis of the income of the estate or trust allocable to each. (5) Impact of tenant's option to acquire property (A) In general No Federal income tax benefit shall fail to be allowable to the taxpayer with respect to any qualified middle-income building merely by reason of an option held by the tenants (in cooperative form or otherwise) or resident management corporation of such building or by a qualified nonprofit organization or government agency to purchase the property or a partnership interest relating to the property after the close of the credit period for a price which is not less than the minimum purchase price determined under subparagraph (B). (B) Minimum purchase price For purposes of subparagraph (A), the minimum purchase price under this subparagraph is an amount equal to the sum of— (i) the principal amount of outstanding indebtedness secured by the building (other than indebtedness incurred within the