26 CFR 1.514(e)-1: (e)-1 Allocation rules.

Where only a portion of property is debt-financed property, proper allocation of the basis, indebtedness, income, and deductions with respect to such property must be made to determine the amount of income or gain derived from such property which is to be treated as unrelated debt-financed income. See examples 2 and 3 of paragraph (b)(1)(iii) of § 1.514(b)-1 and examples 1, (2), and (3) of paragraph (b)(3)(iii) of § 1.514(b)-1 for illustrations of proper allocation.