26 CFR 1.167(a)-5: (a)-5 Apportionment of basis.

In the case of the acquisition on or after March 1, 1913, of a combination of depreciable and nondepreciable property for a lump sum, as for example, buildings and land, the basis for depreciation cannot exceed an amount which bears the same proportion to the lump sum as the value of the depreciable property at the time of acquisition bears to the value of the entire property at that time. In the case of property which is subject to both the allowance for depreciation and amortization, depreciation is allowable only with respect to the portion of the depreciable property which is not subject to the allowance for amortization and may be taken concurrently with the allowance for amortization. After the close of the amortization period or after amortization deductions have been discontinued with respect to any such property, the unrecovered cost or other basis of the depreciable portion of such property will be subject to depreciation. For adjustments to basis, see section 1016 and other applicable provisions of law. For the adjustment to the basis of a structure in the case of a donation of a qualified conservation contribution under section 170(h), see § 1.170A-14(h)(3)(iii).