In addition, a major portion of a building may be treated as a separate building for purposes of this paragraph if the requirements of paragraph (b)(5) of this section are met.
A substantial rehabilitation may vary in degree from gutting and extensive reconstruction of a building's major structural components to the cure of a substantial accumulation of major disrepairs. It may also include renovation, alteration, or remodelling for the conversion of a structurally sound building to a design and condition required for a new use. Cosmetic improvements alone, however, do not qualify as a substantial rehabilitation.
For purposes of this paragraph (c)(3)(ii), use shall mean actual use, whether personal or business.
For purposes of paragraph (c)(6)(ii) of this section, the amount of rehabilitation expenditures treated as incurred by the taxpayer under this paragraph (c)(3)(ii) shall not be considered to be part of the cost of acquiring a building or any interest in the building. The portion of the cost of acquiring a building (or an interest therein) which is not treated under this paragraph as qualified rehabilitation expenditures incurred by the taxpayer is not eligible for a rehabilitation investment credit. See paragraph (c)(6)(ii) of this section.
26 C.F.R. §1.48-11