Section 47 of the Internal Revenue Code of 1986 exempts certified historic structures from meeting the physical test for retention of external walls and internal structural framework specified therein for other rehabilitated buildings. Nevertheless, owners are cautioned that the Standards for Rehabilitation require retention of distinguishing historic materials of external and internal walls as well as structural systems. In limited instances, rehabilitations involving removal of existing external walls, i.e., external walls that detract from the historic character of the structure such as in the case of a nonsignificant later addition or walls that have lost their structural integrity due to deterioration, may be certified as meeting the Standards for Rehabilitation.
36 C.F.R. § 67.7