On any nonconforming structure or portion of a structure containing a nonconforming use, work may be done in any period of 12 consecutive months on ordinary repairs, or on repair or replacement or non-bearing walls, fixtures, wiring, or plumbing, to an extent not exceeding 25% of the current city assessed value of the nonconforming structure or nonconforming portion of the structure as the case may be, provided that the cubic content existing when it became nonconforming shall not be increased. If a nonconforming structure or portion of a structure containing a nonconforming use becomes physically unsafe or unlawful due to a lack of repairs and maintenance, and is declared by the Building Official to be unsafe or unlawful by reason of physical condition, it shall not thereafter be restored, repaired, or rebuilt except in conformity with the regulations of the district in which it is located.