In determining the proportionate value of a destroyed part of a building or other structure, or of additions, alterations or repairs made to a building or other structure, all parts, whether existing, new, or destroyed, and including plumbing, heating and ventilating equipment, electrical wiring and equipment, and other necessary fixed equipment, shall be valued at costs current at the time of evaluation. However, parts which do not serve a required purpose such as decorative painting, molded or cast plaster decorations, and wall, floor or ceiling finishes intended for comfort or enhancement, shall not be included in such valuation.