In construing ss. 161.52-161.58:
(12) "Substantial improvement" means any repair, reconstruction, rehabilitation, or improvement of a structure when the actual cost of the improvement or repair of the structure to its pre-damage condition equals or exceeds 50 percent of the market value of the structure either:
(a) Before the improvement or repair is started; or
(b) If the structure has been damaged and is being restored, before the damage occurred.
The total cost does not include nonstructural interior finishings, including, but not limited to, finish flooring and floor coverings, base molding, nonstructural substrates, drywall, plaster, paneling, wall covering, tapestries, window treatments, decorative masonry, paint, interior doors, tile, cabinets, moldings and millwork, decorative metal work, vanities, electrical receptacles, electrical switches, electrical fixtures, intercoms, communications and sound systems, security systems, HVAC grills and decorative trim, freestanding metal fireplaces, appliances, water closets, tubs and shower enclosures, lavatories, and water heaters, or roof coverings, except when determining whether the structure has been substantially improved as a result of a single improvement or repair.
For the purposes of this definition, "substantial improvement" is considered to occur when the first alteration of any wall, ceiling, floor, or other structural part of the building commences, whether or not that alteration affects the external dimensions of the structure. The term does not, however, include either any project for improvement of a structure to comply with existing state or local health, sanitary, or safety code specifications which are solely necessary to assure safe living conditions or any alteration of a structure listed on the National Register of Historic Places or the State Inventory of Historic Places.