Substantial improvement means any combination of repairs, reconstruction, alteration, rehabilitation, addition, or other improvements to a building, taking place during a ten-year period, in which the cumulative cost equals or exceeds 50 percent of the market value of the building. The market value of the building is the appraised or assessed value of the building, less the land prior to the start of the initial repair or improvement, or, in the case of damage, the value of the building prior to the damage occurring. This term includes structures which have incurred "substantial damage," regardless of the actual repair work performed. 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 building. The term does not, however, include: