1.  The actual age of each improvement made on a parcel of land must be adjusted, for the purpose of computing depreciation, when any addition is made or replacement is made whose cost, added to the cost of any prior replacements, is at least 10 percent of the cost of replacement of the improvement after the work is done. For the purposes of this section, ‘replacement’ does not include changing or adding finish or covering to floors or walls, changing or adding small appliances, or other normal maintenance of the improvement in a good condition.

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2.  Except as otherwise provided in subsection 3, the amount of the reduction must be the product of the prior actual age multiplied by the ratio of the cost of the replacement or addition to the cost of replacement of the improvement after the work is done.

3.  The amount of the reduction for additions which increase the floor area of the improvement may be calculated by multiplying the prior actual age of the improvement by the ratio of the number of square feet of additional floor area to the total number of square feet of the improvement including the addition.