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The depiction of the method in the Tributación manual of assessment looks more complex than it really is.
This is the way to determine depreciation. D is depreciation and it is determined by adding x, the actual age, over estimated useful life (n) to the square of the actual age and the square of the estimated useful life. Tributación lists the useful life ranging from 40 years for a concrete prefabricated structure to 100 years for adobe.
Then the whole thing is divided by 2.
According to this formula a 24-year-old adobe structure would have 15 percent depreciation due to age. Windsor Castle would have no value under this formula.
Finding the actual value (VA) is even more interesting. The depreciation found previously is multiplied by the replacement cost new (Vn) and then everything is multiplied by a fudge factor E, the estimated condition of the property.