
Functional obsolescence is the loss in value due to discordancy with present-day market wants and needs. Functional obsolescence can be indicated by timeworn plumbing, obsolete boilers, outdated lighting fixtures and electrical work, unreasonably high ceilings, bad decorating, and dated architecture (Ratterman). A swimming pool that contributes less to the property value than cost is known as a superadequacy, and may also be considered functional obsolescence.

External obsolescence is the loss of value of the improvement due to an undesirable factor outside the said improvement. Examples of external obsolescence can be shown by inappropriate uses of property, population decreases, construction of landfills or subsidized apartment complexes next to neighborhoods, legislative action, and airport and highway proximity (Williams). Obsolescence that affects the total area of improvement is said to comprise a change in the market.
Depreciation can either be curable or incurable. Curable depreciation is a loss of value that can be amended at a cost less than the increase in improvement value that would result if it were corrected (Ratterman). Incurable depreciation either cannot be rectified or would cost more than any appreciation of improvement value. Depreciation is normally estimated using three different methods which are known as the breakdown method, economic age-life method, and the market extraction method (Ratterman). The most common depreciation calculation method used by appraiser is the economic age-life method, also known as the straight line method. Using the economic age-life method an appraiser would multiply the ratio of effective life to the total economic life by the new replacement cost of the subject. An appraiser’s ability to calculate depreciation is most important when performing the cost approach valuation technique.
Ratterman, M.R. (2009). The Student Handbook to the Appraisal of Real Estate. Chicago, IL: Appraisal Institute.
Williams, Martha R. (1998) Fundamentals of Real Estate Appraisal. Chicago, IL: Real Estate Education Co.
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