The facts are its an issue which is almost impossible to define on a forum. The old text book taught us a theory that seldom works well in a real life event. This is why the cost appraoch in my area tends to almost always be irrelevant as the appraiser is almost always way off on Depreciated Physical Age which he/she then arrives at a Effective age which is not even close. Unfortunate because F & F does not allow a EA that would say most single
family homes are updated or remodeled, often numerous times over say a 75 year period. Ones that have not had any updating, maintenance, may end up as the Depreciated Physical age being similar to the effective age. BUT thats rare unless its near being a tear down .
The real factor is its
economic life and utility life which can be way different than its theoretical effective life. For this reason outside of New Construction the cost appraoch may not work real well, because 90% of all Appraisers simply do not have the construction background to do a cost approach. This is why back in the day we had all been trained it was 30 years remaining life as that was the length of the mortgage being taken out