I feel better that your issue is with the concept as a whole and not just me.
Of course it's not with you. You are just an internet personna who could be posting under an alias. It is most definitly the concepts. It's more what I might call "appraisal reflexes." I hear conclusions based on fishy-sounding assumptions and I challenge the assumptions. I hear conclusions that would have to result from market study and I ask about the market study. Most of us do the same thing, for example, if a reviewer says your value is wrong, you ask for the market analysis that led to that conclusion. I just find when you make the same inquiries about the cost approach, it is surrounded by some sort of defensive shield of dogma that resists all normal inquiry.
Alrighty ... well, at the end of the day, I suspect we agree that we don't agree.
I may have a lot of trouble making myslelf clear, but "agree" and "don't agree" is not the issue. If an appraiser states a conclusion, and that conlcuison is false, I don't really have an oppurtunity to "agree" or "disagree." I focused on one statement, that the build option is available to the "typical" buyer. I have looked into that a few times in the last 15 years. From what I have found, and it should be obvious that this is what would be found, it is a stretch to think that even 10% of home buyers can afford the option. Then there are the physical and legal limitations. I think 2-3% is more is more accurate than 10, and that is still before subtracting for physical limitations, legal limitations, those who just wouldn't do it, and undue delay.
My mentor btw thinks it's a waste of time too--and as such, I was never taught how to construct the approach correctly--not until I took the two classes (Cost from McKissock, and Adv Rept Writing from AI).
Interesting. Just consider my above comment about false statements, and consider the question of how one can "correctly" construct an approach based on a fase premise (other than a necessary hypothetical condition).
FWIW, I have invested considerable time into studying the cost approach. In the 90's, I put together a library of texts going back to the 1800's trying to find the source of all this. I have tried to collect every journal article on the subject. I have published an article on the subject. Also, I took that same McKissock couse (and have the PDF) and the AI course for commercial property within the last three years, and I took the AI "advanced" cost approach class (520? or something like that) way back when.
If there is a "correctly" done cost approach for market value, that is news to me. As far as I can tell, there are some willing to make the assertion that there is a "correctly" done cost approach for market value, but no one has ever published it. And I am not holding my breath.
As to the McKissock course and the "correctly" done cost approach, I recally that course said that "extraction" was a "good" method for finding vacant land value when there is no vacant land. I dont' want to digress too much, but there's another one of those unsupported and unsupportable assertions. I wonder what are some of the "bad" and "just average" methods, and what study was undertaken to determine extractions is "good." But the main point is - getting back to (unsupportable) assertion about the build option being available to the typical buyer - if you have to do extraction or allocation to fabricate a land value, then the build option probably doesn't exist for the typical buyer.
Getting back to the OP and your view that "insurance" is not your business. Then you have a problem if you want to do mortgage appraisals. The clients think replacement cost for insurance is your business and that is why they are asking you to do it. In a way, whether you are an appraiser who thinks the cost approach is relevant to market value 100% of the time, 50% of the time, or 0% of the time - you still have an obligation to identify the client's use correctly and match the scope of work to that. If you read the excerpt from Fannie on how to do that, they are telling you exactly what they require to be insured by the lender.