It would seem to me that there will always be individual variation that even excellent valuers and builders won't pick up so who would I have determining the cost across a large number of properties?
Well, if only there was an expert party involved, that had an interest in such a large number of properties that they could afford to approximate values so that the number they underestimate and overestimate at least stand a chance of cancelling each other out? Someone like... I dunno, the insurance companies?
But no - let's make it every single individuals' responsibility, so that every single individual has to overinsure to be safe. Remember, that individually declared value is NOT how much your house is insured for, it's how much you pay premiums on.
THAT'S the scam.
Cheers - N