... the fact that you cannot build a similar home for the remaining cost compared to the sca . ...
Ok, I'm going to take you seriously on the off-chance that you really are interested in discussing methodology. Are you saying that the sum of your opinion of site value plus your total estimated construction costs is larger than your indicated value from the Sales Comparison Analysis?
If that's the case, I'd be inclined to double-check my numbers. Construction costs can be easily tested for reasonableness by interviewing contractors and owner/builders. (If you regularly do appraisals involving new construction, you're probably doing this anyway. If you never discuss real-world building costs with builders, there's a good chance that you're relying on an inaccurate construction costing method.)
So let's say you've got reliable construction costs and the total of site value plus construction costs is significantly higher than the Sales Comparison indicated value. That leaves you with two possibilities: 1) That your opinion of site value is too high, or 2) That knowledgeable builders are willing to put projects together in exchange for negative profit.
I'm thinking it's pretty obvious that your site value is too high and it's very likely that it's because economic obsolescence has reduced the value of sites between the time of your best site sales data and the date of the appraisal. So, YES, a time adjustment should be made to the land sales data in order to form an opinion of the current site value. And, NO, there shouldn't be any adjustment for economic obsolescence in the Cost Approach section of the URAR form, because it's already reflected in the site value.
But, if it gets to that point, it's most likely that you didn't start with a good analysis of site value. Just as with the Sales Comparison Analysis, it takes more than historic sales data to estimate current values. If your analysis doesn't include current listings for lots and adjustments for current market trends, you're probably off on the cost approach as soon as you enter the very first number - your opinion of current site value.
Once some appraisers learn how to analyze market extracted depreciations from ALL FORMS, the question regarding use of the sales comparison approach in a home of any age will be moot.
Until then, it appears, some will not understand how market derived depreciation can be used to determine individual depreciation amounts that are reflective of the market and therefore supported for their inclusion ....
Are you saying that our profession is lacking in appraisers who know how to identify trends in a population and then apply those trends to the forecasting of individual events? It doesn't work in that direction - correlation doesn't imply causation. With all the data, intelligence, and processing power behind Zillow, Trulia, and Redfin, what's the explanation of their inability to accurately predict sales prices?
If you're saying that many in our profession lack general statistical literacy but that still doesn't stop them from citing vigorously massaged statistical conclusions as if they were fact, I agree 100%.