@Olga Levin
I completely agree with you that it will not make sense to apply the same values in different regions. I was thinking about a database that has different values for different cities which is based on the numbers that local appraisers are using. Do you think that would be illegal?
The powers to be at Fannie Mae & Freddie Mac, and the federal government would very likely take issue with such an approach, even though they track and secure data from appraisals on a national level for their research and review.
It would take out the objectivity of the analysis as well. I live in a town with a median price point of $650,000, and a neighboring town with a median price point of $350,000 (example), the same two car garage adjustment would very likely be different in these two towns which border each other, and say the two properties being appraised could be 3 miles away from each other...the same could hold true for properties within the same town.
Often times adjustments can be based on the price band or level, ie. low end vs. high end, having an understanding of market conditions, gut feel (non quantifiable), market expectations, a test of reasonableness, and a host of other reasons.
There are cost manuals such as Marshall & Swift that can provide you with replacement cost data for various regions that may be somewhat helpful, just an idea and just my opinion !
Have a nice weekend...