I've been doing this my whole career. I never got any pushback, but I wasn't doing a lot of lending work up until 5-7 years ago.
If you overlay the monthly median sale price trend against a linear trendline, what do you see?
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Half the year sales are falling below trend, and half the year they are above trend. This is mostly because of the compositional effects of the housing stock. Nicer/bigger homes selling in March-August compared to September-February. However there are differences in the median seller motivation as well. Neither of these lines perfectly demonstrates price change. In reality, the measured change in the market is something more like the red line.
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Most of the increase is happening March, April, May, then stabilizes, sometimes with a pullback between Thanksgiving thru January (not shown). Let's assume the linear market appreciation is +6%/year, and you are appraising a home in the very early spring market. Appraisers can make linear adjustments, but a January sale is likely going to adjust below trend, and the August sale might be above trend until late March when the pricing surpasses the prior year. So, if you are doing a linear adjustment, you would ideally account for this within your reconciliation. Or you can make non-linear adjustments based what's actually happening (red line), which requires getting on the phone and talking to agents and gridding out sales that are under contract. Once adjusted, these contracts will likely support upward adjustments to the prior year's sales that is above the linear change. That adjustment can be supported by sensitivity, much like many other adjustments are developed. If you're just using the prior year's linear appreciation rate without acknowledging this, you are missing the early spring market, which is why the frequency of low appraisals follows a seasonal trend.
I review appraisals and most appraisers are not doing this level of analysis, and frankly that's ok because you can figure out ways to work through it using reconciliation. But it's important to at least have some knowledge of these trends if you are an appraiser in a heavily seasonal market. Fannie's illustration sucks because it is confusing for a number of reasons. However, I think the point is more to prepare lenders for the possibility of non-linear market adjustments, rather than to show appraisers how to do things.