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The problem is that there is seldom any “usually” to go by. While this was written by a Massachusetts realtor, his cynicism about your question is often justified anywhere in the country:
http://www.maxrealestateexposure.com/real-estate-assessed-value-vs-fair-market-value/
And he doesn’t even discuss how some state regulations may limit, or not, what percentage of market value an assessor can slap on an appraisal. Now does he discuss the typical county reliance on most people’s unwillingness to pursuit a property tax assessment protest, or how corrupt the hearings that sausage-grind such protests can be. I’ve seen “heads we win, tails you lose” processes at work in Texas where California buyers heavily overpay for Texas properties and the assessors will seriously use that against them. But when a buyer benefits from a foreclosure bargain acquisition, the assessors turn a blind eye to granting any value benefit to such buyers.
Even with the article author above not addressing such matters I’ve described, his general conclusion that there is very little dependable relationship between FMV and tax-assessed value holds very true.
–Dee