I am not sure of the method used but this is an interesting subject.
My new house does not have an appraised value yet (just land value for first year). Here's some info on the old house:
Neighbor's home assessed value is 94.5% of his sales price. Ours is 78.9%. He did no upgrades, we did about $65,000 in upgrades. (Both designer homes - back when we still had the street of dreams.) Neighbor on the other side bought a spec home - his is assessed at 81.1% of sales price. One neighbors house cost $100,000 less than ours, the other $70,000 less - so our house may be appraised for less to be in line with others in the neighborhood.
Go to the Sumter County Property Appraiser's web site for lots of info.
http://www.sumterpa.com