My house came with 6.5 acres in a separate parcel. When I purchased it they conveniently switched it from Ag to residential last year and increased it from like $20k assessed value to $150k. That wasn't much fun :) They also accidently counted my outbuilding on BOTH my house parcel and my out lot parcel, so I was double assessed for it. Had I not been watching closely who knows how long it would have been to catch that.
I contested a lot of my rentals when they re-assessed last year. I've had good luck working with them in that regard. If I have an appraisal that says less, or I disagree with the condition, etc. they will adjust. I usually try to be data driven in my contest.
I probably had 10-15 properties this past re-assessment that I got reduced back down.
Bought one for $312k in March 2017, then they assessed later in 2017 for $359k. I disputed and they reduced back down to $312k(less than the $324k it was assessed for when I bought it). I believe it's supposed to be the value of the property on Jan 1 of the assessment year. So even if I had bought in March and completely rehabbed the whole thing, it should be Jan 1 value, not rehabbed value.