The market value of my house has dropped considerably over the last two years, yet the valuation for property taxes continues to increase. I told the county they can buy my house today for what they claim it's worth.
In most areas there is a formal complaint procedure if your house is unfairly valued. When this happened to me once I wrote to the "board of equalization", giving details of the sales prices of similar nearby houses and they agreed to drop my assessment to a much more reasonable value. I was surprised it was so easy.