Depending on where you are, municpal taxes are not based on fair market value (or full cash value). They are based on "assessed" value where the assessed value is the fair market value in a particular year and that year is used for the whole municiplity. Assessed value does not change as market value changes but is held steady for many years. This allows the system to rate the value of your house to other similar houses without having to worry about the volatility of the market. Things that do cause the assessed value to change are changes to the house itself (say adding a swimming pool or doing a renovation or building an addition)