Property tax calculations are often times done by a computer using a coefficient derived from sales figure over the year. That system checks the attributes of a given house–number of bathrooms, square footage, decks, patios, the garage or lack of it, finished or unfinished basement, those sorts of things–and it applies a multiplier coefficient to it.

This is where you do some checking. Lot's of information may be left out such as condition of the house, roof, deck deterioration and so on. Due to certain amenities as well as location variations,  different neighborhoods have premiums or are discounted by market forces. Outside influences such as traffic noise, congestion, odors, graffiti, etc can influence assessments.

For more complete information on how to lower property taxes check into House Tax Ax  These guys really know their stuff.