Property tax is figured by the value  (assessment) of the property, but it's not all that simple, or cut and dry. The reason you have an assessment of your property is to calculate your fair share of  taxes in order for the town to have enough income to run all of its departments. By law, each of Connecticut's 169 towns and cities have to re-valuate properties every four years. Let me debunk one popular myth: Your assessment doesn't change based upon the purchase price of the home- (otherwise, people would sell their home for a dollar, and pay virtually no tax)

The reason for the four year re-valuation is because both residential and commercial property values fluctuate  and cause an imbalance in appropriate rates for both. So in order to keep things in line, the re-assessment  is necessary.

The tax assessment for your home  is based upon a calculation  of square footage, amenities in the house, and location- much like a real estate appraisal, but there are a few major differences that I will try to explain to help you understand.

Here's an example: The town assesses your home at $700,000, and you just bought it for $1,000,000. Did you over pay for the house?  The bank appraised it at $1 million! So what gives? The town thinks your home is worth 30% less than you think it is, and somebody has got to pay!  There IS a simple answer as to why, so don't try to set things straight  at the assessors office. Complain to the tax assessor about being valued too low, and you might just be rewarded with higher taxes. 

The assessment that you receive on your house is 70% of the market value, so if your assessment is $700,000, the town believes it's value to be $1,000,000. The assessment figure and the appraisal figure are NOT THE SAME. The next time you receive your new assessment, if you look closely, there will be an appraisal  amount there, too. They are both on file with the town, and are considered public information.
 
When the town puts together its annual budget, and compiles all of the values, and expected taxes (a/k/a income) the mil rate is set to cover the budget based upon that income to the town. If the town is  coming up short with the budget, then you will see an increase in the mil rate, and an increase in property taxes. I could go on and on about this subject but my post is long enough. I have helped a few clients fight an asessment that was too high, and WE WON. But that's another post for another time.