Firstly, what is your assessment?? You can find your assessment on your tax bill or by calling the town.  That is not the amount that you pay in taxes, nor are you interested in what the assessment is on the land.  Only what is your full market assessment.

Secondly, what ratio does the town use to convert the assessment into a market value?

Do not us the ratio the town gives you on the tax bill, that could be wrong.  If you are in New York go to http://orpts.tax.ny.gov/cfapps/MuniPro/,  if you have a residential property use the Residential Assessment Ratio (RAR).  If you have a commercial property, use the Equalization Ratio.

Divide the assessment by the assessment ratio and that will tell you what the assessor thinks your house is worth.

If the assessor’s value for your house sounds reasonable, and then there is nothing to do.  If you believe your property is worth 10% or less than the assessor’s suggested value then you need to challenge the assessment and taxes.

In simple math this is how it works.  Let us say your assessment is $21,500.  Let us say your property is residential and then you need to use the Residential Assessment Ratio (RAR).  Go to the New York web site above and look for Westchester. Then the Town of Greenburgh. Then Residential Assessment Ratio.  The result of your search should be 2.94.  Take $21,500 divided by 2.94 times 100 equals $731,292.  Can you get $731,000 for your house???  If your house is worth $650,000 or less think about challenging the assessment.  A 10% lowering of your assessment means a 10% reduction of your real estate taxes.  If you taxes go down by 10%, that will mean that your taxes could go down by 10% for many years to come.