First, confirm your figures: don't assume the deed is correct. Bring in an appraiser to measure your square footage (also because if you take action, as described below, you will need to have an appraiser or other similar expert who can write a report and testify if necessary--you cannot rely just on what is in the deed).If the appraisal backs up that your square footage is lower than the town is taxing you based on, you can file a tax appeal. You can get the instructions, etc. for doing this from your town or county clerk's office, but probably want to strongly consider hiring an attorney with tax appeal experience to help you--an experienced attorney will greatly increase your odds of success.