You can file an appeal for free on the county’s Assessment Review Commission website, which also has a step-by-step video on how to do so. It is advisable to file an appeal even if the county’s online application tells you not to, because it may not properly calculate what your property would be valued at during the appeal process.
Under state law, your assessment cannot be increased as a result of an appeal.
You can also attend one of many grievance sessions held by the county, although the presentations do not fully explain the advantages of filing an appeal and make no recommendations on whether to do so. Schedules for these sessions may have been sent to you in the mail by your county legislator and are also available on various county legislators’ websites.
The North Hempstead receiver of taxes, Charles Berman, also holds free workshops open to all, the schedule for which is available on his official website.
Finally, former Assessment Review Commission member Jeff Gold also holds free workshops and has setup a Facebook group, “Nassau - Grieve Your Tax Assessment On Your Own – Free,” to help property owners file challenges.