Grievance Guidelines
Any person who pays property taxes can grieve an assessment, including: property owners, purchasers, tenants who are required to pay property taxes pursuant to a lease or written agreement
Only the assessment on the current tentative assessment roll can be grieved - you can't grieve assessments from prior years.
There is no cost to grieve an assessment and it does not require you to hire a lawyer.
File the grievance form with the assessor or the board of assessment review (BAR) in your city or town.
If you're a property owner who doesn't reside in the municipality where you own property, you have additional rights related to grieving your assessment:
You have the right to attend the hearing of the BAR and to present statements and/or documentation in support of your grievance. You may appear personally, with or without your attorney or other representative
If you are dissatisfied with the decision of the BAR, you may seek judicial review of your assessment.
https://www.tax.ny.gov/pit/property/contest/grievproced.htm
For forms, deadlines and instructions, contact the New York City Tax Commission and Nassau County Assessment Review Commission respectively: