Quote from BuildingLI.com on 10/04/2025,

1. Go to http://www.mynassauproperty.com and search for your property to find your current assessment.
2. Go to https://www.nassaucountyny.gov/arcwebcitizen/Jsp/Login.jsp and register to file grievance usually open from January to March

When filling out the form, you will need to find comparable properties within a quarter mile, that you believe to be similar to your home. 2-3 such properties serves as good justification to reduce your assessment.  Doing this yearly is not likely to offer yearly success