Most of New York files assessment grievances on the fourth Tuesday in May. Nassau does not. The county runs its own calendar through the Assessment Review Commission, and a homeowner working from statewide advice will miss the window by two months.
The four dates
Early January: the tentative roll publishes. The county posts a new assessed value for every property. Yours is on the county's property record page the same week. This is the number a grievance argues with.
Early March: the filing deadline. The statutory date is March 1. The commission has extended it in recent years, to March 31 for the 2027/28 roll, but an extension is discretionary and is announced when it happens. Nothing about a past extension entitles you to the next one.
April of the following year: the final roll. The commission has more than a year to decide. A grievance filed in January 2027 is settled by roughly April 2028, and the result shows up on the tax bills after that.
Thirty days after the final roll: the SCAR window. If the commission's answer is unsatisfactory, an owner of a one- to three-family home can take it to Small Claims Assessment Review for a $30 filing fee. That right expires 30 days after the final roll is filed.
What the lag means for you
The gap between filing and money is long. You argue in January 2027 about a value the county set in January 2027, the commission answers by April 2028, and the reduction reaches your school tax bill in October 2028 and your general tax bill the following January.
Two practical consequences. First, nothing about this is urgent in the way a bill is urgent, which is exactly why people forget it. Second, missing the March deadline does not cost you a month. It costs you the entire cycle, because the next number the county posts is a year away.
Check the county's own calendar
Dates move. The Assessment Review Commission publishes the current year's schedule at nassaucountyny.gov, and that page is the authority, not this one.