Nassau can't raise a home's assessment by more than 6% in a year. There's one big exception: work you did on the house. An addition, a dormer, a finished basement or an in-ground pool goes on the roll at full value, on top of the 6%. That's why a renovation can push your bill up far more than your neighbors saw. Here's how it happens, when it lands, and what you can do.
How the county finds out
When your town issues a building permit, the county sees it. The Department of Assessment says it checks more than 20,000 permits a year. It inspects the finished work and adds the new value to the next roll. You don't report it. The permit does.
When it shows up
The county values every home as of January 2. That value sets the bill for a tax year that starts about 21 months later. Work finished after a January 2 waits for the next one.
Say you finish a dormer in April 2026. The county values it as of January 2, 2027. That value goes on the roll the county publishes that same week. It becomes final in April 2028 and shows up on the October 2028 school tax bill. So the jump often arrives two to three years after the contractor leaves, when you've long stopped thinking about it.
The county has to mail you a notice when it raises your assessment for construction. It comes with the tentative roll in early January. Read it.
What it costs
At the typical Nassau rate, every $10,000 of added market value costs about $178 a year in county, town and school tax, before STAR and other exemptions. Half of all homes pay between $149 and $205 per $10,000. Hempstead sits at the middle, North Hempstead a little lower, Oyster Bay a little higher. So a dormer the county values at $60,000 adds roughly $1,070 a year, every year.
It happens to a lot of homes. On the 2026/27 roll, about 16,000 Nassau homes, one in 20, had their posted assessment go up. The typical rise was $21,000 of market value, or about $500 a year in tax. About 3,100 of those homes jumped by more than 6%, the sign of major work. Their typical rise was $162,000, or close to $4,000 a year.
Which projects raise the assessment
The county publishes a chart. It isn't a complete list, but it shows the pattern. New space and new fixtures count. Like-for-like replacement doesn't.
| Counts | Doesn't count |
|---|---|
| New rooms, dormers, porches | New roof shingles |
| Finished basement or attic | New siding |
| Garage turned into a room | Storm windows and skylights |
| New bathroom or new fixtures | Replacing kitchen or bathroom fixtures |
| Central air | Window air conditioners |
| Fireplace | New oil or gas burner |
| Attached deck, patio or garage | Driveway, fence, landscaping |
| In-ground pool | Small above-ground pool |
| Full gut renovation | Upgraded electrical service |
What you can do
1. Check the numbers. Look up your home on the county's Land Records Viewer. Make sure the new square footage, bathroom count and other facts match what you built. A basement counted twice, or a 300-square-foot addition entered as 500, is a real and fixable error.
2. Grieve the value. The new assessment is still the county's opinion of what your home is worth. You can challenge it with the Assessment Review Commission like any other year. The window for the 2028/29 roll runs from January 4 to March 1, 2027. The 6% cap won't shield the improvement. A grievance can still bring the overall number down to what the house would sell for.
3. Apply for the Home Improvement Exemption. Nassau phases the added value in over eight years instead of taxing it all at once. You have to apply by January 4, 2027. That's the next post.
The tax rate and the counts are from our own study of the county's 2025/26 bills and the 2025/26 and 2026/27 rolls. The rules come from Nassau County's Assessment Review Commission questions page, its Home Improvement Exemption brochure and its assessment calendar.