If you added a dormer, finished the basement or put on an extension, Nassau County will raise your assessment to match. The Home Improvement Exemption softens that. Instead of taxing the new value all at once, the county phases it in over eight years. In the first year none of it is taxed. Each year after that, another eighth of it is. You apply once, it's free, and the deadline for next year's bill is January 4, 2027.
How the phase-in works
The table shows how much of the new value is tax-free each year.
| Year | Tax-free share |
|---|---|
| 1 | 100% |
| 2 | 87.5% |
| 3 | 75% |
| 4 | 62.5% |
| 5 | 50% |
| 6 | 37.5% |
| 7 | 25% |
| 8 | 12.5% |
Say the county decides your extension added $60,000 of market value. At the typical Nassau rate of about $178 a year per $10,000, that's about $1,070 a year in new tax. With the exemption, you pay none of it in year one. In year two you pay about $130 of it. By year eight you pay about $930. From year nine on, you pay the full $1,070. Over the eight years, the exemption saves you about $4,800 on that one project. You never pay less than you did before the work, and the exemption doesn't touch the rest of your assessment.
Who qualifies
- A one- or two-family house. You don't have to live in it.
- Permitted work that added value. Reconstruction, alterations or improvements. Ordinary repairs and replacements don't count, because they don't raise the assessment in the first place.
- Between $3,000 and $80,000 of added market value. Anything above $80,000 is taxed in full from day one. Only the first $80,000 is phased in.
- The house itself. Sheds and swimming pools are left out.
- An older house that's still mostly the old house. The original structure has to be at least five years old, and after the work it still has to be the bigger part of the home. A 1,000 square foot house can't grow past 1,999. Below-grade basement space isn't counted either way.
- Finished and inspected. The work has to be complete, with a certificate of occupancy or other proof. The county inspects and values it before granting the exemption.
The exemption covers county, town and school taxes when you file with the county. School taxes are included only where your district adopted it. Villages and the cities of Glen Cove and Long Beach decide on their own, and you file with them separately for their taxes.
How to apply
1. Finish the work and get the certificate of occupancy. The county won't grant the exemption until the project is done.
2. Fill out state form RP-421-f. It's on the county's exemption forms page. Attach the certificate of occupancy, the building permit, and the contract or receipts showing the cost.
3. Get it to the county by January 4, 2027. Nassau County Department of Assessment, 240 Old Country Road, 4th Floor, Mineola, NY 11501. Phone 516-571-1500. There's no fee, and you don't reapply in later years.
The exemption ends if you sell to anyone other than your heirs. If the deed changes for another reason, call the county so it can update its records.
What changed in 2026
From 2021 through 2025, Nassau granted this exemption on its own, with no application and a far higher cap. The state law behind that version expired on January 1, 2026. For work finished from 2026 on, the standard rules above apply, and you have to apply. If your home already had the automatic exemption, call the county to confirm how many years remain.
If you do nothing
The full added value is taxed from the first bill that shows it, and the 6% yearly cap won't protect you, since improvements sit outside it. Our post on why a renovation raises your bill explains the timing.
The typical rate is from our own study of the county's 2025/26 bills. The rules come from Nassau County's 2027-28 Home Improvement Exemption brochure and the state's form RP-421-f instructions. Applying is free, and you can do it yourself.