Nassau County, N.Y.Good GrievesSources reviewed September 20, 2026

Fixing errors on your Nassau County property record

An argument about value needs evidence and judgment. A wrong fact on the record, such as an extra bathroom or square footage you do not have, is simpler: it can be documented. The route to fixing it depends on the kind of error and the time of year.

What counts as a record error

The Department of Assessment says its responsibilities include collecting and recording property information, maintaining property records and tax maps, and validating building permits so that new construction is included on the next roll. The County then estimates each home’s market value from that information. If the description is wrong, the value may be wrong with it. Common problems to look for:

• Size. Living area that includes space you do not have, or counts an unheated porch or garage as living space.
• Rooms. More bathrooms or bedrooms than the house has.
• Basement and attic. Space recorded as finished that is unfinished, or the reverse.
• Features that are gone. A pool, deck, or outbuilding that was removed.
• Work that was never finished. A permitted addition recorded as built when the project was abandoned or scaled back.
• Property type and class. The number of units, residential or mixed use, or the tax class.
• Lot. Lot size or dimensions that do not match your survey.

Read the record on the County’s property lookup line by line against the house you actually own. Our property checker shows available County figures next to nearby homes, which can make an odd entry stand out, but the County’s own record is the one to correct.

Gather proof before you ask

A request carries more weight when it points to documents rather than memory. Depending on the error, useful evidence includes:

• A survey, for lot size and the footprint of the house and outbuildings.
• Floor plans, or a lender’s appraisal, which usually measures living area and lists rooms.
• Dated photographs of the space in question, such as an unfinished basement or a removed pool.
• Building permit records and certificates of occupancy from your town or village building department, showing what was approved and completed.
• For a recent purchase, the listing, inspection report, and closing documents.

Note the exact discrepancy for each item: what the record says, what is true, and which document shows it.

Route one: ask the Department of Assessment

The Department of Assessment maintains the record, so it is the office to ask. It can be reached at (516) 571-1500, through its online contact form, or at 240 Old Country Road, 4th Floor, Mineola, NY 11501. The Department’s page also invites owners to request a physical inspection by emailing ncfield@nassaucountyny.gov or writing to the same address, including the property address and their contact information.

This route is available year-round, and it fixes the description going forward. What it does not do is guarantee a change to the assessment for a year whose appeal window has passed, or stop the next window from closing. Keep copies of everything you send and note the date.

Route two: file with ARC during the window

If an error inflates the assessment you want reviewed, include it in an appeal to the Assessment Review Commission. ARC’s FAQ describes three forms:

• AR1 to contest the value of a one-, two-, or three-family house or a Class 1 condominium unit. Attach your evidence of the error along with any comparable sales.
• AR2 for the value of other property types.
• AR3 for tax class, exemptions, and other non-value claims. Each claim needs its own application, and ARC says supporting documents for an AR3 claim go directly to the Department of Assessment.

You do not need to show exactly how the County reached its number: ARC’s FAQ says it will consider your claimed value and supporting evidence without requiring you to show why the Department’s estimate was wrong. Filing is free and can be done online, by mail, or in person; see ARC’s forms page and the County’s appeal instructions.

Permits and unrecorded work

Because the Department validates building permits and adds new construction to the next roll, permitted work usually reaches the record in time. Problems tend to come from the edges: a permit for work that was never completed, a later change the permit did not describe, or work done without a permit that the record never picked up.

The first two can support a correction in your favor, and permit records are the evidence. The third cuts the other way. The County’s FAQ says the 6% annual and 20% five-year limits on Class 1 increases do not apply to value added by construction or renovation. An inspection that finds unrecorded work can raise the assessment by more than the cap would otherwise allow. Unpermitted work may also raise questions with your town or village that are separate from taxes. If that applies to you, consider getting advice before inviting an inspection.

Check the record every year

Records change when the County revalues, when permits close, and when ARC decisions are applied. Read your record each January when the tentative roll is published; our tentative roll guide has a six-point check. Then look up your property and compare. This page is general information, not legal or tax advice, and savings are never guaranteed.