A record number of Nassau County property owners filed tax assessment challenges this year after a Newsday investigation revealed the effect County Executive Edward Mangano’s property tax system overhaul has had on those who haven’t filed a challenge since 2010.

County officials extended the deadline to file appeals this year from March 2 to March 10 after Newsday’s February investigation, Separate & Unequal. It was extended again to March 17 after Democrats in the County Legislature called for an extension due to a snowstorm.

The number of properties filing appeals with the county’s Assessment Review Commission rose this year to 216,311 from 183,048 last year, itself a record number. That is an 18.2 percent increase over 2016. Last year, challenges were up by 7.9 percent.

As in the past, homeowners who drove the increase in appeals this year with 32,913 more residential properties filing. Commercial, condominium and utility property filings have remained relatively stable for many years.

Mangano’s overhaul has in effect created two different property assessment systems, and a startling shift in tax burden of $1.7 billion over its first six years.

Property owners who grieved have seen their median tax bills increase by $466, or 5 percent since 2010. The median tax bills of those who did not appeal rose by $2,748, or 35.7 percent, over the same period.

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Mangano initiated the overhaul in an effort to reduce the county’s approximately $100 million annual cost of property tax refunds by settling assessment challenges before tax bills are sent out and refunds become necessary. He also froze other assessments unless a property was renovated, demolished or damaged. The overhaul will save the county between $115 million and $299 million by the end of 2017, depending upon various factors.

To settle challenges more quickly, the county offered filers larger reductions, even though most of their properties were increasing in value. As a result, 78 percent of those who appealed got a reduction.

To make up for the massive reduction in assessed value, governments taxed the remaining value at a higher rate to maintain their budgets. The result was that those who didn’t get a reduction saw large tax bill increases to essentially pay for the tax breaks of those who did.