The minority leader of the Nassau County Legislature, tax attorneys and assessment-reform advocates on Wednesday expressed concern about draft findings by the county comptroller that a new residential assessment method will increase the property-tax burden on middle-class homeowners.

But officials in Executive Edward Mangano's office called the draft audit by Comptroller George Maragos "unfounded," saying one auditor had used incorrect methodology in calculating average home prices.

Legislative Minority Leader Diane Yatauro (D-Glen Cove) said the conclusions in Maragos' draft audit, which was obtained by Newsday, "would be a devastating blow to the average, middle-income homeowner."

The Fair Assessment Committee, composed of residential tax-appeal lawyers, said that while it wasn't endorsing the "accuracy" of the draft audit, "the issue raised is a fundamental issue we have raised for years.

"Until the County adopts an assessment method that is fully transparent and fully accessible to the taxpayer, all of the County's homeowners will be burdened by an inequitable system," the committee said in a statement.

But in a statement, Mangano's press secretary Katie Grilli-Robles said: "The Comptroller's draft audit is unfounded. In its mere six-week review, the Office failed to use experts in the complex field of assessment. In fact, one auditor used the wrong methodology which thereby inaccurately calculated the average price of homes. It is for this reason that the media should not act on draft documents, but rather await a finalized audit."

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Maragos' spokesman has cautioned that the report is still preliminary and subject to change. It was sent to the administration over a week ago for comment and review - though Mangano aide Brian Nevin contended Tuesday that Mangano had not seen it or its findings.

The new system bases assessment on an array of values, including last year's assessment, a recent sale or a reduction as the result of a tax protests, instead of comparable sales, which had been used in the past. Auditors said the tentative assessment roll issued in January was produced by "new and unvalidated methodology" and "appears inequitable, will shift the tax burden to mid-range valued homes and may increase the number of property owners filing grievances."

Auditors estimated that mid-range homes' property taxes will increase by an average $227 next year because of the new system: taxes on high-level homes will drop an average of $173 while the lowest-cost homes will see an overall decrease of an average $55.

Bob Orosz, a member of a volunteer team appointed by Mangano last year to study assessments, said he was disappointed that the comptroller didn't consult with the Assessment Reform Team.

Although he hadn't seen the audit, Orosz said he already had noticed that comparable properties had been dropped from the county assessment website and replaced with an explanation of value that many homeowners don't understand.

"We couldn't figure out our values were back then because of the lack of information," he said. "Now, you've shut the door, closed the blinds and turned off the lights. There's no information. Now we have nothing to go by."