The New York State tax department, which collects both state and city income taxes, is adding a new line this year to 2010 tax forms, asking state residents who own second, or perhaps third and fourth, homes to specify how many days they spent in New York City and a front page article by Cara Buckley in today's edition of The New York Times said that "a number nearing 183 will be a red flag" that could make the taxpayers liable to treated as a "permanent resident" and a bigger tax bill.

A person who spends more than half the year in the city and has a permanent residence in it must pay city and state taxes on every dollar of their income, including money earned outside the city and from personal investments, which can sometimes be substantial.

Thus, the article said, "accountants and tax lawyers have been reminding their multi-homed clients to keep paper trails, as recent cases have shown the extent to which tax auditors and those being audited will go to try to prove - or disprove - permanent residence in the city."

"One couple," the article continued, "in a spirited attempt to claim that they were not subject to $41,000 in New York City income taxes, contended that their million-dollar Manhattan apartment was little more than 'a hotel substitute' and that their 'historic roots' were on Long Island, where they kept a yacht and a 3,500-square-foot home. The well-to-do with more than one home should be warned: it is the equivalent of sending a come-hither look to the tax man....pledging allegiance to the East End or the Constitution State will not save you from a very large bill."

"In one case that reached the state's tax tribunal last fall, the hedge fund billionaire Julian H. Robertson Jr. presented evidence that he had had his assistant painstakingly collect to account for his whereabouts each day, and said that on some late nights he had frantically searched for a car to take him back to Locust Valley, on Long Island, so that he would be outside the city limits before midnight. He convinced the tribunal that he had spent less than half of 2000, the year in question, in the city, and thus did not owe back taxes of $27 million," the article said.

While the question is new on the 2010 tax form, state officials said they had long been on the lookout for residency discrepancies, the article said, adding that "about 230 of the tax department's 1,800 auditors focus on residency cases, and the number of such audits has been inching up, to 2,508 for the 2010 fiscal year, which ended March 31, from 2,000 in fiscal year 2008 (though down from 2,900 six years ago)."

"The taxing authorities do not even have to prove that a person's primary residence is in the five boroughs; they only have to show that the person maintains a "permanent place of abode" in the city, which tax officials interpret as a home that is habitable year-round. Proving the amount of time spent there is up to the taxpayer," the article said.

In a case decided last month, the article continued, John and Laura Barker of New Canaan, Conn., "unsuccessfully appealed the tax department's finding that they owed $1 million to the state because they had a vacation home in Napeague, near Montauk.

Mr. Barker worked as an investment manager in New York City and did not dispute that he spent more than 183 days there, the article said, "but the couple rarely stayed in Napeague and argued that it did not count as a New York State residence because state law says a 'camp or cottage' suitable only for vacation use does not count as a permanent abode....The tax department countered that Mrs. Barker's parents used the place from November to May, that oil was delivered regularly and that the couple had year-round access."