The new state law requiring public disclosure of the sale prices of coop apartments, signed by Governor Pataki last week, may be part of a secret plan to raise taxes a billion dollars a year by extending the mortgage recording tax to co-ops.

For individuals, the tax is between 2% and 2.125% of the amount of a mortgage. Until now, it has applied only to condominiums and houses, but not to co-ops.

When the bill was first announced, some real estate experts worried that the disclosure of prices would mean a loss of privacy, particularly for the celebrity buyers of eight-digit co-op apartments. But now, some suspect that the provision could be a “Trojan horse” that will allow the city to go after more tax revenue in the future by extending the lucrative mortgage recording tax to co-op loans.

A real estate lawyer for Ganfer & Shore LLP, Steven Ganfer, said the law is another step that shrinks the legal gap between co-ops and condos.

“The language of the bill indicates they want to make co-ops look more like real property and condos,” Mr. Ganfer said.

Although Mr. Ganfer said an extension of the mortgage recording tax was probably not “imminent,” he said, “the next step will be to impose a recording fee, and raise a lot of money.”

Currently the tax, which is the source of more than a billion dollars for the city and the state, is only applicable to condominiums and houses. Because co-op buyers purchase shares in a building rather than real property, the “share loans” on their investment are not considered mortgages and are not taxed in the same way.

The proposal to disclose co-op purchase prices was first introduced by the city’s department of finance, which denies that it has anything to do with the mortgage recording tax, but says it is an attempt at increasing transparency and cleaning up its books. In 2002, the sale prices of condos became public even though prices of co-op transactions could not be obtained from the city or state.

The city’s finance commissioner, Martha Stark, said in a statement yesterday: “In 2002 the City received a serious wake up call that anything veiled in secrecy is susceptible to corruption. Making sales prices public information shines a light on the assessment process. It gives everybody the opportunity to understand the relationship between how much a property sells for and how we assess property value.”

The law passed the state Senate and Assembly without any votes in opposition.

Mr. Ganfer, the real estate lawyer, said the law is otherwise unnecessary because the sale prices of co-op apartments are already easy to come by with a little digging.

Periodically, when the city and state have expected a shortfall in tax revenue, politicians have suggested extending the mortgage recording tax to co-ops. During tight fiscal climate of 2003, state officials suggested to the city that extending the mortgage recording tax to co-ops would be a potential source of revenue.

In the Dinkins administration, city officials who pushed for the tax equity between co-ops and condos argued that the differences were a legal technicality, a loophole that allowed co-op owners to duck the mortgage recording tax.

The potential revenues from extending the tax would be enormous. Riding high on the city’s real estate market and the increased volume of mortgage refinancing, the city forecasts more than $1.3 billion in revenue from the tax this year. Some of the revenues from the tax go to the city, other parts go to the Metropolitan Transportation Authority, which is controlled by the state.

Next year, because of fewer expected transactions and an ebb in refinancing, the city projects a 35% drop in mortgage tax revenues, to $882 million.

The president of the Council of New York Cooperatives and Condominiums, a lobbying organization, Marc Luxemburg, said the bill could “possibly” be an entry point to eventually levy the mortgage tax against co-ops.

“The city has talked about taxing co-op loans for years,” Mr. Luxemburg said.

Still, Mr. Luxemburg, whose organization has previously lobbied against attempts to extend the tax to co-ops, did not take a position on the disclosure bill.

“We don’t think it is a big deal,” he said.

A co-op lawyer, Aaron Shmulewitz, of Belkin Burden Wenig & Goldman, said the law would give legislators more ammunition to push for the extension of the mortgage tax.

“Now, the prices being public makes co-ops one shade closer to real property,” Mr. Shmulewitz said. “Years ago, co-ops and condos were extremely distinct. Now the differences have narrowed and in some cases been erased. This is another example of it being erased.”

As an example, he said that in the mid-1980s, the real property transfer tax, the source of hundreds of millions in revenue for the city and the state, was amended to include co-ops transfers.

Mr. Shmulewitz said co-ops and condos “are totally different things,” but with the narrowing legal definition, “there is nothing to prevent the Legislature to deem it as real property,”

An appraiser, Jonathan Miller, said a conservative estimate extending the tax to co-op apartments would double the amount of tax revenue generated. The ratio of co-ops to condos citywide is roughly 75 to 25, according to Mr. Miller.

While new luxury condo projects such as the residences at Time Warner Center and in the new Bloomberg headquarters have attracted attention and high prices recently, many of the city’s most expensive pre-war buildings on Park Avenue, Fifth Avenue, and Central Park West are organized as co-ops. Many of those who own those apartments can afford to pay all cash, but some get loans so they can deduct the interest from their income taxes. And most owners of co-op apartments in other locations in the city, loans are necessary to afford an apartment after years of dramatic increases in housing costs.

Mr. Miller, who writes a quarterly report on the health of the city’s real estate industry, said the city’s budget predictions for lower revenue from real estate taxes “seemed like posturing” that could be used to argue for a tax increase.

A contributing editor for the City Journal, Nicole Gelinas, said a move by the city to extend the mortgage recording tax could be in the works.

“They have grown more dependent on the mortgage recording tax as a source of city revenue, just as the volume has increased so markedly in the last five years,” Ms. Gelinas said.

“I wouldn’t be surprised if they try to close some of the loopholes as they look at the projected revenue decline,” she said.