At the Plaza, of 163 condominiums, only 58, or about one-third of them, now receive the abatement for full-time residents.

Some of the city’s most exclusive addresses, such as 15 Central Park West and One57, receive a far more lucrative tax break — the so-called 421a tax exemption — and so they cannot be tracked this way.

George Sweeting, the deputy director of the Independent Budget Office, said the agency plans to look further into how tracking the tax abatement for primary residents could shed light on the number of pieds-à-terre across the city.

It is an important endeavor, according to Brad Hoylman, the state senator who is proposing legislation to charge an additional tax to owners of expensive pieds-à-terre. According to the proposal, owners of a pied-à-terre priced between $5 million and $6 million would pay 0.5 percent of the amount over the $5 million threshold in annual taxes. This would gradually increase, capping off at properties of more than $25 million, where the owner would pay a $370,000 annual fee plus 4 percent of the amount over $25 million.

While the owners could be wealthy New Yorkers who happen to own a primary residence uptown, say, and a secondary apartment downtown, many who would be targeted by this new tax would be non-New York residents. These nonresidents typically do not pay New York’s hefty income taxes and instead pay just a small fraction of their apartment’s value in property taxes. Property taxes here are based on a complex equation related to rental values and can be very low. At One57, for example, a unit that sold for nearly $3.6 million is estimated by the city to have a market value of just $430,000 when calculating its property tax.

“The pied-à-terre tax is seen by New York’s wealthiest 1 percent as a question of fairness,” said James Parrott, the chief economist at the Fiscal Policy Institute, who first made the proposal for the tax on which Mr. Hoylman based his legislation. “Here they are in the top tax bracket paying the top New York City and New York State income tax rates, but these non-primary residents are just paying property taxes, and the effective property tax rates are so incredibly low.”