The 421a program applies to areas of Manhattan where developers build condominiums on underutilized or unused land. In most of them, the exemption lasts for 10 years, giving owners a 100 percent exemption from any increases in their real estate taxes for two years, then phasing out the exemption by 20 percent every two years over the remaining eight. In Upper Manhattan and the other boroughs, the exemption can last for 15 or 25 years.

In 2008, 421a was overhauled to include, among other things, a requirement that developers set aside 20 percent of their units for affordable housing. As a result, fewer buildings are now entering the program.

There are a number of reasons that new condos have higher property taxes than older ones or comparable co-ops, tax experts said.

Image 497 Greenwich Street, Apt. 7B

Bought in 2005 for $3,180,000

On the market now for $5,250,000

Original monthly taxes: $258

Current monthly taxes: $2,778

Estimated monthly taxes in 2015: $3,675 Credit... Chester Higgins Jr./The New York Times

In New York City, the real estate tax rate for condominiums is 13.353 percent. This number is applied to the assessed value of the property, and the city assesses the properties anew every year. To assess the value of condominiums and co-ops, the city looks at how much income the building would produce if it were a rental.

“By law, we are required to value condominiums as if they were income-producing properties,” said Michael Hyman, a deputy commissioner for tax policy and planning at the city Department of Finance, “so we impute rental incomes even though the buildings are generally owner-occupied.” The law went into effect in 1983, he said. At that time, the city was experiencing many co-op conversions, and the hope was that the legislation would diminish the disparity in value between rentals and co-ops.

Even though now it is condominiums that are making up an increasing proportion of the apartments in the city, the law does not reflect this shift, he said.