The cost to building owners will be high. Mark Chambers, the director of the Mayor’s Office of Sustainability, said the cumulative cost to building owners to make the upgrades needed to meet the caps would exceed $4 billion. He added that building owners would eventually recoup those costs through lower operating expenses.

Many types of buildings were exempted from the caps, including houses of worship and apartment houses with rent-regulated units and other types of affordable housing. Those buildings are required to carry out several energy-saving measures, such as insulating pipes, but those measures fall well short of the costly steps required to meet the caps.

Real estate industry executives say that while they support reducing emissions, they believe too many types of buildings were given exemptions, placing an undue burden for reducing the city’s greenhouse gas output on the remaining buildings.

Ed Ermler is the board president of a group of four co-op apartment buildings with a total of 437 units in Jackson Heights, Queens. He said that in recent years he has invested hundreds of thousands of dollars to install computerized boiler controls and other systems to make the 1950s-era buildings, called Roosevelt Terrace, more energy efficient, and yet the target that the city has set is still “totally unattainable.”

“To get down to even 20 percent from where I am today, with the technology that exists, there’s nothing more that I can do,” Mr. Ermler said. “It’s not like there’s this magic wand.”

He said that the majority of apartment owners in the complex are over 65 years old. “Most are on a fixed income and I have to be very cognizant about anything that I do because I don’t want to put an undue burden on people that can’t afford it.”

Carl Hum, the general counsel for the Real Estate Board of New York, which primarily represents large developers, expressed concern that the exemptions given to thousands of large buildings would undercut the goal.