Ensuring that buyers at his new condominium project would receive tax exemptions under the city’s 421-a program was a tedious process for developer David Kislin.

Satisfying his lenders that 5 Franklin Place would receive the abatement, which gives owners a break on city taxes for up to 10 years, took “lots of planning, lots of legal work, and a lot of attention to detail,” Mr. Kislin, a partner at Sleepy Hudson LLC, said. A key part of the process, he said, was testing and certifying the building’s cement foundations to prove they were poured before the city instituted major changes to the program last month.

With the tax abatement safely in place, sales at 5 Franklin Place are now well under way. But hundreds of other developers seeking to take advantage of expiring 421-a benefits may not be so lucky.

In the past, receiving the tax abatement was simply a matter of building below 96th Street. But now, to be grandfathered into the program, developers must prove that they laid their cement foundations before July 1. The city does not tell them if their project has been accepted until the building is completed, leaving some developers to discover months or years down the line that they did not meet the requirements in time. And in the short term, some developers are discovering that their financing is in jeopardy because they have no assurance of the units’ eligibility for the 421-a benefits, industry insiders say.

As a result, some real estate professionals are predicting a flurry of litigation against the city, as developers who are not approved for the tax abatement file lawsuits.

“People will not be finding out if the work they’ve done is qualified or not for quite some time,” a partner at Kramer Levin Naftalis & Frankel, Jay Neveloff, said. “That’s alarming, and the ramifications are significant.”

The 421-a program was created in 1971 to spur housing development in the city, but was changed this summer after city officials questioned why buyers of expensive condominiums were being awarded tax abatements. The new legislation dramatically increases the number of neighborhoods — including all of Manhattan and areas of Brooklyn — in which developers must now build some low- and middle-income housing to qualify for the benefit.

To be grandfathered in under the old 421-a rules, a developer must have a building permit issued prior to June 30 and must have “gotten in the ground,” or driven some load-bearing material into the ground by that date, a spokesman for the city’s Department of Housing Preservation and Development, Seth Donlan, said. Then the developer must make timely progress on the building until its completion, he said.

In June, developers flooded the Department of Buildings with applications for permits to make the deadline, with 575 permits issued for new residential buildings, more than double the 266 issued in June 2007.

While most developers received permits and were able to begin construction on time, the real problems likely will arise in the future, the president of the Real Estate Board of New York, Steven Spinola, said.

Developers who have begun their construction but don’t continue it at a steady pace will have difficulty receiving 421-a benefits, he said. “If you pour your concrete and then you walk away, that’s when the city is going to raise questions,” he said.

But many developers may not have a choice, as some have not yet secured the full financing to complete their projects, Mr. Neveloff said. “There’s no doubt that there are some people who rushed to get foundations in the ground and are now looking for financing,” he said.

Making the process even more difficult is the fact the city does not tell developers whether they made the deadline to be grandfathered into 421-a. “There are certainly lenders who are going to say, ‘We are not going to finance a project that does not offer a 421-a benefit,'” an attorney at Orrick, Herrington & Sutcliffe, Marc Shapiro, said.

It may be years before developers have proof from the city that their projects meet the deadline. The city does not officially grant 421-a certification until a building has received its temporary certificate of occupancy, which means it is ready to be occupied. “We can’t determine whether they are eligible until they complete the project,” Mr. Donlan said.

Without knowing whether a building will receive the tax abatement, “you’re not going ahead,” a senior bank officer, who requested anonymity, said. “If you don’t know that, you’re taking a chance that the numbers you’ve worked off are not the right numbers.”

Though he was months ahead of the deadline, Mr. Kislin said his lenders were anxious about the tax abatement because it improves cash flow for the project. “For the lenders, it absolutely makes a big difference,” he said. A 421-a exemption “motivates them to get you approved that much quicker.”

Still, it is the newer, smaller developers who have less experience with the city who are likely to find themselves in trouble. “Experienced developers will be, for the most part, quite capable of being certain that they have qualified,” the senior bank officer said. “Those that are not certain would be more likely to have less experience, and/or have legal and architectural representation that is either not of the highest quality, or have simply waited too long to get into the ground.”

Mr. Spinola is urging the city to review the changes to the 421-a program. “There’s been talk about the need to look” at the program because the market has slowed markedly since the changes were instituted, and “we would hope that in conjunction with the state, the city will look at areas that have been negatively affected,” he said.