Mayor Bloomberg will be joined by leaders of the Enterprise Foundation tomorrow to announce a special new $1 billion fund that will help the mayor achieve his goal of building 65,000 units of affordable housing in the city by 2008.

More details emerged yesterday about new fund, which was first reported in The New York Sun Tuesday and has activists for low-cost housing scrambling to see how the money could best be spent. The goal is to give the city a pool of $1 billion with which to build as many as 15,000 low-income units in New York by 2008. One official characterized the announcement as providing in five years the support the foundation had previously given the city over the past 15 years.

“If they are tripling what they are doing, that is a significant increase,” Kathryn Wylde, former president of the Housing Partnership, said. She is now president of its parent organization, the New York City Partnership, a business and civic group. “Having New York as a focus and having them commit to more housing development in New York is all good news,” Ms. Wylde said.

The Enterprise Foundation, a Maryland-based national pioneer in the creation of affordable housing, is focused on the mayor’s $3 billion New Housing Marketplace program. The mayor is determined that the city build or preserve 65,000 new homes and apartments in the five boroughs in the five-year period that began last year.

Housing experts familiar with the foundation’s programs said the biggest chunk of the money will probably come from the group’s syndication of tax credits. The foundation is also likely to collapse other money and donations to put together a $100 million grant in the next five years to supplement the city’s own capital budget for affordable housing.

Officials in the mayor’s office declined to provide details on the plan before its formal announcement. Repeated calls to the Enterprise Foundation also went unreturned.

The story that appeared yesterday in the Sun, which reported the unprecedented size and scope of the Enterprise Foundation housing fund for New York, characterized the bulk of the fund as a grant. Sources yesterday disputed the characterization, saying it is better described as a fund generated largely by the syndication of low-income housing tax credits authorized by the federal government.

Washington created the tax credit program for affordable housing in the mid-1980s to provide an incentive for private investors to develop low-income housing.

National foundations such as Enterprise and Local Initiative Support Corp., known as LISC, generally work as clearinghouses or syndicators, pooling money from other foundations, such as the Ford and Rockefeller Foundations, along with allocations from the federal tax credit program and money from private investors. They leverage that cash to build low-income housing.

A two-bedroom or three-bedroom apartment built under those kinds of programs usually costs about $250,000 a unit. Generally, some part of the construction costs is subsidized. That means the latest $1 billion fund could end up providing the money to build between 10,000 and 15,000 low-income units, according Ms. Wylde.

While the billion-dollar figure sounds immense, Ms. Wylde said it isn’t hard to reach that number if you add how much the foundation can raise from syndicating the tax credits over five years.

That could yield as much as $750 million, she said. Coming up with the balance to get to a billion would not be a daunting task, she said.

Analysts said New York City has typically brought in about $200 million a year in tax credit allocations, and the good news in tomorrow’s announcement will probably be an increase of what has traditionally been earmarked for the city.

“All of this is good news and very positive, but the pot of federal incentives is finite,” Ms. Wylde said. “The difficulty is that we don’t know what to expect in terms of the continuity of the federal support.”

The Enterprise Foundation was the brainchild of James Rouse, a Baltimore developer who began building planned communities and moved into creating housing for the poor. In the past 22 years, Enterprise has had a hand in creating 160,000 units of inexpensive housing across the country and funneled about $5 billion into national affordable-housing efforts.