Mayor Bloomberg said yesterday about one-fifth of a new $1 billion housing fund for New York City will be used to preserve affordable homes for 4,000 low-income senior citizens.

In a move Mr. Bloomberg called a “tremendous vote of confidence” in the city, the Enterprise Foundation, a national pioneer in affordable housing, announced it would create the $1 billion fund to help the city build and preserve 15,000 affordable homes for low-income New Yorkers.

The money is to come from equity, loans, and grants and is expected to create 10,000 new affordable homes for low-income New Yorkers; preserve 5,000 existing affordable apartments, and provide grant dollars and technical assistance for community organizations seeking to build more affordable housing. The new housing fund was first reported in The New York Sun on Tuesday.

“We’re using every means possible to stimulate more affordable housing by creating innovative incentives and removing barriers to development,” Mr. Bloomberg told the foundation’s Annual Network Conference at the Marriott Marquis. “I want to thank the Enterprise Foundation for this tremendous vote of confidence in New York City.”

Mr. Bloomberg provided new details on how the funds would be disbursed.

Enterprise New York, the foundation’s local arm, will start by investing $150 million to jump-start the creation of 2,500 units of supportive housing in 45 buildings around the city. Another $625 million is earmarked to build 7,500 new homes for low-income New Yorkers.

Right now, the new fund has about $317 million in hand, according to the foundation’s senior vice president, Bill Frey. The remainder, about $600 million, will come from grants and proceeds from the syndication of federal tax credits for low-income housing, he said.

Every year, the federal government sends tax credits to individual states to stimulate construction of affordable housing. The credits are doled out based on specific projects by individual developers. Then those developers work with community groups to sell those credits to companies and investors, and that money is used to start projects through syndicators such as the Enterprise Foundation. New York typically gets about $200 million a year in these kinds of credits.

Besides the funds for homes for the low-income elderly, the mayor said money will go to help nonprofit housing developers in the city take advantage of low interest rates to restructure debts and keep rents affordable. Another $25 million is to go toward grants, low-interest lines of credit, and training and technical assistance for community-based nonprofit developers.

The balance of the funds will go toward construction of new affordable housing, including $150 million earmarked to finance development of 2,500 units of supportive housing for the mentally ill homeless, adolescents aging out of the foster-care system, and other people who need a little housing help, the mayor said.

Mr. Bloomberg promised in December 2002 that he would build and preserve 65,000 units of affordable housing for low- and middle-income New Yorkers by 2008. He unveiled the $3 billion New Housing Marketplace program to get the job done. The problem is that for every dollar the city puts into the program, it needs to get $2 from the private sector. Yesterday’s promise of a $1 billion infusion means the city now has almost half the money it needs to reach its goal, officials said.

“During the last 18 years, the Enterprise Foundation has invested $1 billion in affordable housing in New York,” Mr. Bloomberg said yesterday. “Over the next five years, they’re going to match that and help us meet our target of funding 65,000 units by 2008.”

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.

The Enterprise Foundation was the brainchild of James Rouse, a Baltimore developer who, Mr. Bloomberg said yesterday, was an old friend of the mayor’s. Mr. Rouse 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.