Only a tiny percentage of the housing units that would be generated by Mayor Bloomberg’s rezoning plan would be affordable to low- and middle-income New Yorkers, according to a new study from the Pratt Institute released yesterday.

Mr. Bloomberg last week proposed rezoning 35 neighborhoods to help solve the housing crisis that has the city’s vacancy rate at 3%, an all-time low. An author of the study, Brad Lander, said 250,000 units would be required to relieve the housing crisis in New York.

If enacted by the City Council, the rezoning would allow for higher-density residential neighborhoods and would open some industrial areas to residential development. The plan is expected to generate 40,000 to 80,000 new units, but the study suggests only 8% of them would be affordable to low and middle-income New Yorkers.

The data give comfort to Council Member David Yassky and his supporters, who are pushing for guaranteed affordable housing. Mr. Yassky and a clutch of elected officials and community groups joined Mr. Lander yesterday to announce the results of the study. Mr. Yassky supports a policy under which affordable housing requirements would be written into zoning laws, known as “Inclusionary Zoning.”

“If we leave it to market forces, only housing for the affluent will be built,” he said.

Community groups are demanding guarantees of affordable housing. A representative of Brooklyn Community Board 1, Chris Olofsky, promised that if such guarantees were not included in the rezoning plan, the board would reject it.

Community boards and borough presidents are empowered only to make recommendations to the Planning Commission and the City Council, which ultimately make decisions. But elected officials are likely to take notice of community opposition to any plan. Responding to Mr. Bloomberg’s plan, 26 council members have sponsored a resolution calling for the use of Inclusionary Zoning in the new rules.

The Department of Housing Preservation and Development issued a statement yesterday saying it will take a neighborhood-by-neighborhood approach to the rezoning. “The Administration is committed to working with local leadership to maximize opportunities for the creation of affordable housing in ways that do not discourage development,” the statement read.

Developers much prefer Mr. Bloomberg’s proposal of using voluntary incentives such as those that have succeed in Manhattan. Steven Spinola, a spokesman for a developers’ lobbying group, the Real Estate Board, said incentive programs have not had as much of an impact in the outer boroughs because lower rents make it more difficult for builders to recoup their costs. Mr. Spinola disputed the Pratt study, saying developers would build affordable housing under a modified incentive plan.

“If we build more market-rate housing, there will be more housing available,” he said. “Tenants will be in a position to negotiate lower-cost apartments. If the zoning takes place that’s another way to crate more affordable housing.”