With the city's burgeoning population and skyrocketing rents, New Yorkers could soon outgrow New York, housing experts said yesterday.

But one city planner thinks he's found a way to squeeze in more affordable apartments.

Michael Kwartler's idea is to take underused open spaces in existing public housing and put low-rise multifamily dwellings on them.

Kwartler, an urban designer with New York City experience, analyzed 25 of the city's 348 public "towers in the parks."

He found that 65% of them had underused space. By filling it in with townhouses, Kwartler said the city could house up to 15,000 more families. And the city already owns the land, a big cost saving.

Creating mixed-income housing would integrate "superblock" towers into the streetscape, experts said. The effect is "a sense of place" for residents and the erasure of the stigma of public housing.

Kwartler spoke at a gathering of urban planners, architects, economists and policy makers who met at Baruch College to discuss the findings of a 10-month study into affordable housing options in New York City.

The research was sponsored by the office of the public advocate and the Steven L. Newman Real Estate Institute, with funding from the City Council.

"It's an absolutely brilliant idea," said event speaker and urban planner James Stockard.

Kwartler's proposal "would make it less easy to distinguish public housing from the rest of the neighborhood, which is exactly the right thing to do."

Organizers said Kwartler's idea was one of several promising housing options. Others included rezoning manufacturing and commercial districts to include housing and obtaining federal funds to buy housing sites.

Public Advocate Betsy Gotbaum, who cautioned that preserving open spaces and parks was important, was buoyed by the discussion at the forum.

"Beyond its considerable practical value, this study is a sign of hope," she said.