As a boy growing up in Philadelphia, Nathaniel Kahn remembers, he watched in fascination as his famous father, Louis Kahn, tinkered with a large architectural model of a park at the tip of an island. His mother, landscape architect Harriet Pattison, was there, too, fussing with the design. Kahn loved to imagine himself inside the model, playing among the tiny trees that bordered a triangular lawn.

The memory is an especially bittersweet one for Kahn, now 46. The park, a memorial to President Franklin D. Roosevelt, was the last design his father would complete. Just weeks after finishing the model in 1974, the great Philadelphia architect died of a heart attack while returning from a grueling trip to India.

Now, after all those years, it looks like Nathaniel Kahn, a New York filmmaker, may get to visit a real-life version of his parents' wooden model.

A group of New Yorkers says it plans to start construction in October on a long-delayed memorial to Roosevelt, who guided the nation through the trials of the Great Depression and World War II. It would be at the tip of the island that bears his name in New York City's East River, on a 4.5-acre site that has sat vacant since Kahn's death, at age 73.

The prospect of seeing his father's design realized is "enormously exciting," Kahn said. "Lou had so many projects that didn't get made. The Roosevelt memorial meant a great deal to him. It's just amazing that the site is still open."

His only concern is that his mother, 80, who was a quiet force in Kahn's office but never married the architect, hasn't been consulted during the recent process. "I'd be very happy if they would reach out more strongly to my mother, not because she's my mother, but because she was landscape architect for the project," he explained.

The group behind the memorial, Franklin D. Roosevelt Four Freedoms Park, still has other hurdles to overcome. It has raised just $19 million from public and private sources toward the project's $45 million budget. And memorials in New York, like the one to the victims of the World Trade Center attacks, have a way of becoming mired in local politics.

But if the organizers succeed, they could add another masterpiece to Kahn's resume. The design is deceptively simple: The park tapers like a ship, and its triangular lawn funnels into a square, open-air room that offers spectacular views of the river, the Manhattan skyline, and the United Nations, which Roosevelt helped to found.

In those basic geometric forms, Kahn distills a career's worth of ideas about architecture and its timeless power. The park looks as fresh today as it did 35 years ago. It's possible to find resonances of Kahn's renowned Salk Institute, overlooking the Pacific Ocean, in its telescoping vista, as well as hints of what Maya Lin would accomplish with her Vietnam Veterans Memorial in Washington.

Architectural historians already are buzzing about what a new Kahn project would mean for the legacy of the important Philadelphia architect.

Robert McCarter, author of a survey of Kahn's work, considers the Roosevelt memorial one of the architect's "most sublime" compositions, built or unbuilt. "I give it to all my students to study," said McCarter, who runs the graduate architecture program at Washington University in St. Louis.

This isn't the first time, however, that the Roosevelt group has tried to complete the memorial. Construction plans have been thwarted on at least three occasions, sidetracked by New York's 1976 financial crisis, partisan politics, and plain bad luck.

This time, it looks as though the group could pull it off.

"We have all the permitting in place," said Gina Pollara, who is managing the memorial project for New York's Four Freedoms Park. "The project has never been this far advanced."

In June, Four Freedoms Park, which takes its name from Roosevelt's famous 1941 speech, received the go-ahead from the board that runs the planned residential community on Roosevelt Island. Only a few years ago, the group had threatened to take back the memorial site because of the repeated delays.

The political winds also have shifted in the park's favor. Democrats now occupy the White House and New York governor's mansion. Roosevelt's New Deal-style programs are back in favor. And so is Kahn, thanks in part to his son's Academy Award-nominated documentary, My Architect, which tells the story of his three families.

Because the time seems right, Pollara said, the group decided to start construction this fall, even without full funding. "Once people see we are building on the site, they'll be willing to contribute money," she predicted.

Although a Roosevelt memorial opened near the National Mall in 1997, the New York organizers point out that his home state never created its own place of commemoration. They expect the project to qualify for federal stimulus funding, especially since the Obama administration often touts its connection to the Roosevelt era. "We're milking that for all its worth," said Sally Minard, the park's chief executive officer.

The group is so confident that it has started preparing the site, a narrow point of land that reaches out toward the turquoise-and-white slab of the United Nations.

During a visit to the island, Pollara showed off the group's progress. The site has been graded and seeded with temporary grass so that the rough outline of the triangular park envisioned by Kahn and Pattison is visible.

On the east side, facing Queens' emerging skyline, six littleleaf linden trees huddle together, evidence that a claim had been staked. "We're doing a test planting now of the trees to see how they hold up in the wind," Pollara explained.

In the next few days, her group will make a crucial decision about which quarry's salt-and-pepper granite to use for the three-sided room at the end of the island.

The 60-by-60-foot room is an important part of the design. Kahn, who was known for his gnomic utterances, often said that "the room is the beginning of architecture. It is the place of the mind."

The Roosevelt room certainly fulfills that role. By walling off the space with 12-foot-tall slabs of granite, each piece 6 feet thick, Kahn creates an enclosure where visitors can lose themselves in contemplation.

Screened off from the city, they will be alone with the sky and river, heaven and earth. The space is monumental, yet intimate. A bust of Roosevelt, by sculptor Joe Davidson, will be installed along the north wall.

"The room in the garden is such a poetic idea," McCarter said. In his book, he calls it Kahn's "most archaic space" and suggests it was part of his long struggle to create modern architecture that is as enduring as the ancient Greek temples.

Just as important is the procession toward the room. Pattison recalls talking with Kahn about how the landscape would move visitors down to the tip of the island. They wanted people to meander at their own pace, entering the room only when they were ready.

While Pattison frets about the ability of the organizers to maintain the "authenticity" of Kahn's vision, she is eager to see their last project realized. She created the landscape for another of Kahn's celebrated projects, the Kimbell Art Museum in Fort Worth, Texas, but never was credited. "It would be amazing to see this finished," she said.

Although Kahn's design was largely completed before his death, there were still plenty of details to be worked out. The project was transferred to the New York office of Mitchell Giurgola, originally a Philadelphia firm, which had a relationship with Kahn.

Eventually, Paul Broches, a partner at Mitchell Giurgola who had never met Kahn, took responsibility, and landscape architect Lois Sherr Dubin was brought on. The convoluted saga means that three names will appear on the memorial drawings: Louis I. Kahn, his associate David Wisdom, and Mitchell Giurgola Architects.

"The important thing," said Pollara, "is that we are faithful to the Kahn design, and honor both Kahn and FDR."

Kahn's oldest child, Sue Ann Kahn, said she wasn't worried about the current architects' straying from her father's vision. "Knowing my father, he would have changed his mind several times as they were building it," she said. "That's the only part of it that we can't re-create."

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.