Assuming the freedom of a flop, Mr. Leigh and his colleagues wrote it exactly as they pleased. Its theme song (“Dreeeeam the impossible dreeeeeeam!”) became a Broadway classic, as did the play. Its first run lasted for 2,328 performances, and it has been revived and revived and revived ever since. But in the mid-1960s, all of that success came with a bit of a wrinkle, a tax bracket that was as high as 70 percent.

So at the urging of an accountant, Mr. Leigh started buying land; he chose Jackson Township. (He also owns a town house on East 68th Street in Manhattan and a house in France called “l’impossible rêve,” which means “the impossible dream.”) Over the years, he added to that Jackson parcel, bit by bit, as nearby land came up for sale, and today he owns nearly 1,000 acres, making him one of the largest landowners in the area. He has been working on plans to develop parts of the land for more than 20 years.

Sitting in his Manhattan office last week, in front of large picture windows on the 27th floor, Mr. Leigh said he did not expect to see Jackson Twenty-One finished in his lifetime — he will celebrate his 85th birthday at the end of the month with a big party put on by his wife, the artist Abby Leigh — but he does hope to see it started. Much of the infrastructure is in place, and Mr. Leigh says he hopes to break ground on some of the housing in the spring or summer.

According to Jackson Township, Mr. Leigh has so far been approved to develop 150 acres of his land, which may include retail, restaurants, housing and a hotel. But most of the plans remain far from solid. What the Web site displays are not formal offerings but architectural concepts and ideas under consideration, said Tom Bovino, manager of Mr. Leigh’s real estate company, Leigh Realty. They have yet to contract with builders. And even the ever-optimistic Mr. Leigh estimates it will take 25 years to complete the project.

Officials in Jackson Township, however, appear to have caught the optimism — and development — bug.

“He’s committed; he’s got the resources,” said Jose Torres, the municipal administrator of Jackson and a former mayor of Paterson. “This will put us in motion, I believe, and other people will jump aboard.”

Mr. Torres also proclaims himself a fan of the commercial. “It’s catchy!” he said. “Only if you’re nice, right?”

That is precisely what most distinguishes this development and its marketing, its least enforceable premise: all those nice people. There will be no Niceness Monitors or written tests, but Mr. Leigh says he hopes that by actively promoting a friendly atmosphere, by making a certain sweetness feel safe, then just maybe it will stick.