The penthouse atop the seven-story hotel does not match the design that the New York Landmarks Preservation Commission approved in 2004, and the panel held a hearing Tuesday on whether De Niro and his partners should remove the rooftop suite - at an estimated cost of $1.5 million - and start over.
De Niro appeared at the hearing, imploring commission members to give him the benefit of the doubt for any mistakes made in the project.
"We worked on this project a long time to make it as good as we could make it," the "Raging Bull" and "Goodfellas" star testified. "We did it so it would fit into the neighborhood. ... If there are any minor mistakes, my apologies for it, because in any creation there are mistakes."
The hotel is the latest addition to De Niro's growing real estate empire that includes restaurants, hotels and his Tribeca Film Center. The 88-room Greenwich hotel, where rooms start at $625 a night, opened on April 1.
Neighbors including actor-director Ed Burns told commissioners the hotel does indeed fit into the surrounding historic district.
"For me as a lay person, the architecture is beautiful," said Burns, who lives across the street from the hotel.
But Nadezhda Williams of the Historic Districts Council said the penthouse is 1,100 square feet bigger than what the commission approved, sports a mansard roof instead a sloping hipped roof and has decorative details that make it appear "more residential and fussy" than nearby industrial structures.
"Very simply put, this is not the penthouse the commission approved," she said. "The Historic Districts Council urges the commission to require the penthouse that was approved to be built instead."
De Niro's involvement in the neighborhood dates to 1989, when he converted an old coffee warehouse into film offices and the Tribeca Grill restaurant.
He and restaurateur Drew Nieporent later teamed up with other partners to develop Nobu, a perennially hot Asian restaurant that has spawned more than a dozen outposts around the world.
The New York Observer put De Niro at No. 26 on its list of 100 most powerful people in New York real estate, and he is often credited with turning a warehouse district into a pricey loft neighborhood where Mariah Carey has an apartment and Michael Imperioli can been seen playing with his son in the park.
De Niro and partners Jane Rosenthal and Craig Hatkoff founded the Tribeca Film Festival to promote the neighborhood after the 2001 terror attacks.
The new hotel was built on a former parking lot, with the help of $39 million in tax-exempt Liberty Bonds that were awarded after Sept. 11 to spur rebuilding after the attacks.
The restaurant on the ground floor, Ago, is the New York satellite of a Los Angeles eatery popular with celebrities. But it has been panned by critics.
Although it is a brand-new building and not a renovation of a historic structure, the hotel required approval from the landmarks commission because it is in a historic district.
Commissioners made no decision Tuesday but said they would review the hotel again at a future meeting. Some board members said they might approve the penthouse if the architects made alterations such as changing its exterior from stucco to glass and metal.
De Niro was tight-lipped after testifying.
"You know," he said, "it's a process."