Renowned architect Frank Gehry, famous for experimenting with novel building materials and exploding the boundaries of form, has signed on with the city to design a 1-acre playground near the Battery Park Ferry Terminal on the southernmost tip of Manhattan.

Mr. Gehry, whose whimsical-looking concert halls and titanium-covered art museums are cultural landmarks, has agreed to work for free on what will be his first urban playground design. “People are asking us what it’s going to look like,” the parks commissioner, Adrian Benepe, said in an interview. “He said he doesn’t even know what it will look like yet.”

The city has earmarked $4 million for the project, which is part of the largest expansion and revitalization of the Battery since the 1930s, Mr. Benepe said. Plans are currently under way to construct a bikeway connecting the East and West sides of Manhattan, and to restore the historic Castle Clinton, a sandstone fort that is a national monument.

The city hopes to begin construction on the playground late next year, Mr. Benepe said, and expects the space to be open for use in 2009.

“Given the way he has exploded the concept of what performance arts buildings will look like, it certainly will be interesting,” Mr. Benepe said.

Another renowned architect, David Rockwell, is also working pro bono to design a playground near the South Street Seaport.

“Any prominent architect that wants to volunteer their time — we’d love to do it,” Mayor Bloomberg told reporters yesterday.