After a nearly one-year search, the library on Thursday announced that it had picked Foster, of the London-based Foster + Partners, and the recipient of 1999’s Pritzker Prize, to helm a $250 million renovation of the lion-fronted Beaux-Arts building, which sits on Fifth Avenue.

Though details of the plan are scarce, Foster’s main task is to reconfigure a seven-floor space on the western edge of the building, where books are now stored, into one with multiple reading rooms for patrons, plus a circulating collection, so people can check out books; they have not been able to do this since 1970.

In the process, many of the existing floors in the 1.25-million-cubic-foot space, which is located under the Rose Main Reading Room, will likely be removed to create a more open layout, says Paul LeClerc, the NYPL’s chief executive. The renovated library is expected to draw 3.5 million visitors a year—more than double its current 1.2 million.

The non-circulating research collection, meanwhile, will move to an existing three-acre storage area under Bryant Park, which abuts the property.

What will remain, however, are 26 tall vertical windows; the exterior of the marble building, which was completed in 1911 by John Carrère and Thomas Hastings, is protected as a National Historic Landmark.

Foster may also retain some existing bookshelves, whose ends feature decorative cast-iron fasteners, in “a nod to the building’s history,” LeClerc says.

Though LeClerc wouldn’t say which other architects were considered, he acknowledged the initial list was two-dozen-names long.

Foster triumphed, LeClerc says, because of his track record with modern additions to historic structures, like London’s British Museum and Berlin’s Reichstag. In New York, the 2006 Hearst Tower, where Foster topped a stone Art Deco base with a glass high-rise, may have also proved a useful yardstick.

For addressing how he might muffle construction noise from the project so it won’t disturb library visitors, Foster also won points, LeClerc says. The architect also pledged to personally oversee the project, which is expected to last for four years, on a regular basis.

“One of our big questions was, ‘Are you going to make this yours?’ and he said he would,” LeClerc says.

Foster was traveling and unavailable for comment, according to a spokesperson. But in a statement, he hinted at the role computers would play in the renovated space, as the Internet is increasingly used as a research tool.

“We seek to achieve the right balance between history and technology; between access-for-all and scholarship,” Foster said.

The renovation comes as the NYPL undergoes a sweeping $1.2 billion reorganization, which will result in new programming across its 87-branch system, a greater online presence, and two large new libraries, in the Washington Heights section of Manhattan, and on Staten Island.

To foot the bill, it’s selling Midtown’s Donnell Library Center, which will be replaced by a 150-room Orient-Express hotel, with the library relocated to three bottom floors, in a $59 million deal.

And currently on the market is the Mid-Manhattan Library, a six-story tower at across from the main branch. Its collections will be transferred across the street. Also underway is a $500 million fundraising effort that’s already brought in $300 million, LeClerc says.

Still, lining up financing has been challenging, admits LeClerc, who said it partly explains why the Foster selection was delayed from July to October.

“The economy’s problems mean this will take more time than we had originally planned,” LeClerc says. “But it will get done.”