The new Mid-Manhattan Library will open in 2020 with a five-floor open space and a roof terrace. View Full Caption New York Public Library/Mecanoo

MIDTOWN — The Mid-Manhattan Library will close next year to undergo a two-year, $200 million redesign, according to a spokeswoman for the New York Public Library.

The branch, which currently holds the NYPL’s largest collection of books and other materials in circulation, will close in late 2017 and is expected to open by 2020 with an entire floor dedicated to kids and teens, 17,000 square feet set aside for reading and studying, and a publicly accessible roof terrace, according to spokeswoman Angela Montefinise.

The NYPL Board of Trustees on Wednesday allocated $150.7 million for the project, and is fundraising for another $50 million in private funds, according to a statement.

The Mid-Manhattan branch is housed in a 1914 building that was originally designed as a department store. The branch's capacity for 400,000 books will remain the same, Montefinise said.

"The building that was originally designed in 1914 to house the Arnold Constable department store will now really become a library," said lead architect Francine Houben, of the firm Mecanoo.

The project is designed to make the space better function as a library, including the creation of a multi-story, open-plan “long room,” according to Houben.

"By creating the iconic Long Room for the circulating collection, dedicated spaces for children and teens, an adult learning center and business library, plus a rooftop destination for multipurpose use, the building will inspire serendipity and the discovery of all functions of a modern library," he said.

The new interior will have about 35 percent more public space and hundreds more seats than the current version of the branch. Extra space will be created by moving offices to an adjacent building, officials said.

During renovations the branch’s collections and services will be shifted to the Stephen A. Schwarzman Building across Fifth Avenue, and the branch’s Mid-Manhattan Picture Collection will shift permanently to the main branch.

The Mid-Manhattan branch’s community functions, such as IDNYC, will move to the Grand Central branch, while tech classes will move to the Science, Industry and Business Library at Madison Avenue and East 34th Street.