Brad Glatfelter (L) and Marie Pierce of Annandale, Va stand on the moving sidewalk the hallway between the East and West Buildings of the National Gallery of Art while experiencing Multiverse, a light sculpture, by American artist Leo Villareal. (Jonathan Newton/WASHINGTON POST)

The National Gallery of Art announced a $30 million renovation on Tuesday that will add more than 12,260 square feet of exhibition space and a rooftop sculpture garden to its East Building.

That renovation will occur at the same time the museum completes an update to its infrastructure, a process that began with the West Building in 1999. East Building galleries will gradually close from July through December and then remain closed for about three years after renovations begin in January.

The renovated space will include two sky-lit interior Tower Galleries and an outdoor sculpture terrace overlooking Pennsylvania Avenue. The two new galleries will house modern art from the permanent collection, including a possible Rothko room, said Deborah Ziska, a spokeswoman for the National Gallery.

The galleries are at once necessary and aspirational: While the museum can fill the additional space, officials hope the expanded capacity will inspire future donations to the permanent collection.

The $30 million for the project is being donated by a group of well-known Washington philanthropists who were approached by the museum: National Gallery President Victoria Sant and her husband, Roger; board member Mitchell Rales and his wife, Emily; and David Rubenstein, co-chief executive of the Carlyle Group, the D.C.-based private-equity firm. At press time, the gallery had not released information on individual donations. According to the museum’s federal tax filings, this marks one of the largest gifts from private donors in a decade.

The group has made several high-profile contributions in the past year. Victoria and Roger Sant — she has been the museum’s president since 2003; he is a Smithsonian Regent — gave $10 million to the National Museum of Natural History in June to endow its director’s post.

Mitchell Rales serves on the National Gallery and Hirshhorn Museum boards and displays his personal collection at Glenstone museum in Potomac. National Gallery Director Earl A. Powell III called Rales’s collection of works from the post-World War II era “one of the world’s most important.” Last year, the philanthropist announced that he planned to open a gallery the size of the East Building.

Rubenstein’s philanthropic efforts spurred a jaw-dropping announcement last month in which the Kennedy Center said he had donated $50 million for an expansion, the largest financial gift to the center in its history. Rubenstein has been vocal about the need for what he calls “patriotic philanthropy,” or giving to museums, organizations and landmarks that are partially funded by the federal government. The National Gallery certainly falls into this category, receiving the majority of its operating revenue from government grants each year. According to federal tax filings, government grants and contributions to the National Gallery doubled from 2001 to 2011.

In a statement, Powell said of the upcoming expansion: “This gift to the nation by these generous donors will enable us to exhibit more art from our ever-growing modern collection in spaces that will be at once spacious, airy and contemplative.”

There was nowhere to grow but up: The new Tower Galleries will be created out of spaces between the ceilings of existing galleries and the skylights. They will be 23-foot-high hexagons. While the gallery has changed the height of its ceiling before — it is raising the ceiling of one tower for its blockbuster “Diaghilev and the Ballet Russes” exhibition in May — the ceilings will permanently stretch to their highest limit after the renovation.

The West Building, created in 1937, was renovated from 2007 to 2009. In that renovation, galleries were closed in groups, but the building remained opened to visitors and many permanent exhibitions remained on display. In this case, the East Building will close for three years, except for the atrium and office buildings. “We have to do them all at one time,” Ziska said. “That’s how the [East Building] was constructed.”

The East Building, which opened in 1978, allowed the National Gallery to rebrand itself as a much more modern museum. It created an architectural space that enabled the National Gallery to house larger-scale paintings and exhibition pieces that the West Building could not accommodate. The East Building, and the underground connection between the two buildings, created service space for restaurants, gift shops and lectures.

The award-winning I.M. Pei-designed building was a radical departure from John Russell Pope’s conservative West Building design, and it created not only a more flexible gallery space but also a visual imprint that helped the National Gallery attract visitors and international attention. Last year, the East Building accounted for 929,646 of the total 4.2 million visitors.

The National Gallery is also completing an $80 million congressionally funded renovation to the East Building’s marble facade. These repairs began in 2011 and are expected to be completed by the end of this year.

The renovation announcement comes as the National Gallery and the Museum of Contemporary Art in Los Angeles discuss a possible collaboration, first reported by the New York Times. The National Gallery would help with programming at MOCA, which is struggling financially, and the National Gallery could exhibit part of its collection there during the renovation.

“Programming opportunities are being discussed between the National Gallery of Art and L.A. MOCA,” Ziska said. “Our board has not made any formal decision or voted on this,” she said.

David Montgomery contributed to this report.