The grounds maintenance yard at Arlington National Cemetery . The cemetery is expanding to accommodate up to 27,000 more burial sites with its Millennium Project, which will do away with the maintenance yard. (Ricky Carioti/The Washington Post)

Arlington National Cemetery leaders are working on what could be one of the largest expansions in decades, warning that without more burial space the nation’s iconic military burial ground will run out of new grave sites for veterans within a dozen years.

Workers just finished construction on a giant structure that can hold tens of thousands of cremated remains; cemetery officials hope to begin a controversial 27-acre expansion this fall; and they will soon start designing a 38-acre addition around the Air Force Memorial near the Pentagon.

The dramatic expansion plans are part of a whirlwind of modernization and improvements put in place since investigators revealed widespread problems several years ago. The ­changes, including digitized records that allow cemetery visitors to look up burial sites online, have transformed internal operations and the visitor experience at the cemetery, which dates to the Civil War.

But one project — which officials say would beautify the area and add more than 27,000 much-needed interment sites for veterans on the northern side of the grounds — has sparked opposition.

Critics say the Millennium Project, as the 27-acre expansion is called, doesn’t fit the historic site, would damage a stream and raze hundreds of trees in place since the Civil War. And some ask whether it would make more sense to begin planning for the inevitable: the day when the cemetery will be full.

Expansion areas of Arlington National Cemetery

It’s no small question for a place that attracts more than 4 million visitors a year, with graves that span U.S. history, including Revolutionary War soldiers, U.S. presidents, Abner Doubleday, Oliver Wendell Holmes Jr., Joe Louis, Pierre L’Enfant and many thousands of others who served the country.

“I love Arlington. But it’s not big enough for all future wars,” said Rep. Jim Cooper (D-Tenn.), a member of the House Armed Services Committee. “It seems like we need to be preparing for Arlington Two, making sure it’s just as nice and wonderful and historical as Arlington One.”

On Thursday, the cemetery will dedicate a new columbarium court, nearly as long as two football fields, where more than 20,000 cremated remains can be stored. Without it, Arlington would have run out of what cemetery officials call “niche” space by 2016.

Now the focus is on new grave sites with the Millennium Project. “This is important,” said Kathryn Condon, executive director of Army National Military Cemeteries, “because if we do nothing today, we will run out of in-ground burial space in 2025.”

There are about 22 million living U.S. veterans now, said Paige Lowther, of the National Cemetery Administration at the Department of Veterans Affairs, and each day many hundreds die.

Nationally, only a little more than half of the VA’s 131 national cemeteries — administered separately from Arlington, which is run by the U.S. Army — are fully open for new burials.

The VA is planning to expand, as well, with five enormous new cemeteries.

But Arlington’s history sets it apart. Federal troops took over the Arlington House estate that had been home to Confederate Gen. Robert E. Lee and his family during the Civil War and began burying Union soldiers there, including 1,800 killed at Bull Run.

Arlington conducts 27 to 30 interments a day on its formal grounds, where magnolias bloom over seemingly endless rows of white headstones, caissons roll past and Old Guard soldiers give three-volley salutes as a final honor.

Some families wait six months for an in-ground burial there.

On the southern side of Arlington, heavy machinery is demolishing a 1 million-square-foot Navy Annex office complex on a hill overlooking the Pentagon to make more space for another expansion.

Cemetery officials are starting the $19 million planning and design for that this year and project that the three projects together would allow new veteran interments into the 2050s.

After that, there’s nowhere else to grow.

“Eventually, Arlington is going to close because there is no more space. That’s a given,” said Joe Davis, a spokesman for Veterans of Foreign Wars, which supports the expansions. “We want Arlington to continue to serve as the final resting place for all of our nation’s fallen warriors for as long as it possibly can.

“But Arlington does have a life span.”

A few years ago, Arlington officials were scrambling to explain to furious lawmakers and distraught families a rash of burial problems. Condon, who is stepping down at the end of the month, was brought in June 2010 after investigators found scores of unmarked and mismarked graves, urns that had been unearthed and dumped in a dirt pile, and millions of dollars wasted in botched information-technology contracts.

She has overseen changes large and small: overhauling burial procedures; fixing the leaky roof in the visitors center; adding recycled-material sidewalks so that grieving relatives are not walking in the roads; modernizing the eternal flame at John F. Kennedy’s grave site (requiring the recent installation of a temporary burner); taking down the “Keep Off The Grass” signs so visitors can explore.

Condon has also been working to get the cemetery listed on the National Register of Historic Places. And she oversaw the effort to move the cemetery from an antiquated index-card record-keeping system to digital mapping of grave sites — complete with an app for visitors to find family remains and historical points of interest.

Last month, John E. Hamilton, VFW’s commander in chief, sent a letter to Condon thanking her for “the extraordinary accomplishments . . . under intense scrutiny.” VFW had repeatedly called for management of the cemetery to be transferred to Veterans Affairs, but, he wrote, “our trust in Arlington has returned, and it’s due to your leadership.”

Condon also inherited strong opposition to earlier Millennium Project plans. In 2010, cemetery leaders asked the Army Corps of Engineers’ Norfolk District for “a more environmentally friendly design,” cemetery officials said.

But critics such as H. Hedrick Belin, president of Potomac Conservancy, said last month that the recent plans would “totally devastate the topography of the region,” tearing down trees and leaving “some real questions about the impact on water quality in the Chesapeake Bay.” The stream valley would be altered, he said.

“We want to preserve this place, a wonderful place to reflect on their sacrifices,” said Rob Nieweg of the National Trust for Historic Preservation. “We oppose what we consider to be a ham-handed design.”

“Arlington wants to bulldoze [one section] essentially, remove 800 native trees and an extraordinary amount of earth.”

Those woods that one section would replace are historic, he argued. “These are the last remaining bits of the Arlington estate. The Custis-Lee mansion at one point was surrounded by 1,100 acres.”

Stephen Van Hoven, the cemetery’s urban forester, said the work will preserve the stand of trees next to Arlington House, so the view from that site will remain the same.

Lt. Col. David Fedroff of the Army Corps said the design requires cutting into the slope for safety reasons; a cemetery can’t be put on a steep hill.

The project, which will be submitted to the National Capital Planning Commission for final approval in June or July, would also remove invasive species that choke native plants and vines that strangle trees, Van Hoven said. Plans call for nearly 800 new trees, thousands of bushes and seedlings, efforts to improve water quality and restoration of the eroded streambed.

All the trees have been numbered with small metal tags, which helps track the different species. After earlier criticism, the design of a road was changed to save some trees. Orange ties around trunks mark a wetlands area as off-limits, and an arched bridge design allows animals to cross the stream; earlier plans had concrete embedded in the water, project manager Greg Hegge said.

Hegge said cemetery officials think that they have addressed almost all of the criticisms, although “some individuals have expressed such a degree of concern over the living trees on the site that they will accept no tree removal.” The officials won’t be able to satisfy those concerns, he said.

“And we owe it to our veterans and our loved ones who served this country to make that balance,” Condon added.

But Arlington officials would not have so many environmental issues to juggle, said Cooper, the congressman from Tennessee, if they were not trying to expand into land so ill-suited for a cemetery. He wondered what the cost per grave will be and asked whether it is truly patriotic to spend so much.

“I don’t mind paying top dollar, and our veterans deserve to be honored,” he said. “But anything we purchase — literally — would be cheaper than what we’re doing now.”

Cemetery officials should be looking ahead and finding a beautiful new site, he said, rather than staying in denial.

The Millennium Project has cost $17 million so far, including planning and design, storm-water retention and stabilizing the stream, and $82 million has been budgeted for construction, Arlington officials said. The new columbarium court cost nearly $16 million. And it’s too soon to know how much the Navy Annex project might cost.

“It’s tough,” said Davis, the VFW spokesman. “That’s where everyone wants to go. Even people whose immediate family doesn’t live anywhere near here. Because they’re surrounded by all their brothers.

“My parents are buried there. I want to go there — knock on wood they have space for me.”