GUANTANAMO BAY, Cuba — Although the Pentagon estimates that no more than 80 of the 400 or so terrorism detainees currently held here will ever be brought to trial, it is moving forward with a proposal to build a $125 million legal complex.

Air Force Colonel Morris Davis, chief prosecutor of the suspected Al Qaeda and Taliban supporters, says he expects to file charges against 10 to 20 prisoners soon after new rules for trying detainees are presented to Defense Secretary Gates in mid-January.

The Supreme Court in June found the Bush administration’s military tribunal system unconstitutional, and Congress passed the Military Commission Act in September to replace it. But less than 20% of the prisoners held at the U.S. naval facility here are expected to faces charges under the new commissions. “At the end of the day, I think the total will be about 75, give or take a few,” Colonel Davis said.

Much of the work involving the war-crimes process is done in Washington or in other American-based offices of the military’s judicial network — not at Guantanamo Bay.

Still, Colonel Davis says, there is just one courtroom here, in a converted airfield terminal that also houses legal staff and a high-security lockup for the accused. The new compound would have three courtrooms, restaurants, parking, and accommodations for at least 800 people.

“It’s going to take longer to do these trials one at a time in one courtroom,” Colonel Davis said. A more rigorous pace could be undertaken if the legal complex is ready by July, as the Pentagon envisions.

The deputy assistant secretary of defense for detainee affairs, Cully Stimson, insists the legal facility is vital to bringing terror suspects to justice. Even if the trials were held back to back, he said, it would take more than a decade to prosecute an expected 60 to 80 detainees using one courtroom.

“What we’re asking for is a very modest proposal,” Mr. Stimson said. “We’re fiscal conservatives by definition. We’re not building the Taj Mahal here.

“We simply want to carry out Congress’s will by holding accountable those people who have committed war crimes.”

Doubts about the future of Guantanamo and the logic of investing in an operation many American allies want to see shut down may doom the building project.

Calling the legal complex “a massive boondoggle,” the American Civil Liberties Union has urged the incoming Democrat-controlled Congress to deny funding.

“No one thinks more than a few dozen detainees will ever be tried there,” the ACLU legislative counsel, Chris Anders, said. “Given what shaky ground they are on legally, I just don’t see the next Congress authorizing any significant construction for additional courtrooms.”

There is nothing in either the Military Commissions Act or in the rules governing courts martial that requires war-crimes trials to be conducted in a formal courtroom — or even at this remote naval base in southern Cuba, Mr. Anders said.

The Pentagon earlier this month backed down from a plan to fast-track the legal compound without approval from Congress; the project is expected to be part of the Defense Department’s supplemental funding request in February or March.

“Due to the extensive size, scope, and complexity of the trials by military commission, additional infrastructure and personnel will be required at Guantanamo to proceed with justice as expeditiously as possible,” a Pentagon spokesman, Navy Commander Jeffrey Gordon, said. “We want these procedures to be full and fair, and do not want the lack of facilities to be a reason to delay the process.”

Colonel Davis says he expects to have cases ready to try by March. But the staff judge advocate, Navy Commander Pat McCarthy, said no one is certain how long the rule-writing and legal challenges to the Military Commissions Act could take.

Among those now at Guantanamo are 14 prisoners deemed by the American government to be “high-value” detainees who were transferred from secret CIA prisons — including the alleged mastermind of the September 11, 2001, attacks, Khalid Sheikh Mohammed.