The National Capital Planning Commission is scheduled on Thursday to take a final vote approving President Trump’s ballroom, clearing the last review for a major addition to the White House that was publicly unveiled in detail only in January. Last month, another panel led by the president’s allies, the Commission of Fine Arts, discussed the ballroom for 12 minutes before unanimously approving it.

The hurried reviews, with construction cranes already swiveling above the White House grounds, are an abrupt departure from how new monuments, museums and even modest renovations have been designed and refined in the capital for decades. And the ballroom will be worse off for it, architects warn.

Take the White House fence, a far more modest part of the complex that received more probing attention from both commissions when it was rebuilt during Mr. Trump’s first term.

Such details affect how people passing by experience these iconic places, and how each structure fits into a capital city that has been planned around civic symbols and sightlines since the 1790s. The deliberation is also an expression of democracy, said Carol Quillen, the president and chief executive of the National Trust for Historic Preservation, which has sued the administration over the ballroom.

“Even if we are slow and we make mistakes and we fight, that process has meaning to us,” Ms. Quillen said. No project belonging to the public should be the vision of just one man, she said.

That is, however, how the ballroom has often been described.

“President Trump is the best builder and developer in the entire world, and the American people can rest well knowing that this project is in his hands,” Davis Ingle, a White House spokesman, said in a statement. Past administrations and presidents have wanted a ballroom for more than 150 years, he said, and Mr. Trump will accomplish it.

But in the sprint to complete it before the end of his term, the addition appears to have compressed the normal design evolution for any project.

As recently as October, the president was still increasing the ballroom’s capacity, the kind of decision needed at the concept stage. And the White House has said it plans to begin building in the spring, a timeline that would mean construction documents would have to be prepared even as the design was still under review. (Before a judge demanded in December that the project seek review by these two commissions, the administration appeared poised to skip them entirely.)

“The timeline never made any sense to me,” said Thomas Gallas, a former member of the planning commission and an architect. A building on this scale might take its architects and engineers 18 months to two years from initial concept to completed construction documents, he said.

Reviews by the planning commission generally follow similar steps, with major projects seeking feedback on initial concepts, then approval of preliminary plans, and then final approval. The public process for the Fed renovations took two years, the African American history museum even longer:

For the ballroom, the planning commission never had a say on the concept design. And this week, it will vote on a combined preliminary and final review, a move more common for antenna replacements or new security bollards. The Commission of Fine Arts did something similar in February.

Rodney Mims Cook Jr., the Trump-appointed chair of the arts panel, countered that the group had significant input, including in unofficial meetings with Mr. Trump and in feedback objecting to a large pediment previously planned for the top of the ballroom’s south portico. “We asked him to tone down the porch,” he said. “We asked him to remove the pediments. We asked him for landscape. All of that he did.”

Will Scharf, the chair of the planning commission and the White House staff secretary, said his commission had handled the ballroom with the same deliberative pace it has other analogous projects, like an overhaul of the Capital One Arena and the plan for a new R.F.K. Stadium. Those projects, he said, share the ballroom’s sense of urgency and ready funding (characteristics a memorial or museum may not have).

“If not for President Trump, his desire to move quickly, and his raising the money to fund this, a project like this could languish for years with no decision or action,” Mr. Scharf said. “And we could still be debating it at N.C.P.C. meetings 20 years from now.”

Some big projects in Washington have been bogged down for years. And it’s certainly possible that the White House fence would have been just fine with five inches between the pickets, and that the African American history museum would have looked nice with a Custom Artisan #4 finish instead.

But it’s harder to argue that a major addition to the White House needs swifter public scrutiny than its fence (these commissions have meanwhile continued to push back on projects that are not the president’s personal priorities). Many concerns about the ballroom are also not minor ones. And without further work, the details provoking those concerns will become lasting features of the capital.

For starters, the ballroom is set to become the dominant anchor at the end of Pennsylvania Avenue, a link planned by Pierre Charles L’Enfant to connect the Capitol and the White House.

“The ballroom is literally an imposition between two branches of our government,” said David Scott Parker, an architect on the board of the National Trust for Historic Preservation, and one of more than 30,000 people who wrote to the planning commission objecting to the building.

The proposed East Wing is about 60 percent larger than the White House residence by floor area. But by cubic volume, and including the porticos, it’s more than three times as large because of the ballroom’s vast ceiling height. Viewed from the south, the ballroom’s size will make it the dominant building of the White House complex, with a portico bigger than that of the residence and a lopsided appearance disrupting any symmetry with the West Wing.

The south portico, which was not part of the addition’s initial design, also has no doors into the ballroom. And all of the columns will block views and daylight from inside.

During the planning commission review earlier this month, the project’s architect, Shalom Baranes, acknowledged that the south portico was more ornamental than functional.

“Is it an absolutely essential part of the program? I would say no, it’s not,” he said. “Really it’s an aesthetic decision to have it there.”

That decision, however, is part of the reason the White House driveway planned by the famed landscape architect Frederick Law Olmsted must be rerouted, breaking its symmetry (the kind of detail the planning commission might have dwelled on in the past).

Inside the East Wing, the ballroom itself is far larger than industry standards suggest is necessary for 1,000 guests (by that standard, it might fit 1,500 people). Mr. Baranes said the extra space was needed to accommodate TV cameras, journalists, security and ceremonial processions. But one result is that events with fewer than 1,000 people could feel empty.

The commercial kitchen and first lady’s office suite on the lower level are likewise supersized. And on the second-floor colonnade connecting the ballroom to the executive residence, a blank wall of faux windows will face the north (the direction from which most tourists get a glimpse of the White House). Behind them is a row of bathroom stalls.

Many criticisms of the building, Mr. Scharf said, fail to acknowledge that the White House has continually evolved since its beginning. “As our country’s developed, so too has the White House complex,” he said, adding that he would vote on the project this week after having read every one of the letters the commission received. “I see the ballroom project as a natural extension of that history.”

Most of the concerns that have been raised touch not on how the building will be used inside, but on how it will face the public. That makes seemingly prosaic matters — the height of the roofline, the jog in the road, the square footage of the ballroom — also symbolic ones.

“This is the People’s House, this is not Donald Trump’s, or Joe Biden’s or the next president’s,” said Phil Mendelson, who sits on the planning commission in his role as the chairman of the D.C. Council. He has been a lone objector trying to raise these questions before the commission.

Now, barring intervention by the courts, time is apparently up to resolve them.

“I still don’t understand,” Mr. Mendelson said, “why the ceiling height has to be 40 feet.”