The National Capital Planning Commission, which is chaired by one of Trump's former lawyers, deliberated and then voted on Thursday to approve the East Wing Modernisation Project.
Trump says the $US400 million ($A580 million), 8400sqm ballroom will be a privately financed defining addition to the White House and a lasting symbol of his presidency.
"I am honoured to be the first President to finally get this much-needed project, which is on time and under budget, underway," (sic) Trump said in a post on Truth Social.
Phil Mendelson, a member of the commission and chairman of the Council of the District of Columbia, criticised the speed of the process, saying it was rushed.
"I think that we can have a ballroom. To me that's not the issue," he said.
"It's just too large. And if we can get the same program, but not as tall, not competing in height with the main structure, and a condensed footprint, we are better for that."
More than a dozen protesters gathered outside the commission building ahead of the vote, holding signs that read, "Hands off the people's house", among other messages, and displaying a stack of boxes they said contained 35,000 comments from the public, with 97 per cent of them against the project.
"The American people have weighed in on this project, and they hate it," Jon Golinger, democracy advocate with Public Citizen, said.
"(Trump) needs to put the White House back the way the people gave it to him."
He said a vote to approve the project could be legally vulnerable to challenge.
The commission is one of two federal bodies, along with the US Commission of Fine Arts, assigned a role in overseeing key DC-area building projects.
The Justice Department appealed Tuesday's ruling that the president cannot construct his planned ballroom on the site of the White House's demolished East Wing without approval from Congress.
The federal judge in the case granted a request for a preliminary injunction by the National Trust for Historic Preservation, a non-profit organisation that brought a lawsuit alleging Trump exceeded his authority when he razed the historic East Wing and launched construction on the new building.
