Donald Trump has said that the new ballroom which he is adding to the White House campus is set to be 40 per cent bigger than initially expected.

The "top of the line" ballroom will now have a capacity of 900 people, after it was initially announced that it would hold up to 650 people, the president said in a telephone interview with NBC News.

"We're making it a little bigger," the president said, adding that it will be "as good as it can get anywhere in the world."

The ballroom is intended to provide a new space where foreign leaders and other dignitaries will enjoy dinners and receptions on the White House grounds, at a cost of $200 million.

Trump reiterated that he will help to pay personally for the addition while other private donors, described as "patriots" by the White House, will also contribute.

As a former real estate developer, the president has been closely involved with the ballroom's production, and has repeatedly expressed his excitement about the project.

"Right there you see all the trucks. They just started construction of the new ballroom for the White House," he said, as trucks carried out what is the largest renovation of the White House's exterior since Franklin D. Roosevelt expanded the East Wing in 1942.

Trump hopes to use the massive, gilded ballroom for state dinners and other events before the end of his term in 2029, White House Press Secretary Karoline Leavitt previously said in July.

The White House is "currently unable to host major functions honoring world leaders and other countries without having to install a large and unsightly tent approximately 100 yards away from the main building entrance," she said.

The ballroom is being built on the site of the White House's East Wing, which currently stands between the Executive Residence and Treasury Department headquarters.

The East Wing currently houses offices for the First Lady Melania Trump and her staff, the White House Military Office, the White House Visitor Office, and other White House support functions. These have been temporarily relocated during the construction.

Trump himself will be enjoying lavish foreign hospitality this week, when he makes an unprecedented second State Visit to the UK.

"I look forward to saying hello to Charles and Camilla," Trump told NBC News ahead of his visit to Windsor Castle. "It's going to be incredible. It's such a great honor."

Trump's grand ballroom is going to be even grander - 40% larger than original plan