The tycoon has vowed his company will do “no new foreign deals” while he is president, but he has left “new” and “deals” open to interpretation and those words are drawing scrutiny.
A spokeswoman for the golf resort said the expansion was not a new deal but just another construction phase that was included in the broad plan approved by the local government in 2008.
Some lawyers who specialise in government ethics are not convinced.
Richard Painter, who served as chief ethics lawyer for President George W Bush, said there were so many pending issues before local governments at each stage of a real estate project that it was like a new deal every time.
“Each phase requires building permits, each phase requires financing, each phase poses additional conflicts of interest,” said Mr Painter, a Trump critic who has urged him to sell his company.
“Americans don’t want their president or any other high ranking official dependent on a foreign government for a building permit.”
The Scottish expansion is especially worrying, Mr Painter said, because the US will need to negotiate a trade deal with the United Kingdom now that it has voted to leave the European Union.
Mr Painter said: “In that context, who is going to deny a permit to the president of the United States?”
In addition to its Scottish resort, the Trump Organisation has several other uncompleted international projects, including a hotel in Vancouver, British Columbia, a golf course in Dubai and two resorts in Indonesia.
His partner in Indonesia, billionaire developer Hary Tanoesoedibjo, plans to attend Mr Trump’s inauguration on Friday and an official inauguration ball.
The Trump Organisation did not respond to requests for comment, except for the brief statement from its resort spokeswoman.
Mr Trump’s original 2008 proposal for the resort, the Trump International Golf Links, included plans to add a 450-room hotel, a second golf course, 500 luxury homes and 900 timeshare apartments.
A recent article in the Guardian newspaper cited new plans for about twice the number of homes and timeshare apartments.
Officials at Aberdeenshire Council could not confirm the Guardian figures but did say plans for the second golf course were still under review and had not been approved.
A website run by the council shows the Trump Organisation has had to undergo many reviews since its 2008 plan was approved, including for small projects, such as erecting a flagpole, and building a wall and adding six rooms to a hotel already there.
The resort has had a troubled history.
It has faced fierce opposition from locals, including a fisherman who became a national hero for refusing a £562,000 offer from Mr Trump to buy his land.
Environmentalists protested about possible damage to Aberdeen’s dramatic dunes which overlook the wind-swept North Sea. A documentary was shot called Tripping Up Trump.
The pledge of “no new foreign deals” came at Mr Trump’s press conference last week, in which he unveiled several other steps he would take to allay concerns that he could be tempted to put his private financial interests ahead of the public good.
Matthew Sanderson, a former legal adviser to several Republican presidential campaigns, was “full of unanswered questions and malleable commitments that leave the public guessing”.
Mr Sanderson said the Scottish expansion showed that Mr Trump would need to provide more details.
“His conflict situation is extraordinary and only extraordinary levels of transparency can combat the appearance that his new office is enriching him,” Mr Sanderson said.
To make questions of possible self-dealing go away, many ethics lawyers have urged Mr Trump to follow the example of past presidents by selling his ownership in his company and putting the cash into a blind trust overseen by an independent manager.