After years of negotiations and delays, construction is underway on the first of several major projects that represent a massive overhaul at O’Hare International Airport.

Construction started in earnest last month on what will become the airport’s new Concourse Dthe city said as city and airline officials gathered for an official groundbreaking event Monday. Construction on the new terminal is expected to take about three years, with a planned completion date in late 2028. It will add 19 new gates to the airport.

Building the terminal marks the first major step in the airport expansion, first negotiated by then-Mayor Rahm Emanuel in 2018.

The centerpiece of the planned overhaul is the Global Terminal, a replacement of O’Hare’s current Terminal 2 designed by a team led by architect Jeanne Gang.

Also part of the planned $8.2 billion revamp is the construction of a second satellite terminal, Concourse E, and a tunnel connecting the concourses.

The terminal overhaul, which was marked last year by contentious negotiations between the city and O’Hare’s two main carriers, United and American, has faced the threat of ballooning construction costs and schedule delays. Initially, the construction plan called for both satellite concourses to be built first so that they could then be used by the airlines for gate space during the Terminal 2 rebuild.

But the airlines, which are footing much of the bill for the construction, pushed for the build of the Global Terminal to be moved up, hoping to ensure the centerpiece wouldn’t be threatened by increasing costs.

Last spring, the city and the airlines reached an agreement: The Aviation Department could begin building the first satellite terminal while planning a phased construction of the Global Terminal. Then — if enough money was left over — it could construct the second satellite terminal and a tunnel connecting the two concourses.

Aviation Department spokesperson Kevin Bargnes said Monday the plan was still to complete the entire overhaul.

“Mayor (Brandon) Johnson and (CDA) Commissioner (Michael) McMurray are committed to building the entire ORDNext program, including both Concourses D and E, the new O’Hare Global Terminal, and the tunnel connecting these facilities,” he said.

The CDA will share start dates for construction of Concourse E and the Global Terminal later this year, Bargnes said, after the department finishes “a comprehensive review of operational and financial implications of different phasing proposals for the program.”

The Aviation Department maintained the project would cost $8.2 billion — on-budget and just shy of the $8.5 billion it was initially pegged at years ago — despite earlier projected budget overruns.

In 2023, the airlines said they’d received an estimate that would have put the project $1.5 billion over budget in 2018 dollars. And last year, Bloomberg reported the overhaul was $2 billion over budget, citing sources familiar with the matter. The city told Bloomberg then it had made cuts in the overhaul plan to make it “on-budget.” But at the time, the airlines cast doubt on the city’s estimates.

“The project remains on budget,” Bargnes said Monday. “Over the past year, the CDA has worked closely with airline partners, as well as design, construction, and engineering teams, to maintain cost discipline and find cost efficiencies, particularly in utility systems.”

At the official groundbreaking Monday, representatives from both United and American joined city officials to mark the start of construction. Johnson, in his remarks, called the airport “one of our city’s most powerful economic engines.”

Aviation Department Commissioner McMurray, whom Johnson appointed to the post in May, said the overhaul would help minimize taxi times at the airport and deliver a “smoother passenger experience.”

The new Concourse D — designed by Chicago-based Skidmore, Owings & Merrill along with Ross Barney Architects, Juan Gabriel Moreno Architects, and Arup — will include a 40-foot-high atrium, commercial space and a children’s play area. Scott Duncan, design partner at Skidmore Owings , described the new terminal as a “large, domed, multi-tiered space.”

Construction on the new concourse will start with six months of excavation, the city said, with vertical construction expected to start next spring.

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