(Bloomberg) -- United Airlines Holdings Inc. and American Airlines Group Inc. have reached an agreement to press ahead with an $8.5 billion revamp of Chicago’s O’Hare International Airport, the world’s fourth busiest airport.

The deal comes after months of disagreements over ballooning costs for the project, which was first announced in 2018. In separate statements, the dual hub partners said they and the city have found a solution to avoid further delays and keep costs in check — although it won’t be easy. 

“The path ahead will be challenging, and we’ll all need to continue working together to get this project completed within the allotted budget,” American said in a statement.

Mayor Brandon Johnson praised the agreement. “We look forward to prioritizing the O’Hare Global Terminal to bring benefits to the traveling public sooner, while providing thousands of jobs and contracting opportunities to Chicago,” the mayor’s office said in a statement. 

Chicago unveiled the expansion plan for O’Hare in 2018, vowing to transform the aging hub into a global showcase. But six years later, very little has been done and estimates for the project were running some $2 billion over budget.

Under the agreement, the airlines will need to approve any plans to cut the scope of the project or add to the budget, according to United. Construction will also happen in phases, with priority being given to the construction of one of two satellite concourses.

The Chicago Department of Aviation will also plan a phased construction of the Global Terminal, which would replace the existing terminal two.

“The Global Terminal has always been the centerpiece of this program and the crucial piece needed to ensure Chicago maintains its status as a global hub,” United said in a statement. “Under this new phasing plan, we can deliver the most impactful elements of the modernization project the quickest.”

The full scope of the agreement has yet to be announced by either the city or the airlines, and it’s not yet clear whether it will include the 25% gate increase that had previously been a part of the project. Reaching that target is critical, Senator Tammy Duckworth said at a groundbreaking event for improvements to Terminal 3 last month. 

“The end goal is always to get the point where we have the 25% increase,” she said. “We can’t go through all of this and only have three additional gates.”

(Updates to add Chicago mayor’s statement and background.)

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