The board overseeing Washington Dulles International Airport approved a $19.9 billion overhaul plan on Wednesday, marking one of the largest airport redevelopment efforts in U.S. history.

The plan adds $15.5 billion in new budget authority to the $4.4 billion previously approved for the project. It covers construction of new concourses, renovation of the main terminal, an underground AeroTrain extension, and additional moving walkways to replace older people-mover vehicles. The full buildout spans roughly 15 years, with some elements not completed until at least 2039, and adds about 5 million square feet of new or renovated space.

Funding for the additional amount comes mostly from debt. The authority plans to issue approximately $14.2 billion in new municipal bonds, draw on $150 million in grants, and use $1.1 billion in passenger facility charges to support debt service on another $1.1 billion in bonds. Officials have emphasized that the project relies on airport-generated revenue and bonds rather than direct federal taxpayer appropriations.

The overhaul aims to modernize facilities at Dulles, which serves as a major hub for United Airlines. Earlier announcements highlighted support from the airline and federal officials for the transformation. Some observers have raised questions about potential effects on airline costs and passenger fees as a result of the increased capital spending.

The Metropolitan Washington Airports Authority manages both Dulles and Reagan National Airport. The board considered the measure during virtual meetings on Wednesday after reviewing detailed meeting materials outlining the scope and financing.