For Reagan National Airport passengers who have suffered the indignity of Gate 35X — the jockeying for position in a crowded room with limited seating followed by more jockeying for the shuttle to a plane — relief is coming. Officials expect to open a new concourse in 2021 that will replace the airport’s long-standing practice of busing travelers to their planes.

Those who have passed through National over the past few months have no doubt seen the signage and scaffolding and have navigated the ever-shifting traffic lanes and ride-hailing pickup zones that have come with the construction. Airport officials say it’s part of the challenge of trying to construct new buildings at the same time you’re operating a full-service airport.

“Overall, people have adapted pretty well,” said Richard Golinowski, acting vice president for operations support for the Metropolitan Washington Airports Authority, which manages National and Dulles International airports.

The $1­ billion program, dubbed Project Journey has two parts: the new concourse and an expanded security screening area. It is being paid for in part by fees collected from airline passengers when they buy their tickets.

“With passenger growth setting new records at Reagan National Airport, Project Journey was developed to transform the passenger experience and provide new levels of safety, security and convenience for travelers in Terminal B and C,” Golinowski said. “New security checkpoints and a 14-gate concourse with jetbridges will give passengers more freedom of movement around the terminal while providing more space to relax and enjoy the airport before and after their flight.”

But the project already has run into scheduling difficulties.

Airport officials had originally planned to open the new security checkpoints in fall 2020, several months before the new concourse was expected to become active. But unanticipated problems at the construction site have put that portion of the project more than a year behind schedule. It is now scheduled to open in the latter part of 2021.

At a November briefing, Kevin Sharkey, vice president and general manager of the Virginia and Washington division of Turner Construction, the project’s lead contractor, told board members that the company was working to make up the lost time.

“We have a schedule, but not a recovery schedule,” he told board members. “We are working on a recovery schedule.”

Board members were not pleased.

“We need to focus on every possible mitigation measure,” said William E. Sudow, a Virginia representative on the board. He warned Sharkey the board would have a “laser focus” on the project.

Added District representative Warner Session: “There’s an urgent concern about project delay.”

Golinowski said the delays were caused by “unforeseen conditions” at the building site. Among them, old utilities that had to be relocated and unexpected soil conditions. The contractor had to revise foundation and steel structure elements for the project. Weather also played a role. He said the airports authority continues to work closely with the contractor on schedule issues.

Work on the new concourse, however, continues to move forward. The building is slated to open in July 2021.

The concourse, which will connect to Terminal C, will have 14 gates, an American Airlines Admirals Club lounge, and more than 14,000 square feet of additional retail and food offerings. The 230,000-square foot building will sit on the north end of the airport on a site previously occupied by MWAA’s corporate offices and two hangars.

On a recent day, workers moved around the interior of the new building doing various tasks. Steel beams outlined where each gate will be located and crews have begun installing glass windows. Other workers cleared patches of ice that had formed on the concrete floor from the recent cold snap.

The building’s design will mirror that of Terminals B and C with glass walls and domed ceilings, said Rob Yingling, an airport spokesman. From the new concourse, travelers will have expansive views of the D.C. skyline, including the U.S. Capitol dome.

As part of the project, security checkpoints housed in National Hall, the glass-enclosed walkway of Terminals B and C, will move to a new building that will be built above the roadway in front of the two terminals. Once completed, the number of security checkpoints will increase to 28 from 20.

Like many airports built before the 9/11 terrorist attacks, National has had to incorporate security checkpoints into its existing space. At an airport that was built to serve 15 million travelers, but now handles more than 23 million, that poses significant challenges.

Construction of a new commuter terminal has long been on National’s list of needed improvements, but it has taken decades to move forward on the project in part because it was difficult to get airlines to agree on a plan and how to pay for it.

In the late 1990s, then-USAir announced plans to build a $16.2 million commuter terminal at the airport, but 15 months after winning approval, the airline scrapped the project because of financing difficulties.

Project Journey is the first major addition to National since the 1997 opening of Terminals B and C, which added 1.1 million square feet to the airport. The terminal was designed by César Pelli, who once worked for Eero Saarinen, the Finnish American architect responsible for Dulles Airport’s distinctive terminal. As the cost of the project at National escalated, it drew criticism from airline officials and members of Congress, who scoffed at the design, which included 74 “Jeffersonian” domes. Ultimately, officials cut costs by reducing the size of the building and reducing the number of domes by 10. In the end, the project cost $450 million and opened at least three years behind schedule.