American Airlines is expanding at DFW International Airport, with plans to move into the unoccupied Terminal E satellite building to add dozens of daily regional flights.

The Fort Worth-based carrier finalized a lease with the airport Thursday that will see $20 million invested to update the terminal and convert an existing nine mainline gates into 15 regional gates.

American said the additional space is expected to come online in the spring of 2019 and will allow the carrier to ramp up its summer schedule at DFW from about 800 peak daily departures to nearly 900 daily departures.

“American is committed to ensuring DFW remains a premier gateway for many more years to come,” American’s vice president for DFW operations Cedric Rockamore said in a letter to employees Friday morning.

The satellite building, connected to the main Terminal E by an underground walkway, was built in the 1980s to add more gates to what was then a Delta Air Lines hub.

More recently, the building’s gates had been used to house airlines that were displaced during renovations being made to Terminal E as part of the airport’s $1.9 billion renewal and improvement project.

With renovations in the main terminal complete, American will move into the satellite building and make some improvements of its own.

Those will include renovated ticketing and gate areas, new carpet and surfaces, new jet bridges, and new elevators, escalators and moving walkways.

American will oversee the improvements and will be reimbursed by the airport up to $20 million under the terms of the lease agreement. The carrier is also investing $25 million of its own funds for American-specific improvements, according to the airport.

DFW Airport is American’s largest hub, with nearly 57 million passengers enplaned in 2017. That represented about 85 percent of the airport’s traffic.

American said it would move some regional flights off their current gates in Terminal B to the satellite building once complete. The company plans to put its Embraer E140 and E145 operations, jets that seat between 44 and 50 passengers, at the new satellite building.

The airline is also looking at extending a connector tunnel that would allow for more international arrivals at Terminal B. The tunnel, currently accessible at three Terminal B gates, allows arriving passengers to walk to the main international gateway in Terminal D to go through customs.

American is conducting a feasibility study looking at extending the tunnel to three additional gates, which would allow it to move some international flights on narrow-body aircraft to Terminal B, freeing up space for larger planes at Terminal D, according to airport documents.