DEN airport has broken ground on construction of a $1.5 billion expansion of its three concourses that will increase the number of gates by nearly a third.The net 39-gate expansion will add 12 gates, including international arrival-capable gates, to concourse A, 11 gates to concourse B and 16 gates to concourse C by the spring of 2021, the airport says.

Denver handled 61.4 million passengers in 2017, up 5.3% year-over-year, airport data shows. Passenger traffic increased 5.9% to 25.9 million on the largest carrier United Airlines, and 6.7% to 18.2 million on the second largest, Southwest Airlines.

United will use the 11 gates that will be added to concourse B, including four on the west side of the facility that open in 2020. It will also use 4-6 of the gates being added to concourse A, the airline's Denver chief pilot Rob Biddle told pilots earlier in May.

"Denver's future is akin to what Hartsfield is for Delta," United chief executive Oscar Munoz told local employees during the week of 7 May. "[Denver is] going to be for United the Hartsfield."

He was referring to Delta Air Lines' Atlanta hub, which is the largest in the world and widely considered one of the most efficient.

Southwest supports the expansion and is working with the city to determine how many of the new gates it will use.

Already in 2018, flights at Denver are scheduled to increase by 3.3% year-over-year, and seats by 3.5%. United is scheduled to increase flights by 10.9% and seats by 7.2%, while Southwest will reduce flights by 1.7% and seats by 0.5%.

New routes are driving a significant portion of that growth, including new service on Allegiant to AVL, Frontier Airlines to BUF and MYR, Norwegian to CDG and United to JAX and LHR.