Denver International Airport has been racking up new flight routes and new airlines, and a spokeswoman says that demand is behind a decision to expand a long-planned gate addition by 50 percent.

DIA now plans to add 39 gates by 2021 across all three of its concourses under newly released contract documents, up from 26 gates that had been planned as of early August. Spokeswoman Stacey Stegman confirmed the change to The Denver Post.

Those additions adds up to a maximum $1.5 billion project cost — making the gate expansion a mega-project in line with the $1.8 billion public-private partnership contract the city approved in mid-August for an overhaul of the terminal.

That controversial terminal deal — with Ferrovial Airports, the lead private partner — included a major four-year, $650 million renovation of the terminal to be overseen by Centennial-based Saunders Construction. It includes the relocation of security screening to the upper level. Ferrovial then will privately manage expanded concession spaces in that building for three decades.

For the new gates expansion, cost estimates weren’t yet available in August when DIA signed a $45 million contract with WSP USA to serve as that separate project’s program manager.

“The build-out is more (than earlier), and that’s because the airlines are all growing,” Stegman said. “Almost every single airline wants to grow in Denver,” from major carriers such as United, Southwest and Frontier to smaller airlines.

Last year, 58.3 million travelers passed through the airport, which opened in 1995 and originally was built to accommodate 50 million a year. DIA officials have been eyeing a capacity of 80 million passengers a year with the new projects.

Stegman said some estimates for the project still were being fine-tuned, even as DIA submits new contracts to the Denver City Council.

The airport this week filed summaries of four proposed contracts with design and construction firms in the council in anticipation of a hearing next week. The council committee that handles airport matters is set to consider the contracts at 1:30 p.m. Wednesday.

Given their size, the contracts likely are in for heavy scrutiny by council members. Final votes could be possible as soon as Nov. 13 if the committee advances them next week.

“As someone who’s been all over DIA since there was nothing but a grange hall out there in the prairie, I understood there would come a day when this expansion would be necessary,” said Councilman Kevin Flynn, a former transportation reporter for the Rocky Mountain News. “We’re fortunate the airport was designed specifically for this type of expansion, but what I look forward to determining is whether this is the time and the price.”

The expansion will make use of the “telescoping” design of DIA’s three original concourses, Stegman said. The long buildings are connected to the terminal and each other by the underground passenger train system at their centers, and their original designs built expansion capability into both ends of each concourse.

Councilman Wayne New, the vice chair of the Business, Arts, Workforce & Aeronautical Services Committee, said he understood the basis for the expansion, but he urged DIA officials to provide more projections and data to the public in coming weeks.

“I want to see how they justify the number of gates they’re recommending,” New said. “I’m sure they have that information — they just ought to show it.”

Besides new gates, the project calls for new concessions spaces and new public facilities.

Two contracts for architectural and design work, each worth $65 million, are under consideration with Jacobs Engineering and HNTB Corp. And DIA is proposing two construction contracts: $655 million with a Holder-FCI joint venture, and $700 million proposed with a Turner-Flatiron joint venture.

All four contracts would carry five-year terms, with up to three one-year extensions possible if the projects take longer or the airport expands the scope.

DIA currently has 111 regular gates and 42 “apron load” positions, where passengers sometimes board smaller planes from the tarmac. The addition of 39 gates would increase the number of regular gates by 35 percent.

Airport officials argue that the demand-driven projects are worthwhile because they will boost income both for the airport and for its airlines.

Council president Albus Brooks agrees.

“This expansion is what the airlines have been asking for and adds capacity so planes are not waiting on the runway,” he said. “Airports across the nation are investing billions into their infrastructure, but few are adding capacity by expanding gates. This gives DEN a competitive advantage.”

The airport generates its own income from various passenger and airline fees as well as tenant rents, so it doesn’t draw on taxpayer money for improvement projects such as the gate expansion.

DIA has undertaken prior expansions, and it has added the apron gates. But this project would constitute a major addition.

Stegman said the concourses’ telescoping design makes expansions less costly than at some major airports that are planning gate additions. For example: Los Angeles International Airport is spending $1.6 billion on a new terminal that will add just 12 gates.

DIA has a $3 billion-plus expansion plan for the next six years or so, Stegman said. It includes the terminal project’s renovation work, the concourse expansions, expanded roadways and preparations for a seventh runway.