The growth at Austin’s airport continues to be so relentless — departures and arrivals are up 51 percent since 2009 — that before officials even started construction on the latest parking garage, they decided to add another level.

Construction of what will now be a 6,000-space, $250 million garage and an adjoining office building could begin in a little over a month, they said.

And a $350 million project to add nine passenger gates on the main terminal’s east side now needs additional funding, officials said, for more international gates, a concourse club for one airline and replacement of several of the downstairs baggage carousels.

These changes at Austin-Bergstrom International Airport, which by law is a self-sustaining city enterprise, will come before the Austin City Council on Thursday. The council will be asked to authorize a construction contract for the new six-story garage on the current site of surface Lot A, and the 70,000-square-foot airport administration building at the new garage’s west end.

Surface Lot A, located on the north side of the Austin-Bergstrom International Airport terminal, is where the proposed 6,000-space parking garage would be built.

American-Statesman Staff

The council will also consider increasing appropriations for the airport by $72 million — $44 million for the garage project and $28 million for the added terminal costs — money that the airport would borrow by issuing revenue bonds and pay back from airline, passenger, parking and concession fees.

The projects come on top of other expansions in the past five years: the “east infill” project for the terminal, a second parking garage that opened in 2015, an added surface lot east of Presidential Boulevard used by airport employees, a second hotel on the airport grounds, the reopening in April of the “South Terminal” for smaller airlines, and 2,000 new surface parking spaces to the north of the Airport Hilton.

WATCH: Get a sneak peek of Austin’s retro-themed South Terminal

“It is astonishing,” said Shane Harbinson, the city’s Aviation Department assistant director. “We are one of the fastest-growing airports in the U.S.”

Passenger traffic, 12.4 million in 2016, could top 13 million this year, officials said.

A growing airport

American-Statesman Staff

The garage and office project had been projected to cost $206 million, Harbinson said. But the airport’s rapid growth, even as the project was being designed, led to the addition of another floor with about 1,000 parking spaces, explaining the $44 million increase. Once the garage is finished, along with the surface lots to the north, the airport would have at least 17,000 parking spaces, not including thousands of other spaces in the private Parking Spot and FastPark lots off the airport property.

“We’re attracting more people from outside the direct Austin area, in the 10-county service area, and those people are parking longer,” Harbinson said. The airport has solicited construction bids for the garage, he said, and, if the council approves Thursday, “they’d be ready to start construction by the end of April.”

The garage should be complete in about two years, with the office building opening about six months later.

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The office building, officials said, would allow about 50 airport administrative staffers to move there from inside the terminal on the inaccessible, secure side of the building (on a mezzanine level above the passenger gates). Dozens more city aviation staff members would move to the new offices from an existing building on Spirit of Texas Drive. And Harbinson said the new building would have room to accommodate some tenants in the terminal, including airline staff and officials with the federal Transportation Security Administration and Federal Aviation Administration.

Expanding the airport

American-Statesman Staff

Moving out the city and federal workers, he said, would free up rentable space on the terminal mezzanine for, among other possibilities, a “common-use lounge” where passengers for a daily fee could get away to a quiet space while waiting for their planes.

As for the terminal expansion, work began last year to increase the apron space east of the terminal, which will allow more overnight parking of jet liners and accommodate the larger Boeing 787 planes coming into more common use. Crews are also extending the concourse’s east end.

With international air traffic on the rise at Austin-Bergstrom, including some planes with 300 passengers or more, Harbinson said the airlines wanted at least two of the nine new gates to accommodate two jetways for a single airplane. That allows faster boarding and disembarking, he said. With the $28 million in added money for the project, he said, six of the nine added gates would now be outfitted for both international travel (with secure passage to and from customs) and, with access adjustments to the rest of the concourse, for domestic flights.

That east end of the terminal will also include, Harbinson said, a new members-only club for a “legacy carrier” at the airport at a construction cost of $5 million. Asked which airline, Harbinson said that, with the contract still under negotiation, the city at this point won’t disclose the company’s name.

That $5 million, and more, would be recovered by the airport in additional fees paid by the airline on a per-square-foot basis, he said.

The airport, with some of the additional funding for the terminal project, would also replace five of the seven baggage carousels on the lower floor. The original carousels have been in place since the airport opened in 1999.

“None of the manufacturers want to bid on renovating those,” Harbinson said. “So we’re going to replace them.”

Correction: This article has been updated to reflect the private Parking Spot and FastPark lots have thousands of parking spaces, not a lower number.