Hartsfield-Jackson spokesman Reese McCranie said the existing decks are decades old and not structured to support the weight of four additional levels.
A key reason the project is expected to take so long to complete is that the airport must keep parking available for travelers while it demolishes each deck and rebuilds at the same site.
"That's a huge challenge," said Hartsfield-Jackson development program director of project management David Pino. "That is the task."
Maintaining traffic flows -- and revenue -- "is of paramount importance," Pino said.
To provide parking during the work, the airport plans to build a third parking deck next to the Georgia International Convention Center to accommodate about 6,000 cars. People using that deck will have to take the existing SkyTrain people-mover to get from that parking structure to the terminal.
"People won't be impacted until the fall of 2017, " McCranie said. "We will have a replacement deck in place once we start construction and replacement of the parking structures."
The new parking decks will double in size to eight levels, with a planned 15,000 parking spaces on the Terminal North side and 17,000 parking spaces on the Terminal South side. Also planned are pedestrian bridges on level four -- which would allow travelers to avoid walking across the busy roadway.
Today, the parking garages have four levels, with a total of about 13,000 spaces in the decks out of a total of 29,000 public spaces at the airport.
The airport also plans to build a third parking deck to accommodate about 6,000 cars while the old parking decks are being rebuilt.