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Work will begin next month on the project that will fill an empty space in the centre of the terminal, an area passengers do not usually see.
The project is needed after the increasing numbers meant there was an "unacceptable" lack of room in the departure lounge and long queues through the terminal.
The airport now deals with 1,035,645 passengers a year - a 47% increase since the terminal was built in 2006 - and during its busiest hour hosts more than 500.
It takes 1734 more jets a year than it did in 2006, a 46% increase.
Airport chief executive Richard Roberts said the extension would be on the first floor only.
It would triple the floor area of the departure lounge and more than double the security screening area.
He said the project was expected to be finished by the end of next year.
It would be "quite complicated" in terms of effects on passengers, but the airport would keep the public informed.
Mr Roberts said the airport had experienced "significant growth", particularly over the last three years, meaning the building had been under significant pressure.
A process of design was begun, as well as consultation with airlines, something required under the Airport Authorities Act.
Those processes were completed, and "we have a project on our hands".
Tenders were out for concrete and steel work this week.
Mr Roberts said once the project was complete, passengers would have facilities "which should be expected by any customer in any facility".
"It's going to be a pretty cool space.
"At the moment it is not acceptable, and it hasn't been for a while."
The airport had been planning the project as quickly as it could, but there were "a lot of hurdles to jump".
Mr Roberts said the project had been designed to keep up with growth until about 2026.
"We couldn't future-proof it 'til 2030 or 2050, because then our customers would say `but we don't need that now, so we're not paying for that'."
Construction should start in mid-October but by early 2019 the work will be more obvious.