The airport's original annual capacity was set at 24 million passengers when the project was drafted in 2000. It was increased to 27 million in 2006.
Now, because of the capital's surge in popularity as a tourist destination, experts predict BER will have to take at least 15 percent more passengers when the tape is finally cut.
"After current calculation of the flight schedule forecast, Berlin Brandenburg Airport will open with approximately 31 million passengers and will post a constant increase in traffic growth in the following years," Tagesspiegel newspaper quoted from a leaked document used by BER's advisory board.
The data were reportedly compiled by the Munich-based firm Intraplan for the project.
"We have considerably amended upwards the air traffic forecast for the capital region," confirmed Intraplan chief Markus Schubert, without citing exact figures.
BER, which is supposed to take over traffic from the two existing airports at Tegel and Schönefeld, was originally slated to open in 2012.
But amid a deluge of scandals over mismanagement and corruption, massive construction faults and delays saw the €4.7-billion project pushed back to 2016. Another €1.1 billion have also been requested to finish the work.
Schönefeld back on board?
Meanwhile, Berlin Brandenburg Airport CEO Hartmut Mehdorn, wants Schönefeld to take passenger overflow from 2016.
While Tegel is scheduled to shut down completely, Schönefeld is due to be used in future for government and military flights instead of commercial traffic.
But federal authorities have not yet given BER the go-ahead to use the airport under a so-called "double roof" air traffic handling concept.
Passenger volumes at the two existing facilities doubled over the past decade as Berlin became Europe's third most popular tourist destination after London and Paris.
Last year, Schönefeld, with a capacity of seven million passengers, saw 6.7 million pass through. Tegel carried nearly 20 million passengers in the same period – an increase of eight percent over 2011 and nearly double the airport's original intended capacity of 12 million.