The Port of Seattle plans to spend $400 million to build a car-rental garage for the Sea-Tac Airport, a $94 million increase than what was originally planned, the Seattle Post-Intelligencer reports.

In essence, the garage would be a way for the Sea-Tac airport to house 45 million passengers and their vehicles by 2020. It will also reduce curbside congestion.

The garage would be a 2.1 million-square-foot multi-story building that would free 3,200 parking spaces where rental agencies currently park their vehicles. The building would also cut rental-car related traffic down 20 percent.

Presently, 12 percent of traffic through the airport is made up of car rental-related vehicles.

The goal is to have shuttle buses running on compressed natural gas will take passengers to the airport on a 10- to 15-minute ride. By moving rental car companies to a consolidated garage, the port estimates that 90 percent of carbon monoxide and particulate gas emissions will be cut.

Rental agency representatives were vague on whether agencies would privately develop a parking garage. Marshall Fein, general counsel of Advantage Rent A Car and vice chairman of the rental car-company committee says that he is not sure if companies can legally cover the bill by charging customers the $4-per-day facility fee taxed by the Port of Seattle.

According to the Seattle Post-Intelligencer, the port’s five-member elected commission has not approved the construction of the building. Before the latest cost estimate, construction was scheduled for 2011.