The main entrance to the Postal Service’s regional automated facility on Corliss Street in Providence. The facility was built in 1960, and was the first automated post office in the United States.

The Providence Journal / Andrew Dickerman

PROVIDENCE — The state’s biggest post office is planning to get bigger — much bigger.

The U.S. Postal Service is planning a major expansion of the massive Processing and Distribution Center on Corliss Street, which serves as the hub for the postal network for Southern New England.

The federal agency wants to build a 153,000-square-foot structure next to the existing 245,000-square-foot building to house three enormous machines used to sort large envelopes and magazines.

The Flat Sequencing Sorter machines take up 30,000 square feet each, according to Bob Conroy, activation coordinator for the local USPS.

While the volume of first-class mail — bills, letters and the like — is declining as that traffic switches to e-mail, the amount of larger “flat” mail, such as magazines and catalogs, is increasing.

“This is just a more efficient way for us to process the magazines,” Conroy said.

Similar expansions are under way at USPS facilities across the nation, said Christine Dugas, spokeswoman for the USPS.

“This is part of a national plan to provide the most efficient and cost-effective automated processing in the world,” she said.

The Providence proposal is in its early stages, Dugas said, and still requires approvals at the national level before it can proceed. There are no cost estimates available, and a definite timetable has not been completed, though she said that they would hope to have the building up by 2010.

The elbow-shaped building would be built on land north of the main processing facility that is partially vacant and partially occupied by a parking lot. The facility would run along the West River Street side of the postal complex. The expansion is not expected to add new employees to the plant, which employs about 2,000 workers, Dugas said.

Last night, Postal Service representatives briefed the City Plan Commission on the project. As a federal agency, the Postal Service is not required to go through a local approval process, but the Postal Service has opted to undergo the city approval process regardless.

“In what may be a bit of a coup, we’ve convinced the federal government to go through our planning process,” said Robert Azar, director of current planning for the city.

Dugas said the Postal Service wants to be a good neighbor, and work with the city.

“We don’t have to, but we certainly always try to,” she said. “This is just part of the process to keep the town informed.”

That said, city planners weren’t going to let the Postal Service off scot-free just for being nice. They asked — politely — to see more significant tree coverage throughout the project, and were also hoping that a “green” roof could be installed to reduce runoff and save energy.

When Postal Service representatives said that the decision on the type of roof is made at the national level, Plan Commission staff and members said they would approach the Rhode Island congressional delegation to lobby the Postal Service on the city’s behalf.

The City Plan Commission then gave the project master plan and preliminary approvals. The project will return for a final approval, and it is possible construction could begin before the year is up, USPS officials said.

The Providence facility was built in 1960, and was the first automated post office in the United States. Today, the facility is the leading plant for the Southeast New England district, which includes Rhode Island, Cape Cod and Southeastern Massachusetts as far north as Hull. It sorts 3 million pieces of mail per day.

dbarbari@projo.com