Fairfield's former downtown Post Office will soon be a place to get a burger instead of the day's mail.

Plan B Burger Bar, a chain with five restaurants in Connecticut and one in Massachusetts, plans to open its seventh eatery in the former post office at 1262 Post Road this autumn.

"We're shooting for early fall, so probably Septemberish," Al Gamble, the chain's co-owner, said Thursday.

1262 Post Road Holdings LLC, an affiliate of Wilton real estate investment firm Westport Capital Partners, bought the brick building a year ago from the U.S. Postal Service for $4.3 million with plans to expand the 11,130-square-foot building for retail use.

The federal agency moved its downtown post office to a smaller storefront at 1300 Post Road as part of an overall downsizing plan.

1262 Post Road Holdings decided to adapt the structure for multiple-tenant use when a single occupant could not be found to take the space, Steve Grathwohl, an asset manager with Burr Street Equities hired by 1262 Post Road Holdings, told the Fairfield Citizen.

"We were like any other landlord," he said Thursday. "We were hoping Apple would move in and take the whole space, but that didn't happen."

Grathwohl said the plan was recently approved by the Zoning Board of Appeals for parking needed for multiple-tenant use. The site will have about 60 spaces of off-street parking, he said.

The project, which still needs to receive site plan approval from Town Plan and Zoning, includes expanding the building to about 15,000 square feet, Grathwohl said.

Plan B would occupy 4,600 square feet of space in the front of the building facing the street, while another three or four tenants would lease the rear area of the structure, he said Thursday.

"When we decided to go from single-tenant to multiple-tenant use, it was clear that the front portion would be really good for a restaurant," he told the Citizen. "Al (Gamble) is a very experienced restaurateur and operator who is very easy to deal with. I think it's going to be a great addition for downtown."

Grathwohl said the facade of the original 3,600-square-foot post office, built in 1936 and expanded in the 1950s, will be preserved as part of the plan.

Gamble said Fairfield was chosen as the site for its newest Plan B Burger restaurant to cater to educated consumers who will appreciate business' use of hormone-free, antibiotic-free beef, which is ground fresh in-house twice a day. "We love the whole demographic and the community," he said. "We're very excited about it and are looking forward to getting to know the arts, the businesses and the colleges."

In addition to its burgers, the Plan B signature menu items include Lobster Mac & Cheese and Beef Wellington Bites, he said.

"We also have a lot of organic greens in our salads," he said.

Gamble said the Fairfield outlet, which will have about 85 workers and serve alcohol, will have about 140 seats in its dining room and another 20 to 24 seats on a planned patio. A 10-year lease with an option to renew for another 10 years has been signed for the property, he said.

Gamble, of Cromwell, founded Plan B Burger Bar in 2006 with Shawn Skehan, a fellow restaurateur from Simsbury, with the opening of its West Hartford location. They own other outlets in Stamford, Milford, Glastonbury, Simsbury, and Springfield, Mass., with plans to open two more sites in Washington, D.C., and Alexandria, Va.

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