Clarification: This story was updated at 11:35 a.m., Sept. 30, 2011, to clarify that Jon Jennings is a minority owner of the Maine Red Claws basketball team.

The Forefront, a proposed $100 million development at Thompson’s Point, may soon get even larger.

The developers are considering adding a 2,300-person capacity outdoor amphitheater, which would connect to their 2,500-seat indoor concert hall.

An amphitheater could attract more concert-goers and musical acts that tour primarily in the summer and prefer outdoor venues, said Jon Jennings, one of the development’s three principal partners.

Many cities, including Boston, Chicago, Bangor and Mansfield, Mass., already offer outdoor facilities that attract those acts, he said. The city held outdoor concerts at the Maine State Pier this summer, but The Forefront amphitheater would be designed specifically for concerts.

“There’s been a lot of interest in having an outdoor concert venue in Portland for a very long time,” said Jennings, who is also a minority owner of the Maine Red Claws basketball team. “This would answer that need.”

Jennings described the 2,500-seat indoor concert hall as being “like an airplane hangar” in the center of the Thompson’s Point property.

When musical acts want an outdoor venue, they could open the northeastern wall — “like large hangar doors where the airplanes come out” — which would lead to a slightly sloped grass hill.

An extra 2,300 people could sit on that slope and look down into the open-ended building, which would expand the total venue capacity from 2,500 to 4,800.

The idea for the amphitheater arose last year, when Greg Mitchell, the city’s economic development director, connected The Forefront developers with a Massachusetts company that wanted to provide an outdoor concert venue in the Greater Portland area, Jennings said. The company books and manages concert venues, Mitchell said.

Neither man would name the company because The Forefront hasn’t signed any agreements with them yet. But if they don’t reach a deal, the Forefront developers might still add an amphitheater, Jennings said.

The amphitheater was one of several changes presented this week to city officials at a Planning Board workshop.

Other proposed changes include:

Moving the 718-space parking garage to the northeastern portion of the property. The city asked for that change, so the project’s parking garage and large surface lot would be less visible from Interstate 295. The change also would allow Forefront parking to be used for overflow parking for the adjacent Portland Transportation Center, Jennings said.

Moving two office buildings to the northwestern edge of the property, facing I-295.

Adding a plaza and green space between the center of the offices, event center and hotel on the Thompson’s Point peninsula. That will give it “more of a campus feel,” Jennings said.

The two sides continue to trade ideas on traffic-control issues. A traffic study commissioned by the developers at the city’s request recommended a number of changes to reduce bottlenecks, including adding an additional exit lane onto Thompson’s Point Road from the Fore River Parkway.

The study also recommended a left-turn lane from Thompson’s Point Road onto I-295; significant signs for parking, buildings and directions to various streets; construction of a westbound right turn lane on Congress Street at Stevens Avenue; and a handful of other changes.

Parking also continues to be a much-discussed topic. The project currently has 1,454 parking spaces scattered throughout the 27.6-acre property, including the 718-vehicle parking garage.

Jennings said that’s enough for most events, including Red Claws games, a 2,500-person concert and the daily happenings at the property’s proposed hotel and office buildings. But for 4,800-person concerts, the property would need more than 1,700 parking spaces to accommodate everyone, based on 3.5 people per car, said Steve Bushey, a civil engineer working on the project.

That’s about 250 more parking spaces than what the project currently proposes.

Furthermore, planning board Chair Joe Lewis said he was “deeply skeptical” of the 3.5 person-per-car figure.

“I think two is probably a more realistic number,” he said.

Bushey said they based the 3.5 number on studies from other concert venues and Red Claws games. Lewis has asked to see those studies.

Jennings said they’re looking at options to provide more parking for major events, but also noted that venues such as Fenway Park and Hadlock Field can’t provide parking for sold-out games, and customers park in nearby lots and spaces.

With the changes, the project will now cost about $105 million, about $5 million more than previous estimates, Jennings said. If The Forefront gets the necessary permits, the developers expect to start demolition and construction by spring.

Phase 1 of the project will take 18 to 24 months to complete, they said, but the office buildings and hotel may open before that timeline expires.

The Red Claws have a five-year contract at the Portland Expo through the end of the 2013-2014 season. Using that timeline, they could move to Thompson’s Point in the fall of 2014.

But the contract also enables the Red Claws to leave early without penalty if the Thompson’s Point event center is ready for Fall 2013.

Staff Writer Jason Singer can be reached at 791-6437 or:

[email protected]