When Mohegan Sun filed its application for a western Massachusetts casino license in January, some feared its $600 million plan for the town of Palmer might seem puny alongside pricier projects proposed for Springfield and West Springfield.

But since then, Mohegan Sun Massachusetts has bulked up considerably.

On Friday, it announced the addition of a water park and a second hotel, pushing total investment in the project to nearly $1 billion — three weeks after unveiling a 300,000-square-foot retail component that raised the original price tag to nearly $800 million.

MGM Resorts International's Springfield project is priced at $800 million, while Hard Rock International's plan to develop a West Springfield site tips the scales at $700 million.

"We think we're far ahead of everybody," said Mitchell Etess, chief executive officer of the Mohegan Tribal Gaming Authority.

The Palmer project's latest feature is "more than a water park," Etess said. "It's an indoor-outdoor adventure park, part of an overall destination experience."

The 70,000-square-foot development would include a 250-room hotel separate from the main casino hotel. The outdoor portion of the park would wind through wooded terrain off Route 32 in Palmer and provide such activities as kayaking and zip-lining.

Boston-based Finard Properties, identified as the developer of Mohegan Sun Massachusetts' retail complex, also would develop the water park and resort hotel.

"We've really been paying attention to the goals of the legislation," Etess said, referring to the Bay State's 2011 expanded-gambling law that authorized one resort casino in each of three geographic regions as well as one slots-only facility. "This is beyond the scope of anything you're seeing proposed anywhere in the state. It's a really big step in the direction of making this an overall destination, something that's going to draw people from out of state."

He said the water park development would appeal to a slightly older demographic than existing water parks, of which there are only a few in the Northeast, including the Pump House Waterpark at the Jay Peak resort in Jay, Vt., and the Cape Codder Resort & Spa in Hyannis, Mass.

In 2007, the Mashantucket Pequot Tribe announced a preliminary agreement with Great Wolf Resorts of Madison, Wis., to develop an indoor water park near the tribe's Foxwoods Resorts Casino. That project never materialized.

Etess said Mohegan Sun Massachusetts approached Finard about developing the water park resort.

Brigade Capital Management, a New York-based investment company, would help finance the original $600 million portion of the overall project, which includes a casino with 3,000 slot machines and 80 table games, two restaurants, a quick-service food court, a bar and lounge with an entertainment stage and a 300-room casino hotel with a fitness center, a ballroom and meeting rooms.

Mohegan Sun Massachusetts officials have said they expect to reach a required "host community agreement" with Palmer officials in July, which would enable the town to conduct a referendum on the project in September. Projects that advance that far in the approval process are expected to have to file final applications with the state by year's end.

The Massachusetts Gaming Commission wants to award the license for a western Massachusetts casino by the end of February 2014.

b.hallenbeck@theday.com