A plan by the Army Corps of Engineers to pay for and elevate buildings along 28 miles of Rhode Island’s southern coast is being challenged by one of the state’s largest environmental groups.

Save The Bay says the proposal to raise 341 homes between Westerly and Narragansett to guard against the impacts of climate change is inadequate and flawed.

One of the main concerns is the Army Corps’ use of outdated data to estimate sea-level rise. The Army Corps projects that coastal water will climb 4.4 inches within 50 years because of climate change. Data from the National Oceanic and Atmospheric Administration (NOAA) projects sea-level rise of 2 feet within the next 32 years and up to 7 feet by 2100.

“Frankly, by underestimating sea level rise, the study also underestimates everything else — the potential harm to coastal homeowners, the number of structures impacted, and the costs of raising and maintaining roads and septic systems and providing other services to these communities,” Jonathan Stone, executive director of Save The Bay, wrote in a letter to the Army Corps.

Coastal Resources Management Council (CRMC) executive director Grover Fugate also has questioned the sea-level rise data. During an October CRMC meeting, Fugate said a greater number of the 4,000 coastal buildings between Westerly and Narragansett could be at risk from the impacts of climate change.

“We believe there should be more (homes) eligible for that project,” Fugate said.

If the project is approved, it would cost $58 million to elevate structures 12 to 18 feet. The Army Corps is offering to subsidize 65 percent of the cost with federal funds set aside for adaptation work after Hurricane Sandy. Property owners would pay the rest. The state is considering plans to offer low-interest loans to help coastal property owners finance the projects, according to CRMC.

“This project would represent a significant investment of public funds, and it is irresponsible not to utilize the most current data available,” Stone wrote.

Save The Bay also is criticizing the Army Corps for not taking a harder look at relocating waterfront buildings — a practice known as retreat — that are in danger of harm from storm surges and an eroding shoreline.

Save The Bay’s letter noted that the Army Corps set a precedent for retreat in the 1980s, when it acquired 61 homes along flood-prone portions of the Pawtuxet River in Warwick.

“Save The Bay submits that the alternative of retreat must be fully developed and considered,” Stone wrote.

Save The Bay also wants more solutions for protecting the hotels, restaurants and small businesses along the 28 miles of shoreline the report examined.

“Any flood mitigation plan for the study area is not complete without addressing both residential and commercial properties and infrastructure needed to serve them,” according to the letter.

Save The Bay filed its letter just before the public comment period ended Nov. 21. The Army Corps said it will most likely hold public meetings in each community after the plans are approved.

Christopher Hatfield, project manager with the Army Corps, said there might be adjustments to the plan, including a review of the sea-level-rise estimates, during an agency meeting in January or February.

“We’ll have to have our discussion and see how we handle it,” he said.

Hatfield will discuss the project at a public CRMC meeting on Dec. 1 at 6 p.m.