A plan to renovate one of the oldest houses in Southampton Village is the latest source of concern for residents and preservationists, who say the proposal could damage the character of the village and threaten Lake Agawam.

The owner of 472 First Neck Lane is seeking to add six bathrooms, two bedrooms, a gym and a garage to the 5,695-square-foot lakefront house, which was built around 1882.

Blueprints show the addition — which would start within a mandated wetlands setback of 150 feet — would nearly double the footprint of the 2 1⁄2-story, seven-bedroom, eight-bathroom house in the village’s historic district. The proposed 7,556-square-foot mansion would include 14 bathrooms, nine bedrooms and 12 other rooms.

A section of the main house that was not part of the original structure and a nonhistoric second house on the property will both be razed if the plan is approved by the village’s board of architectural review and historic preservation.

Architect Anne Surchin, a former Southampton resident and co-author of “Houses of the Hamptons, 1880-1930,” said the plans seem to be “engulfing the existing house,” which was named “Mocomanto” after an American Indian who sold the land to English settlers.

John Bennett, an attorney representing the homeowner, whom he did not identify, said “the main portion of the house is really being lovingly preserved,” noting plans would revert it to “more historically accurate” architecture and that the addition would be at least a story shorter than the existing house.

advertisement | advertise on newsday

Mocomanto — which already has sections within 150 feet of wetlands — has been altered multiple times since it was built by Frederick Betts, a founder of the Southampton Village summer colony, archival photos show.

The property was purchased for $10.7 million in December 2012 by 472 First Neck Lane LLC.

Before the renovation can proceed, the homeowner must obtain a dimensional variance, a special exception and a wetlands special permit from the village zoning board of appeals.

Opponents have already taken out an ad against the plans in a local paper and have started a “Save Mocomanto” website. They said they plan to speak out at a June 22 meeting of the zoning board of appeals.

Tom Edmonds, executive director of the Southampton Historical Museum, said he has “never seen so many people so upset” by a home renovation proposal, noting the house “adds to the atmosphere around Lake Agawam.”

Joyce Giuffra, who lives next door to the property, said she is concerned that a home with nine full bathrooms and five half-bathrooms could stress Lake Agawam, which is still under a Suffolk County Department of Health advisory for a blue-green algae bloom.

“We really are concerned that the massive expansion of historic homes around the lake will turn it into a cesspool and tax the rather fragile state of the lake,” she said.

Bennett said that “anybody who’s concerned about Lake Agawam should be standing up and applauding” because plans would replace two antiquated septic wastewater systems within 200 feet of the lake with a “brand new, cutting edge” wastewater system 230 feet away. They would also establish a 40-foot wetlands buffer of native vegetation and relocate or remove some features within 100 feet of wetlands.

advertisement | advertise on newsday

Stony Brook University Professor Christopher Gobler, who has been studying Lake Agawam for 15 years, said that a new septic system could reduce the amount of nitrogen — the main cause of toxic algae blooms — that are emitted from the house’s wastewater into the lake. But even if the septic system does not, “this one home is not going to tip the scales” of the lake’s health unless it is the start of a trend of much bigger houses nearby, he said.