The debate opens a window onto the growing pains of communities like Concord, where the population nearly tripled - to 17,000 - between 1990 and 2010.
"They have no more open space left in the town," said Diana McCarthy, a conservationist who lives in neighboring Chadds Ford Township. "It had just been decimated through development and it happened very quickly."
The plans submitted last week by Woodlawn Trustees Inc., the Delaware company that owns the property, are the latest step in a years-long fight in the Brandywine Valley.
In 2013, Woodlawn sold the bulk of its land to a conservancy group that transferred it to the National Park Service. Renamed the First State National Monument, that parcel is now federally protected from development.
But the adjoining property in Concord had been set aside for development, said Vernon Green, Woodlawn senior vice president and chief operating officer. Though protecting open space is part of Woodlawn's mission, Green said not all of the real estate company's land was intended for preservation.
"That's often the part that I think is most misunderstood," Green said. "Over the years, Woodlawn has developed parts of its land as well as preserving parts of its land. It appears that some people thought that anything that Woodlawn owned was going to be preserved."
Residents who have hiked, biked, and ridden horses on the long-public trails said they were surprised to learn it could be developed.
"The thing that keeps me going is that I biked in this place 20 years ago and I was amazed that a place like this would exist in an area that's so dense," said Ken Hemphill, who is leading a group to oppose the development. "And when I found that they were going to develop it after riding past wildlife refuge signs for 20 years, I was, like, in shock."
Woodlawn initially submitted plans for more than 400 houses and commercial development, but withdrew them last year after hundreds of residents packed meetings to protest.
"We definitely listened to the people with their comments and we just felt it would be best to rethink the plan and do something that was less intense," Green said.
Under the plans submitted last week, Green said, most of the public trails would remain. Houses would be built on fields and space between the trails, he said, and the development would be surrounded by open space.
The housing proposal will go before the township Board of Supervisors for a vote in the fall, said Township Solicitor Hugh Donaghue. Township supervisors did not return messages left last week.
Green said a proposal for commercial development along Route 202 on the site was still planned but would proceed after the housing development.
In an effort to protect the site, McCarthy and her husband, Jack Michel, whose family has owned land in the area since 1957, founded the Beaver Valley Conservancy. They hope the organization can establish partnerships to buy the land from Woodlawn.
For Michel, development would mean losing the history of the land, which has long been home to mills and farms.
"To pave over these 323 acres that are right next to the new national monument really diminishes by half the story that needs to be told about that monument," Michel said.
Old farmhouses and barns that Woodlawn now rents out on its property would likely be demolished, Green said.
As Woodlawn proposes yet another housing development - 42 percent of the houses in Concord Township were built since 2000, according to census data - residents are questioning whether the community can afford to lose more open space.
Jason Hoover of Wilmington, who has been active in opposing Woodlawn's plans, said he liked to run on the Woodlawn trails because its fields and rolling hills offered an escape from suburban sprawl.
"It looks like you're in the middle of Pennsylvania or something," he said. "Not Delaware County right off 202."