Eastleigh Borough Council in Hampshire, which identified North Stoneham Park as a site for new housing, recently approved the idea for its draft local plan. This triggered a campaign to save the 40ha of rolling landscape.
Harry Willis Fleming, whose family sold the land in the 1950s, said the park, now part council-owned, retains features of Capability Brown's 18th century design.
But apart from a spurt of restoration in the early 2000s, much of the parkland has been largely ignored and its history forgotten.
"It's partly for that reason the landscape was seen as an easy option for building on," said Willis Fleming, who helped to set up an historical trust to raise awareness of the park. "But there's a huge opportunity to use it for recreation and educational purposes," he added.
Gilly Drummond, president of charity the Association of Gardens Trusts, said: "The park was designed by Capability Brown but is in decline, in divided ownership and under threat of development for 1,300 houses. Sadly, when parks decline, developers sharpen their claws."
An Eastleigh Borough Council spokesman said: "The draft local plan is out to consultation until October, when it will go to the Planning Inspectorate for determination, and we expect to hear back next year."
He added: "As far as I know, the housing is proposed across areas of which North Stoneham Park is one of several elements."