An environmental group is vowing to fight plans for 2,500 homes on the Althorp Estate, near Northampton.
Earl Spencer has an estate at Althorp
The scheme for Dallington Grange is backed by the government, but campaigners are determined to stop the houses being built on greenfield land.
Paul Witherington, from Save Our Soil, said: "We believe government policy is ill-advised and stupid."
The project is part of a large development covering Northamptonshire and Milton Keynes/south Midlands.
Mr Witherington said the rate of development was unsustainable, because "it is equivalent to six towns the size of Northampton over 25 years".
"We think if that happens it will destroy our environment."
Developers plan to build two schools, a shopping complex and commercial facilities at the Althorp site.
Earl Spencer's Althorp estate owns two-thirds of the land. The other third is owned by Northampton Borough Council.
Campaigners have distributed thousands of leaflets to publicise public meetings about the scheme this week.