Furious residents will start a protest campaign tomorrow amid fears 3,000 homes will be built near where they live.
The idea of the site sprang from talks between Horsham, Crawley, and Mid Sussex Councils to find ways of meeting housing demand in central Sussex.
The earmarked land is near Ifield in Crawley but firm plans have not yet been agreed.
The Community Against Urban Sprawl group wants the idea scrapped.
It fears a swathe of Mid Sussex will be swallowed up by the homes and has led to the community being dubbed Crawsham, a mixture of Crawley and Horsham.
Spokesman Henry Vatcher, of Rectory Lane, Ifield, said: "We are trying to get it rejected at an early stage. Thankfully we got wind of it.
"When this sort of thing reaches the stage of discussing planning permission we feel that would be far too late."
The protesters, many holding placards and posters, will stage a rally outside Crawley Town Hall tomorrow between 1pm and 7.30pm.
They plan to repeat the demonstration at the venue on Monday.
The protests are being held before Crawley Council's executive and planning committees discuss the issue.
More than 300 people attended an exhibition about the scheme at Ifield Barn Theatre, Ifield.
Mr Vatcher said: "The idea of the strategic gap between the two towns is to keep their character and identity.
"If we get urban sprawl then you really do start to lose that."
The group says building homes for Horsham people on the edge of Crawley makes no practical sense.
Other sites in and around the Crawley area are up for consideration. They include land near Pound Hill, Maidenbower, and Bewbush.
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