Campaigners hoping to protect a green space from being turned into homes believe they have found evidence of an "incredibly rare" bat species inhabiting at the site.
The Deer Park Action Group in Old Catton carried out a bat survey at the Deer Park in late summer.
It comes after the Grade II* listed green space was purchased from the Buxton family by housing company Devlin Developments in April.
A planning application is expected to be submitted to Broadland District Council within a year.
The Deer Park Action Group used bat monitoring equipment to detect sonars from different types of species.
The group added its results have revealed a thriving population in the Deer Park with six species found altogether.
They claim this includes a species which the Wildlife Trust says is "incredibly rare" - a barbastelle bat.
There are estimated to be 5,000 barbastelle bats across the UK.
Bat survey organiser Christian Clarke, who is a member of the action group, said: "To be involved in this project has been thrilling from start to finish.
"The results we have just shows the importance of the Deer Park to the wide variety of wildlife that inhabit it.
"There could be as few as 5,000 barbastelle bats in the UK and to have one of their habitats be under threat of development is unthinkable.
"If this isn’t a clear indication that the Deer Park must be preserved, what is?"
Natural England will be formally approached for consultation when a planning application is submitted to verify the findings.
The other species thought to be found during the survey included pipistrelles, the noctule and brown long eared bat.
The bat recorders were placed overnight in two gardens in Burma Road and The Warren to record the soundwaves of the bats for the survey.
Kathy Branson, chairwoman of the action group, said: "We have found several bats on the Deer Park. Most exciting is the barbastelle as it is red listed and at risk.
"It also looks like a tiny vampire bat, which is highly appropriate for Halloween."
Devlin Developments has been contacted for comment.
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