The 61 bats were discovered in stone crevices and arches of a balcony at Seaton Delaval Hall, Northumberland, during an ecology survey.
Once a grand baroque stately home belonging to the flamboyant Delaval family, the 18th century property has a lively past of costume balls, elaborate practical jokes and spectacular theatrical productions.
But it was partially destroyed by a fire in 1822 and was bought by the National Trust in 2009 – which undertook much-needed conservation work.
David Bullock, head of nature at the National Trust, was surprised that the pipistrelles were choosing to roost in light and dry crevices as they typically like cool, moist and dark hibernation sites.
“I’ve never encountered hibernating pipistrelles in such numbers before,” he said.
“The cavities in the stones in Seaton Delaval Hall’s central hall provide one of the few known hibernation sites of what could be hundreds of bats.”
The bats first came to roost in the hall after it was ravaged by the fire and left exposed to the elements for more than 40 years.
Seaton Delaval Hall was recently awarded £3.7m National Lottery funding for repairs and restoration, as well as moves to improve visitor facilities.
Bat ecologist Tina Wiffen said: “We discovered the bats when we were undertaking an ecological survey to assess the possibility of introducing new art and visitor information installations into the central hall of the building – a project being supported by the National Lottery.
“On finding the bats, we conducted a formal survey and at least 60 bats were counted in February and 61 in early March – with more visits then needed for verification.”
It is likely that even more bats are hidden in deeper crevices in the former party house, Ms Wiffen added.
Pipistrelles are the commonest British bats weighing around 5g – the same as a 20p piece.
A single pipistrelle can eat 3,000 tiny insects in one night.