Experts who worked on an initial restoration of the Grade II*-listed mansion ( pictured) in Market Drayton, Shropshire, have hit out at the way in which the building is currently being renovated.
And fears have been stoked by rumours that the local architect asked to draw up plans for a modern extension, Rodney Bellamy, has been dropped and that the property could go back on the market.
Former Soane Museum director Margaret Richardson, a key member of The Pell Wall Hall Preservation Trust (PWHPT), which rescued the derelict building in the late '80s, said: 'The situation is pretty awful really - this is what happens when you lose control over the project. It's awfully sad.'
Between 1989 and 2003, the PWHPT put £1 million of English Heritage money into trying to 'authentically' restore the once-ruined
But, due to a lack of funds, the charity sold the property and trust chiefs launched a search to find a buyer who could complete the meticulous work they had undertaken.
However, the new owners, Kay Johnson and Chris Vassiliou, have unveiled a series of proposals for the building which have shocked trust members.
Among them are plans to build a steel and glass balcony on the side of the house - a move which was expected to get the committee go ahead last night (14 December), but was deferred to allow members to visit the site.
Other proposals already considered by North Shropshire District Council include two applications for interior work
as well as plans for a portico - all of which have been turned down because the schemes were 'not historically authentic'.
Architect John Wibberley, a Soane expert and a founder of the trust, along with fellow architect Barry Clayton, is especially worried about the number of 'inappropriate' applications being submitted.
'It's a scattergun approach in which [the owners] hope they get at least a few proposals approved. Some of the additions are simply inappropriate,' he said.
Wibberley admitted a complete restoration of the mansion, which was seriously damaged by fire after years of neglect in 1986, will be a huge undertaking.
But he said he had been disappointed by the approach of the new owners, who were selected from more than 1,000 expressions of interest when the property was first put up for sale. He said: 'The applications have come as a surprise to us.
'We sold it in good faith and had all sorts of assurances. But they haven't done what they said they intended to.'
The estate agent Jackson Stops & Staff, which originally sold the house, has denied the property is back on the market.