More than 5,000 people who pledged support in the fight to save a Victorian mansion from demolition have been asked to dig deep in the face of a looming legal battle.

The anonymous owners of Athlone House, on the edge of Hampstead Heath, are to take their bid to knock down the 19th-century mansion to a planning inspector after Camden Council failed to make a decision on its application within the legal time limit.

The Highgate Society has now launched an appeal for funds to cover its legal costs in order to carry on its tenacious battle to save the 1870 house from being replaced with an opulent eight-bedroom home with a basement swimming pool and ballroom.

Susan Rose, chairman of the Highgate Conservation Area Advisory Committee and a member of the Highgate Society, said: “We can’t be sure that we can get support pro bono. You can’t expect highly-paid professionals to give up their time. We’ve relied on people’s good nature but when push comes to shove, barristers have to eat like anyone else.”

Athlone House Limited bought the mansion in 2005, but plans to demolish it were rejected by the council in 2010 and the Planning Inspectorate in 2011 on appeal.

Thousands upon thousands of people, including Monty Python legend and film director Terry Gilliam, have signed a petition calling for the council to refuse the application after it was lodged at the town hall in November last year.

The council could still give the proposals the green light at a planning committee meeting scheduled for July 31.

But an approval would not eliminate the society’s need to fund legal representation, as campaigners have already decided they will seek expert advice on whether to take the local authority’s decision to the High Court and seek a judicial review. Dr Rose has estimated the campaign group needs to raise about £20,000.

Highgate Society vice-president Michael Hammerson said: “If every one of the 5,000 people who signed the petition could give us £10, that would cover our legal costs. Talking to people, they seem as angry at what’s going on as we do, so we hope they will respond to our appeal.”

A spokesman on behalf of the Athlone House team said: “We remain committed to building a house of the highest quality on this important site and we are disappointed that we have had to appeal for a second time.

“The revised proposal, whilst maintaining the same architectural quality, is substantially smaller, and there have been some significant design changes.

“National planning policy has moved on since the previous appeal and we are now confident that our reduced scheme is eminently supportable.

“Although we are disappointed that this revised application has gone to appeal, a public inquiry will give all interested parties the opportunity to take part and make their views known to the planning inspector.”

Cllr Phil Jones, cabinet member for regeneration, transport and planning, said: “The latest application has raised a number of complex issues, in addition to some 4,000 representations from local residents to consider. I am very disappointed that the applicant has appealed rather than let the council decide.

“The proposal will still be presented to the council’s planning committee so that the community can have their say and the views of the committee can be given to the planning inspector as part of the appeal. The council will be vigorously defending the appeal.”

n To donate, contact the Highgate Society at highgatesociety.com/pages/contact-us.php.