There is no justification for demolishing a much-loved Highgate mansion because it can be restored to its former glory, a conservation expert has told an inquiry.

Developers are appealing to the Planning Inspectorate to knock down Athlone House on the edge of Hampstead Heath and replace it with an opulent eight-bedroom family home.

Campaigners fighting to save the former hospital and RAF intelligence base urged planning inspector Colin Ball to refuse the plans on Thursday as they made their case for why the house should be refurbished and not demolished.

Conservation engineer Jon Avent, who spoke on behalf of the Athlone House Working Group (AHWG), told the inquiry: “It is eminently suitable for refurbishment.”

Mr Avent, a director of a Bath engineering firm, concluded: “I considered there to be no justification for the demolition of Athlone House and in its current condition, the building is well-suited to repair, restoration and occupation as a fine domestic dwelling reflecting its historical significance.”

He told the inquiry that the house has been “neglected” by the mysterious owner, thought to be a foreign billionaire.

Mr Avent was one of four experts assembled by AHWG to face off against developers Athlone House Limited at the 12-day inquiry, at the Wesley Hotel in Euston.

All of them have offered their services free of charge.

Developers Athlone House Limited have argued that restoring the house is not a viable option because the necessary repairs are too extensive.

But campaigners want to see it stick to a historic obligation known as a section 106 agreement made in 2005 which required it to restore the house.

Several of the AHWG experts accused the owners of “wilfully neglecting” its duty to make sure the house is not falling into disrepair, the inquiry heard.

Landscape historian Deborah Wolton, who has written two books on Hampstead Heath, told the hearing that Athlone House is considered a “landmark” on the boundary of Hampstead Heath.

She said: “[There is] a in feeling of outrage at the threat hanging over it.”

Ms Wolton, of Kentish Town, added: “The neglect casts a shadow over Athlone House.

“We all, walkers of the Heath, would much rather see it restored.”

Athlone House Limited is appealing against a failure by Camden Council last summer to rule on the proposals in time.

The council later refused the plans at a committee meeting.

The proposals attracted more than 5,000 objections.

The inquiry continues.