A countess has joined the latest basement battle to hit Hampstead after she objected to her neighbour’s plans for an underground cinema and games room.

The Countess of Listowel, Lady Pamela Hare, has joined dozens of residents fighting plans to dig 3.75m under a £3.6million property in Pilgrim’s Lane to install the leisure area and a library.

The application relates to a house that is well-known for a ship’s mermaid figurehead that protrudes from its front and was installed by its former owner, the late Norwegian businesswoman Ingeborg Lane.

In a written objection to Camden Council, the countess, mother of the Earl of Listowel, who faced opposition from residents in Highgate over his plans to develop Swain’s Lane, said: “I pass the house every day with my dog walking to Hampstead Heath.

“I think the house is perfect as it is. It’s a house of character.

“I have great admiration for the previous owner, a dear sweet lady, and no doubt she would turn in her grave.”

The basement would extend under the property’s footprint and into the back garden, behind the countess’s home.

Objections from 58 people have been sent to the council, which rejected a previous application incorporating a swimming pool in 2011.

The site was bought in 2010 following the death of Ms Lane.

Oliver Froment, 63, managing director of a financial services firm who lives next door, fears his home will collapse if the plan goes ahead.

“I’m just so concerned,” he said. “There are so many horror stories in Hampstead about houses that have been damaged by basement excavations.

“This has been dragging on for months now and the council really should have rejected it already.”

Mr Froment, who lives with his wife and two young children, believes he is at greater risk because part of his home is suspended above a driveway.

“If they get it wrong, my house will literally collapse,” he added.

He has brought in an expert to carry out an independent basement impact assessment. In the report, ground engineering advisor Dr Michael de Freitas wrote: “The application as submitted contains no evidence showing that the nature of the ground on site has been understood.”

Michael Doyle, the developer’s agent, said: “Camden Council has commissioned an independent review of our original basement impact assessment, which also looked at the neighbour’s questions, and it mostly supported the work we have done.

“It raised some further questions that we are trying to address now.”

Cllr Valerie Leach, Camden Council’s cabinet member for regeneration and growth, said: “It is essential that the views of residents are taken into account and at present we have received a number of objections and reports that are being reviewed to ensure that planning officers can make a recommendation on the application.”