The owner of a listed 18th Century townhouse in London's Mayfair has been ordered to destroy an unauthorised basement, and told to restore original features removed during a refurbishment.
Westminster City Council said the extensive alterations, which involved altering ceiling heights, were denied planning permission, but completed regardless.
The owner of the property, estimated to be worth around £5m, appealed the decision, but was not successful.
The owner has been approached for comment.
A planning application was submitted by the owner for the works in 2010, but it was refused after the Planning Inspectorate deemed it would harm the building's "special architectural and historic interest".
However, in 2020, the council discovered that the owner had ignored the ruling and had excavated the existing basement to create a large sub-basement underneath.
The council said it also discovered that historic features such as panelling and fireplaces had been removed.
Westminster City Council said the entire new sub-basement must now be re-filled. The space currently houses a cinema, gym and sauna.
The owner must also reroute services such as electrics and plumbing and restore fixtures and fittings to their original listed status.
It is thought the cost of the works, which were given a 12-month deadline for completion, will run into hundreds of thousands of pounds.
Labour councillor Geoff Barraclough, Westminster City Council's cabinet member for planning and economic development, said: "I hope this outcome sends a clear message: those who ignore planning rules will be held accountable.
"It is simply not acceptable to carry out works that have been explicitly refused listed building consent."
Barraclough said the owner showed a "complete disregard" for the historic significance of the property and the council's decision.
He added: "We remain committed to safeguarding Westminster's unique architectural heritage."
