The owner of a Grade II listed building has been fined £45,000 for causing “irreversible damage” to his home.

Maurice Lawee, of Wildwood Road, Hampstead Garden Suburb, pleaded guilty to four offences of carrying out unauthorised work to a listed building at Willesden Magistrates’ Court on Tuesday, in a prosecution brought by Barnet Council.

The court heard that Mr Lawee bought his house in the suburb’s conservation area for £3.7m in 2008.

In 2010, he was granted listed building consent for an extension to his basement, which included installing a lift a garage conversion.

However, complaints were received by the Hampstead Garden Suburb Trust that Mr Lawee was carrying out unauthorised work on the house, which was listed in 1996.

This included building two large conservatories, removing inside walls, the original staircase and doors, and joining outside buildings into the main structure of the house. There were also complaints he had harmed the brickwork.

Conservation officers visited the site and ordered work to stop.

Despite repeated assurances by Mr Lawee and his architect that they would stop, further visits showed they had carried on regardless.

Due to the extent of the unauthorised work to a listed building, Barnet Council decided to prosecute Mr Lawee.

Mr Lawee was fined a total of £45,000, and ordered to pay the council’s costs of £14,000. It is the third highest fine to be issued in a magistrates’ court for this particular type of offence.

Mr Lawee has since applied for retrospective consent for the works, some of which have been granted and others refused.

David Davidson, architectural adviser for the Hampstead Garden Suburb Trust, said: “The Hampstead Garden Suburb Trust is very pleased that the court has taken these offences seriously and imposed such a substantial fine on the building owner.

“The suburb is an area of international significance in the history of town planning and architecture, and alterations to its buildings need to be carefully considered in order to retain its special characteristics.

“The result will hopefully deter other owners of listed buildings from similar actions and ensure that they get the necessary approvals before carrying out works of alteration.”

The solicitor from Barnet Council’s shared legal practice, HB Public Law, said: “Irreversible damage has been caused to the brickwork and outhouse and original features appear to have been destroyed or removed from the building.”

Leader of Barnet Council, Councillor Richard Cornelius, said Mr Lawee had shown a “blatant disregard” for the fact he was living in a conservation area.

Cllr Cornelius added: “Planning laws are there for a reason and the high level of fine reflects the seriousness of this breach.”