Residents who build without seeking planning consent from Barnet Council have been told they will have the offending structures demolished or removed - and be billed for the trouble.
The warning comes after a Finchley homeowner, who flouted planning regulations, paid the price when the wall he built was knocked down by council contractors.
The wall was demolished last week after the Finchley resident broke the law by ignoring instructions to remove it.
Around five workmen pulled down the wall, railings and adjoining gates belonging to the large home in Hendon Avenue, after the council served a notice in January 2004 demanding a reduction in its height to less than a metre.
Although the householder appealed, the decision was upheld by an independent planning inspector in February 2005.
At the end of last month, the council's planning and environment committee gave the go ahead to demolish the wall under the Town and Country Planning Act.
The homeowner was billed £3,200 to cover the cost of the demolition.
Cabinet member for environmental protection, Councillor Melvin Cohen, said: "We are committed to ensuring that our borough remains an attractive place so structures that are out of place will not be granted planning permission.
"But for an applicant to totally ignore our instructions is utterly unacceptable and we simply will not tolerate it.
"Where there is a clear breach of planning regulations we can and will take enforcement action. We have made every effort and given the applicant every opportunity to put this unauthorised work right.
"As they have shown themselves to be completely unwilling to do so, we'll do it for them and they can look forward to receiving a bill for our trouble."
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