An extension that is the pride and joy of a Belsize Park homeowner could be bulldozed after neighbours complained the “monstrosity” has been built illegally.

The over-the-fence row has broken out in Lancaster Grove after chartered surveyor Barrier Tankel complained that his neighbour’s basement and ground floor extension was four times larger than the original drawings.

He claims the new structure is a gross invasion of his privacy. But neighbour Michal Silver has denied the claim, insisting the extension building is actually smaller than the first plans drawn up.

Camden Council has launched an investigation into the dispute and a bid for retrospective planning permission has been submitted.

Mrs Silver, who has lived in her Lancaster Grove home for 15 years, could be forced to pull down her living and television room extension should the council decide she has contravened planning laws.

A council enforcement officer is considering the case and 17 neighbours had lodged objections with the council about the extension as the Ham&High went to press.

Mr Tankel, who built his home in Lancaster Grove 27 years ago, has issued a plea to residents to have their say on the extension.

“If this is allowed to go ahead you could end up with a whole row of these monstrosities,” said the 70-year-old.

“This is a conservation area and the architecture around here is fantastic, so for someone to build this here is just such a shame.”

But Mrs Silver, a textile designer, said the dispute was over the style of the building work and not the substance of planning laws.

“I don’t like his house either so it does come down to taste I think,” she said.

“I have sat there for years looking at the house he built, but this is London and there will always be something you don’t like – perhaps Barrie should move to the countryside.”

She added that she had offered to plant mature trees to reduce the impact on her neighbour’s property and that green ivy would soften the look of the extension within two years.

She said: “As a textile designer the building is something I am proud of and the last thing I want to do is build something which does not fit in with the area.

“In a couple of years when the ivy has grown it will look like part of everything that’s going on around here.”

Council planners are welcoming comments on the application until May 7.