Barbara Glosby, of the Ethelred Estate, challenged Lambeth LBC's controversial decision on 30 January to allow 1,460 square metres of Lambeth Walk Park to be given over to children from the Walnut Tree Walk Primary School.
The development would open the way for the school's merger with the Vauxhall Primary School whose valuable site, also owned by the council, would become vacant.
To replace the public's loss of some of the parkland, the council said it would provide 'replacement' open space on 'derelict waste land' incorporating a local youth club.
Mrs Glosby's barrister, William Upton, argued the 'replacement' site was already designated as part of the park and had been used for purposes ancillary to the park for around 30 years.
It could not be a 'replacement' for lost public open space, he told the court, as it was already itself classified as open space.
After a day of legal submissions, Deputy High Court Judge Michael Rich QC overturned the planning permission and ordered the London Borough of Lambeth to reconsider the matter.
He found the council had not properly considered the history of the
'replacement' site and the implications of its loss to the youth club.
He emphasised the land had originally been given to Lambeth by the Greater London Council to be used as a public open space. It had been used for an adventure playground before the youth club moved in.
But the judge said he wanted to make it 'absolutely clear' that his decision did not mean the council had to come to a different conclusion when it reconsidered the planning application.
The scheme would involve the loss of 1,460 square metres of public open space in the park, but council officers said the 1,600 square metres of 'replacement' open space on offer was not only larger but would be 'equally advantageous'
But Mrs Glosby's flat is two to three minutes walk from the park and she regularly uses it. She claims the extension would jeopardise her enjoyment of her home and property and reduce its market value.
She is a leading member of campaign group, The Friends of Lambeth Walk Open Space.
Lambeth's barrister, Peter Harrision, said the effect of the scheme would merely be to alter the shape of the park, which would neither close or be reduced in size.
Rejecting claims that the council's decision breached Mrs Glosby's human rights, he said she had no civil right to live near a park of a certain size or shape.
But judge Rich ruled on Monday that, whether or not the 'replacement' land on offer was itself public open space, the council had failed to adequately consider whether it was appropriate.
STRAND NEWS SERVICE