The court's ruling was a triumph for local campaigner, Felicity Irving, who is determined to preserve the 0.77-acre meadow for use by Cuckfield residents.
Known locally as the 'play meadow', she said it had for generations been used by village children and dog walkers and as an outdoor Sunday school for nearby Grade 1-listed Holy Trinity Church.
But all that came to an end when Mid Sussex District Council, which owns the meadow, "locked out" residents in 2013, the court heard.
The council then granted itself planning permission for a house on the site in January this year.
However last week Mrs Justice Lang overturned the permission, saying councillors had been "seriously misled" into granting it.
Their decision was also fatally inconsistent with previous refusals of planning consent for single dwellings in the countryside, she ruled.
One of the council's planning officers had recommended in a report that planning consent for the new house be granted.
The report said that the development was "acceptable" and would cause "less than substantial harm" to the conservation area.
It also stated that the "public benefits" of a "well-designed dwelling" on the meadow outweighed any harm that would be caused.
Upholding Irving's challenge, however, the judge said the officer's report failed to mention two key local countryside protection policies.
And its statement that the proposal complied with the local development plan "when read as a whole" was "seriously misleading", she ruled.
There was also an "unexplained inconsistency" with previous council decisions to refuse consent for two similar developments, she added.
The judge noted that the council could be expected to "make a profit" from selling the meadow with planning permission or with a new house built on it.
But it had not put that forward "as a reason in favour of granting itself planning permission."
The judge stopped short of finding that the council's decision was irrational, but noted that assessment of the public benefit of developing a countryside meadow in a conservation area was "a critical issue."
The new house, she added, would "occupy most of the open site and block the fine views of the countryside" from a neighbouring footpath.
Had councillors been correctly advised in the officer's report, their decision may well have been different, she told the court.
The planning permission, granted on January 18 2019, was quashed.