Combative residents' group the Heath and Hampstead Society want the courts to overturn Camden Council's consent for the controversial residential scheme (pictured) which has been branded 'obtrusive' and 'unsuitable' for the upmarket north-west London district.
Backed by former RIBA president Marco Goldschmied and comedian Sacha Baron Cohen (aka Ali G), the campaigners have fought a long, high-profile battle to scupper the plans to replace an existing 1951 house on the site with a higher two/three storey home.
The society believes the council contravened rules governing Metropolitan Open Land when it granted permission for the development in the leafy Vale of Health area back in January.
It also feels a number of objections about the project, including the loss of views of the heath, were ignored.
Next month the society will ask the High Court for leave to challenge the decision - the first step in a potentially costly judicial review process.
Tony Hillier, chairman of the Heath and Hampstead Society, said: 'Camden Council made the decision on legally faulty grounds. We are pretty confident we have a good case and the chances are we will win.'
It is not the first time the society has turned to the judicial review process, having helped swimmers at the local Hampstead Pond fight the authorities.
Hillier is hopeful the community will rally round to support the legal campaign.
'We hope the residents of Camden will dip into their pockets to fight this decision on a matter of principle.'
He added: 'If they continue with the scheme and demolish the existing house, they could end up with no permission and no building, which would be a bit of a risk.'
James Gorst refused to comment.