The Twentieth Century Society has scored a significant victory in its battle to save Connell, Ward and Lucas's 1936 Greenside.
Runnymede council has admitted that it acted illegally by giving Listed Building Consent for the demolition of the Grade II-listed house in Surrey.
The society plans to force a judicial review against the council's original decision with victory 'now a mere formality', it claims.
The legal case against the council's consent relies on objections to its ruling that the building 'was beyond economic repair'. The planning committee had failed, the society's director Catherine Croft argued, to properly assess Greenside's architectural and financial value.
Croft told the AJ that the society was pleased that the council now agrees with this argument.
'We are delighted that the council has admitted it was in the wrong, ' she said.
'Hopefully this means that the case will not be contested. This was a very big step for us. We couldn't have done it without the financial support of our members and the public, ' she added.
The building's rescue is not guaranteed, however, since Greenside's owner could fight to defend the original planning permission.