A planning application to demolish Ripon's Old Lecture Block has been refused by Harrogate Borough Council.
Developers Graycliffe Homes said they submitted the application after ground investigations revealed serious gypsum deposits beneath the site on College Road - the firm had originally planned to convert it into apartments.
But a decision notice from chief planner Gary Bell states that the submitted gypsum report from the developers "does not unequivocally conclude that it is necessary to demolish the Old Lecture Building for reasons relating to serious gypsum deposits, and therefore is not considered to provide justification of the demolition of the building."
The notice goes on to identify the historic importance of the building. It reads: "It is a non-designated heritage asset, of importance to the 20th century history of the city of Ripon. The proposed demolition is considered to result in substantial harm to the significance of the conservation area which is a designated heritage asset, failing to preserve or enhance its character or appearance.
"It has not been demonstrated that the harm to the significance of the heritage assets is outweighed by any public benefit."
Ripon Civic Society has welcomed the news that the former girls' high school has been saved from demolition, but chairman David Winpenny said the fight to protect the historic building is not over.
He said: "The Civic Society welcomes this unequivocal statement of the value of the building. We are very aware, however, that the refusal of permission to demolish is by no means the end of the story.
"The current state of the building has rightly been condemned by many, especially those living close to it. They deserve better than having a derelict structure on their doorstep.
"We therefore look to Harrogate Borough Council to work swiftly with the owners to have the building put back into a sound condition, and to find a long-term use for it - the Society has previously agreed with a proposal to convert it into apartments, if no educational or public use can be found for it.
"Powers are available to the Council to enforce appropriate action. It is more than time this long saga was brought to a satisfactory end."