Excellent - Thanks Stella
I have now downloaded the document and translated it if nyone want a copy by mail. (not sure what the copyright implications are here).
Regardless of whether an illegal structure is adjacent to a historic monument, building without a permit carries civil and criminal penalties, including heavy fines.
Both the Gendarmerie can be called in to act and the municipality has the right to demolish and recover costs.
The same here.
We were at a dinner party last week and our Mayor was telling the story of someone who has been forced to demolish the building he erected without planning permission.
I would go to your local Prefecture if your local Mairie seems disinterested.