thanks for responses, the person at Avranches who we presented the declaration prealable too, just took the picture of the cabin (which was 1 page of a 10 page document and did not include dimensions) to his boss and 5 mins later came back and said it was simply not possible in our region, size immaterial (I had specifically offered the dimension sheet before he left), he even said a garage no problem but not an apparently habitable building.