The advice given to me by my mairie is that inside I can do what I like, but if I change the appearance of the property from the outside then I will need his permission. If you have enough windows for any planned conversion you may be OK, but if you want to enlarge them then you will need permission. You pay your rates based on the sq meterage of living space, so if you convert the loft without changing the appearance then you should inform the Mairie, but then again..... What they don't know.......