Hi Sally…
It can be really easy… but, of course, every property and its situation is different.
Many things will only need the Déclaration Préalable (via the Mairie) … some will need more involved forms…
each project is “judged” at the local Mairie and the appropriate forms will need to be completed, lodged and accepted…
It’s the Mairie who tell us folk what we need to do… 
but they are generally very helpful.
adding an extra bedroom might also impact SPANC requirements to conform with norms (assuming not connected to public drainage system) 
Chatting things through at the Mairie… can have unexpected results…
On one occasion the Owner planned to do such and such…
and when talking at the Mairie… he couldn’t understand why it was being queried.
Aha… turns out he was planning to match the existing finish/colour… and the Urbanisme lady gently asked… Do you really want it to be “that particular colour”…
But, then it will match… don’t you want that ??? he asked, somewhat bemusedly.
We would rather you changed the colour completely, if at all possible, since the existing colour is not correct for this region and was done without permission many years ago…
Ah well… he was relieved to be able to agree to change the colour of the whole thing… and earned the gratitude of the whole Mairie … 
There will be a new sewage system going in to cover the stables and the barn conversion.
you seem to have all your bases covered 
as the illustrious Dick Strawbridge says… « you should not be able to see the sky through your roof »
They have made life interesting insofar as they are only 4ft 10 inches from the floor so we had to either go with 3 staircases to the bedrooms or lower the whole floor or expect everyone to limbo dance under them to get into the bedrooms.
Bedrooms will look the same as yours, the dinning room will have the floor taken away and be full height to the apex, with a floating walkway connecting two of the bedrooms.
The lights/holes you see in the last picture are not in the roof but old beam slots that run through the wall, as the barn was part of a castle with another building and turret attached at some point in the distant past.
a useful reference:
