I'm studying for my real estate license in France right now, so I can translate for you what the books say:
- You need a permis de construire for all new construction over 20 m2 floor surface (40 m2 in an urban zone with a PLU), or
- If you create an opening to the exterior, enlargen one, or modify the load-bearing structures of an existing structure while changing its "destination" or usage (ie. farm building to habitation).
- If the floor surface of the building(s) exceeds 170 m2 cumulative, then you must have an architect oversee the construction/renovation.
- You only need a déclaration préalable de travaux if: the work creates less than 20 m2 of floor space (40 m2 in urban zone) and does not bring the entire structure over 170 m2; you are just modifying the exterior (and not creating any openings); you are changing the "destination" or usage of the building (without modifying any load-bearing structures).
Hope this helps,