It depends on which area you are in, how well you speak French and what the attitude it towards foreign homeowners in your area. Plus, what their policies are on changing the look of buildings/building new structures so that it fits with the existing surroundings.
I was working on a house renovation in France and the owners wanted to change the garage entrance which was situated under the house - ie. the house was on a higher level and the car port was almost under the front patio (hard to explain) - anyway, the existing gates were horrible old metal link fencing and they wanted to replace that with nice wooden fronted doors that opened in a louvre fashion. But as they were near a church, they weren't allowed!! Honestly, the French are a law unto themselves so unlike other European countries ie. the UK, there's no set rules that all regions stick to. It's a bit hit and miss, but make sure you play by the rules they have and speak good French and try and be as humble as possible. The more they think they're getting resistance, especially from foreign homeowners, the more they'll dig their heels in! So, hope that helps a bit, I expect it's stuff you already know anyway!