I'm Dutch, what David says is true, rules are unclear in France, but i'd add that it's easier in France to get yourself on land ,for cheaper than in the Netherlands. I know someone who's bought a few hectares and build an atelier. Which is a space that can be max what was it? 4 x 6 m or 4 x 7, can't remember, you're supposed to not stay in it for one night a month, but because he is friendly with the mayor, he's added bits and bobs, left and right , nobody checks. Building so small and then having thick strawbay walls is not very practical though.
The thing is as well they do not like to legalize you after a while, because when they do you can claim your
water rights and the commune has to pay for a pipe to your house and you can demand electricity which EDF (
energy company of France) ,has to pay, but still, they don't like it.
And then a new mayor can come ,which can not be friendly with foreigners or alternative lifestyles.
But on the other hand they're happy with whoever wants to live there, because young people leave for the cities and old folk remain. The elderly like capable strong friendly young helpfull people around.
I've heard that when you buy a ruin and it's registered on the cadastre(kadaster) which you can check at the mairie you have the right to build it back as was.
Your best bet is focus on a region you really like and meet some dutch/belgian/english/alternative french people who are already there, they've probably got the best valid info for that region and usually are happy to help newcomers.
There's a lot of contradicting info on cob building, according to a friend of mine who has built two cob houses, that is because the earth and clay are different everywhere and lots of websites just parrot content, because it looks cool on their blog to have something on it about cob building, without ever having done it themselves.