Wells in the back garden
So, the water diviner has been to your garden and has told you that there's water there or not that land and you want to build a well. Well, that is if there's not one already. Most country gardens already have a well in them. You can even get them inside the house. As the people became richer, the wells that were already built and that were just outside the back of the house got incorporated into the scullery or the something that was built as the house got extended over the centuries. Pull up a tomette in the scullery, and there it is. There are many of those old fashioned pumps in the inner courtyard of houses in villages in France, the water running underneath the house that has been extended bit by bit. They go pretty deep as well and can sometimes be on average about 10 metres. The local village square probably still has the remnants of the wells, or a fountain that is no longer used or connected, perhaps. During the Great War and WWII, the villagers would hide weapons down them. You never know what might be lurking down yours.
But, if you don't have a well, then you can't just go digging one and extracting the water for your own use. This is France, and administration will have to be informed of what you intend to do. You'll need one of those now infamous CERFA forms. CERFA, the Centre d'enregistrement et de révision des formulaires administratifs, the Centre for the registering and revision of administrative forms, the beginning of administrative life as we know it in this country. There's one for everything and anything, even if you haven't imagined it yet.
The local Mairie has to be informed and there has to be that obligatory month before you can start the work, waiting to hear back if they disagree, or not. There's a special CERFA just for wells, in fact.
Building a new well can cost in the region of 2-9,000 euros depending on what the land is like. If you do want to know if there is a well in your garden or water running under it, then you could always look at the plans of the Cadastre. If not, and no we're not joking, get a water diviner in and do it with a stick or a pendulum held over the map or the land itself.
Any wells that are already existing before 31st December 2008 should have been declared to the Sate already, but as is often the case, it was not done. New legislation came in as from 1st January 2009, meaning that all new constructions had to be declared and authorised. Anything that is below 1,000m3 of water is considered as being for domestic use. Beyond that, it will be classified as professional, and may not be authorised.