Hi, and welcome to the forum!

When you are considering the purchase of a plot of land, the property listing should give you some indication regarding what part of the land is buildable. For details, go to the local mairie (town hall) where you should be able to consult the cadastre records for that plot and determine how that land is "zoned" under the current planning scheme. That will determine to a large extent what and where you can build.

The building rules can be pretty elaborate (and often down right aggravating), as they govern how windows can be placed (nothing looking directly into a neighbor's windows), height of the building and even what sort of tiles you are allowed to use on the roof. Some building companies have a catalog of the ready-made plans and styles like what you are looking for, otherwise part of an architect's job is to design you something that meets the local regulations (not always an easy task).

Getting building permission can be a nightmare - which is no doubt why you hear so many stories of people doing clandestine remodeling projects. Like most things administrative, it can pay to put in some time up front with the local town hall folks, trying to understand the peculiarities of the land use regulations. Ultimately, it's the departement that approves (or not) your plans, but having the good will of the local mayor and his fonctionnaires is always helpful!