France's planning rules - sound pretty straightforward
3.1.2. Planning Rules
This part of the plan will set out the general planning rules that will apply within each development area. In particular, it will set out rules on change of use, permitted height of buildings, building arrangement, any architectural requirements, and public utility services and requirements.
It will also set out the maximum permitted density of development on a site for new and existing buildings. These rules are called the coefficient d’occupation des sols (COS).Using these rules it is possible to calculate the density of permitted development on a particular site, e.g. on a 800m² site a COS of 0,25 allows construction of 200m² of net surface area, called surface hors oeuvre nette (SHON).
If the property is located on a lotissement (housing development) then, in addition to the general planning rules, there are specific rules that apply to the lotissement, which are set out in a cahier de charges.
If you are considering buying such a property then it is important you read and understand these rules as they govern the management of the whole development.