If you want to change the external appearance of your French property you will need an outline planning permission or Certificat d'Urbanisme Positif. The application can be made by either the Notary or a Land Surveyor (Geomètre), and is lodged at the Mairie (Town Hall), for consideration by the Mayor and the town or village council.
Despite the Mayor's authority the application is then passed up to the departmental planning office or Direction Departmentale de l'Equipement. Given their final approval the mayor issues the required certificate. Matters may be complicated if your property is close to, say, a listed historical monument in which case the application will also be considered by l'Architecte des Batiments de France.
These certificates are valid for one year from the date of issue and are automatically renewable for a further year, although the extension has to be applied for. The actual building permission, then, has to be applied for within 24 months of the date of issue of the certificate.
Given that the net surface area to be built upon does not exceed 170 m2 you do not need to call upon the services of an architect to make the application for building permission. Net in this case means the area of the floors excluding those parts not fit for residential use, terraces and balconies. Agricultural holdings, farms and buildings are covered by different regulations.
The town hall then displays your application and, within 15 days writes to you giving the date by which you can expect to receive a decision and stating that if you receive no reply by that date you can assume that you have tacit approval. This process should normally take two months.
Given that you obtain approval, tacit or written, you have two years to carry out the works approved. A 12 month extension can be obtained upon application.
When you finally get started you must advise the authorities that you have done so, in triplicate, and you must post a sign giving the details of your authorisation. Should you wish to extend an existing construction no authorisation is required if the area concerned is less than 20 m2. Nor does an application have to be made for an open-air swimming pool.
This should only be regarded as a brief explanation of the very complex system that is the French Planning Law. Details for your own particular circumstances should be obtained from a Notaire or Geomètre.