Replacing doors, windows and shutters
The first consideration for doors, windows and shutters is that they need to be in character with the building, in terms of their style, and their shape and size. Study some of the other buildings of a similar age and style in your region, preferably those that have not yet been renovated, to determine the traditional style.
Adding new openings in an existing building
All new openings in walls for doors and windows require that a declaration of work be given to the local mairie.
If you are adding new openings to an existing building, so that doors and windows can be added, the big challenge is to add these new openings so that they appear natural for the building. With a project such as a barn conversion this is a significant problem, because the barns didn't originally have these openings, so who can say what is 'authentic'.
Look for clues. The age of the building is of course the biggest clue of all, and its style of construction. You will find that windows above a certain size were never added, or that windows were never symmetrical with each other.
Stone lintels were usually only used for openings up to a metre wide or so, with wood lintels being used for any larger openings. The need to have natural light in a building is seen as being important nowadays, but there was a good reason for having small windows - it helps keep the interior of the property cool in the summer and warm in the winter. Double-glazed windows and special types of glass have reduced this need, but not eliminated it completely.
If you are renovating a house in France remember that windows usually open inwards, and shutters open outwards, from inside the building. So if you put a sink or a bed just in front of a window, it will be difficult to open and close the shutters!
Likewise, a kitchen window behind a sink can not be opened if the tap is tall or the window is low (special low profile taps are available if you have this problem).
House insurance policies in France usually demand that shutters remain closed when the property is vacated and that all accessible windows should have shutters, so you cannot decide to just leave the shutters open. Some insurance policies demand that a shutter is closed if it exists, but no problem if there is not a shutter at all!
You can buy attachments for shutters that make them easier to open and close without dangling yourself from the window, but these may not be beautiful and will not always solve the problem. More recent and contemporary houses often have electronic shutters that roll down from the top of the window, but these would not look good on most older houses.
One frequent problem we had was getting the window and door openings to be the size shown on the plans. Working with stone, the mason claimed it was impossible to be so exact. So instead of being able to buy 'standard size' doors and windows, we had to pay extra for 'custom made' fittings.
Although they are very close in size to the plans, a couple of centimetres difference adds significantly to the cost.
I heard of someone who demanded that all their windows be remade by the builder so that standard size windows would fit, but I don't know if that was for a new house. I doubt if our mason would (or could) redo the openings with two centimetres difference in size without making the alteration look worse than the original.
Each region of France has its own style of shutters, doors and windows, unique to the region, town or department, so when you buy shutters in the local discount DIY superstore you should be sure that you are buying the correct style: look at some other properties in the region before you buy.
Note that shutters were not traditionally painted in bright colours - again, there will be a local tradition to guide you. Some regions and areas will have colour charts and other guidelines about colours that they have decided are acceptable - our own local village was classified as one of the 'most beautiful villages in France a few years ago and the rules become much more strict as a result.
Similarly, you can look through the brochures at alumnium and pvc windows but might be told that your commune only allows wood frmed windows - check with the maire before orderng lots of expensive doors and windows.
Many new windows are supplied or available with little bars that divide the panes of glass into squares. I have been told that these little panes did not existed on buildings constructed before the 19th century, so they are an example of something that looks quaint and original but is not.
It is the same situation with shutters: a 17th century house was not usually designed to have shutters fitted, but nowadays almost every historic house in France has shutters.
So the cute houses and villages that you see as you travel around France might look very picturesque and traditional, but would perhaps be unfamiliar to a visitor from the 16th centuy!