Virtually all building works, from new-builds to renovation, require some sort of planning permission in France. Securing the necessary permissions need not be an obstacle course provided you consult the relevant planning bodies as early as possible and follow their advice.
As a first step, when buying land, you need to assure yourself that it can be built on. Agricultural land may be designated for that use only, while a plot of land within a new housing development ('lotissement') can clearly be built on, subject to final approval of the proposed structure. Home land zoned for building may be earmarked for a much needed road or public building, and not necessarily be available for housing.
By visiting your local Mairie, you can consult and copy maps and plans, and find out what land has outline planning permission or a 'certifcat d'urbanisme'. This means in theory that the land can be built on, but this is not guaranteed, and will depend on a detailed planning application being approved, and a building permit eventually being issued.
In order to ensure the success of your planning application, you can consult the area planning department (D D E), where you can talk to an official who deals specifically with your town or village, and knows the locality well. For larger projects (above 170 m²) an architect is required - and is advisable in virtually all cases (see below) - to draw up and submit plans. At this stage, your first ideas can be discussed with the D D E and if necessary altered, and their advice is invariably accurate and helpful.
Every building plot has a designated S H O N which is the maximum building surface area allowed in relation to the size of the plot. You cannot expect to plant a huge building on a tiny plot, and there will also be restrictions on height, number of storeys, proximity to the street and neighbouring properties. Further regulations concern size and position of windows (in relation to neighbouring property), balconies and terraces, swimming pools and septic tanks.
If your building project comes within a conservation area or is close to a historic monument, then the advice and permission of an organisation known as 'Batiments de France' (equivalent to English Heritage) have to be sought. Again this is best done by an experienced architect at the earliest stage, as he will be familiar with what is likely to be approved and what is not. There may be quite detailed regulations concerning types of materials that can be used, roof tiles, windows and doors, building height, slope of the roof etc.
Some smaller works such as building extensions and modernisation may only require a 'déclaration des travaux' (notice of works) rather than a request for a building permit, but it is always advisable to ask first to avoid any costly mistakes later, such as an order to dismantle the structure and start again.
The building permit ('permis de construire') is usually granted for two years and has to be renewed if you have not started the works by then. Some local authorities are more assiduous than others in checking building progress and ensuring that a building is being built according to plan. They will issue a certificate of completion at the end of the project. Some one-off taxes are payable on completion, and there will be charges for connexions to the electrical and gas services, water, sanitation and telephone.
If you live in a co-ownership building, such as a block of apartments, many alterations such as closing-in a balcony or combining two apartments to make one, also require the permission of the co-owners and the management company (known as the 'syndic'). Some syndics are more strict than others but all have rules about external painting, colour of blinds or shutters, and maintenance of the common parts (stairwells, corridors, lift, outside grounds, parking etc).
For new-builds, cost savings can be made by using an architect rather than buying your home from a developer's catalogue, and you will end up with a house that is purposely designed for you and your family. Architects can also make best use of unusual or awkward sites - such as a sloping surface - and some I have encountered have succesfully designed a three or four bedroom house on a plot of just 200m².
It goes without saying that building work shuld be carried out by properly qualified artisans who carry correct insurance and provide a 10-year guarantee. There is also a trend towards self-build, under an architect's supervision. Which ever route you choose, you need to consider the eventual attractiveness of the property to a potential buyer, should you decide to sell at some future date.