Please, please , please listen folks,
Whilst France has a set of national planning regulations( which regularly change without notice ) some Mairies are still operating on the ' c'est comme ca ' mode. This is quickly dying out as younger maires are being elected who are very aware of how they can raise money for their communes and what their liabilities are - so beware.
The actual regulations state that ANY structure which is bigger than 2 square metres requires either a déclaration préalable or a permis de construire.
When people joyfully state that you do not need permission for a shed under 20m2 what they are actually meaning is that you do not need a permis de construire. You will still need to submit a déclaration préalable informing the commune of the works that you are intending to carry out the same way you would if you were intending to install rooflights or put a porch on the front of your house !
Please be aware that your garage/shed area footprint( emprise au sol) or floor area,( surface de plancher) , whichever is the larger, ( measured from a line dropped vertically from the edge of the roof overhang in all directions ) is now added to the footprint/floor area of your house and other annexes. If the sum of these is over 170 m2 then no matter that the garage/shed is under 20m2 the situation will change from a simple déclaration préalable to a permis de construire and you will have to have a member of the ordre des architectes to submit your application.
The only thing on wheels that you are allowed to store on your property permenantly ( other than cars,motorcycles etc ) is a caravan that you use for your holidays which moves for periods of the year. Mobile homes are only allowed on sites that are specifically registered for them although some mairies may turn a blind eye to one being lived in during a renovation project.
If you want to see a list of what permissions you need for what proposal please go to www.brittanyarchitecturalservices.fr and look in the planning section. We have been dealing with the vaguries of the french planning system since 1985. If you have a question please email us or give us a ring - it is much easier to help you do the right thing than to fish you out of a hole afterwards - advice is free !