Planting can sometimes be like waving a sabre in front of your neighbour if the correct distances are not respected! Here are a few points to make sure that you do not make a mistake, remember that it is always better to discuss things than to battle over the garden fence...
The basic rules
The law says that if there is local precedent then this will always take priority over written legislation. Thus in a locality it may be acceptable to plant your hedge right on the boundary line whilst elsewhere it is normal to only plant a low hedge in these circumstances. Only your local town hall can tell you if such practices are present in your community. To be accepted these practices must be constant meaning that they are still used. If a local practice is no longer used then the legislation applies.
In the absence of any other local practice a hedge must be planted at least 50 cm from the boundary line (it is the boundary marking between properties that is important). In this case, the hedge must not exceed 2 metres in height. If you want a hedge that is higher than 2 metres then it must be planted at least 2 metres from the boundary.
An exception is made for terraced or attached housing. In this case, it is the specific rules relating to that housing and planting that override the law. In other words if a private rule contradicts the law then it is the rule and not the law that is applied.
The law is no botanist!
The law (and the courts!) makes no distinction between what is a tree (a thuja, for example) and a “grass” in the botanical sense. Palm trees, bamboos and cordylines are in fact « grasses » in botanical terms. Therefore, if a bamboo grows higher than 2 metres it can be breaking the law if it is planted too close to the boundary. Likewise, a climbing plant that is grown on a high structure too close to the boundary could also be breaking the law.
Be careful! In Belgium, the rules are slightly different and it is the type of plant grown that decides whether there is an infraction or not. The law recognizes the difference between short stemmed trees, which will not exceed 3 metres in height (low shrubs like the spiraea), and those with tall stems (those that will exceed 3 m when adult, like the poplar). If it is a tree that is classed as long stemmed then it is not even worth cutting the top of because it will still break the law (whilst in France such an action would make the tree conform to the law)