Certain classes of aircraft are completely excluded from the regulation such as model aircraft and hang gliders; see Article 2 of the EC Regulation for details. However, aircraft with a maximum take-off mass of less than 500kg, which are not used for commercial purposes or are used for local flight instruction which does not entail the crossing of international borders, are only excluded from the regulation in respect of the insurance obligations relating to war risk and terrorism. This means that aircraft in this category do need to have minimum levels of cover in respect of third party and passenger liability.
To check the minimum requirements for an individual aircraft please see the estimate on the G-INFO site.