Subsidiarity is a principle of the EU which holds that the union should only act when its objective cannot be sufficiently achieved by individual member states acting alone. The principle of subsidiarity, much as the principle of conferral, safeguards member states’ rights, and governs the scope of the EU’s lawful authority. The principle ensures that Member States are allowed to make their own actions and decisions, but also authorizes the EU to intervene when a member state cannot sufficiently achieve an objective on their own, ‘by reason of the scale and effects of the proposed action.’ This principle is important because it keeps the power close to the people, and ensures that the EU does not override state power. Relevant sections of treaties concerning the principles of subsidiarity are Article 5(3) of the TEU and Protocol (No 2).
Foster, N. (2008). EU law (1st ed.). Oxford: Oxford University Press.