Exempted development is development for which planning permission is not required. Categories of exempted development are set out in planning law. Relevant exemptions in relation to domestic developments are outlined in the leaflets.
There are usually certain thresholds relating to, for example, size or height. Where these thresholds are exceeded, the exemptions no longer apply. The purpose of exemption is to avoid controls on developments of a minor nature, such as small extensions to houses.
If you are unsure if your development requires planning permission, you can submit an Application for Declaration of Exempted Development under Section 5 of the Planning & Development Act 2000.