In most cases, a building under 2.5m in height can be built without the need for planning permission. This means that you can build a small outbuilding, a garden shed, or a similar structure without having to go through the formal planning process. However, there are still regulations that you need to follow, such as building regulations. You should also make sure that your intended structure is not located in a special designated area, such as a conservation area or national park, as different regulations may apply. It is important to check with your local council to ensure that your proposed development is allowed under permitted development rights before starting any building work, as failure to do so could result in costly penalties and fines.