Not all extensions require planning permission. 

There are certain rights called "Permitted Development Rights" that enable you to carry out certain developments on your property without the need for planning permission.

They are mainly centred around the dimensions of the proposal, its position relative to existing structures, and its proximity to boundaries. 

These guidelines can get complicated sometimes, so you might want to get a Lawful Development Certificate issued by the planning department to ensure that your proposed development is agreed upon.

This, however, is not a requirement and you can skip it if you’re sure about your project, but we urge you to check with a professional before moving ahead, as building without planning permission when it is required can lead to enforcement action.

Whether you require a planning application or not, you almost always need to get building regulations approval prior to construction. Hence, that would involve getting some plans drawn up.