Garden Buildings are considered Permitted Development and do NOT require planning permission as long as:

The garden building is less than 2m from a boundary. The building is to be placed less than 2.0m from the boundary of the property with a maximum overall height of no more than 2.5m from existing ground level
It is not to be used as self-contained living accommodation or have an antenna*
No more than 50% of area of land around a property can be taken up with garden buildings
If your garden building does NOT meet the above criteria, e.g. if you want a garden building taller than 2.5m and situated less than 2.0m from a boundary of your property, then you would NEED to apply for planning permission
If, however, your garden building DOES comply with the above criteria, you won’t need to apply for planning permission and you can have a garden building up to 30m2 (internal size). Anything bigger than 30m2 will need to go through building regulations