Usually, no. You can build a structure in your garden without planning permission providing it is less than 2.5m eaves height and less than 15 square metres footprint. You can have a garden room up to 30 square metres if it is more than 1m away from the boundary.
It is always worth checking with your local council if you are unsure, as every local area has their own regulations. We can help guide you through the process if you have a larger project which does require planning.