In many instances you can build a garden room without the need to apply for Planning Permission, but you do need to comply with the Permitted Development rules.
These rules dictate where you can position different shape buildings and the maximum height different elements of the building can be.
Our sister site the Garden Office Guide has put together a short video that looks at these rules.
This video is intended as a useful visual guide, but is not intended as the definitive source, you will find that here.
The Garden Office Guide is a specialist website for garden office buyers.