A must for anyone considering buying a garden room, is a visit to the governments Planning Portal, they have a useful tool for indentifying if your garden room needs planning permission, or falls within the householders permitted development rights.
The tool shows various different scenarios in 3D, for instance if you live in a listed building the tool clearly shows that outbuildings don’t fall within permitted development rules so planning permission will be required.
An easier format to understand than a list of rules, you can access the tool by clicking here.