Many garden rooms are built tight to the fence to maximise space. This is possible under Permitted Development if your room is no higher than 2.5m.
The thing is that building tight to a fence leaves little room for fixing the external cladding. Modular garden rooms that are built off-site will come in small panels (max 1200mm wide) and already have the cladding fitted, so it is just a case of slotting them into position.
When building from scratch on-site, you will probably be building in larger wall sections, so will need to consider how you will fix the sheathing, breather membrane, battens and cladding and be able to lift it into position correctly. After all, combined this will be quite a weight! You also need to think about the corner intersection with the rear wall, considering you can only access these walls from one side.
Garden Fortress build their garden rooms from scratch on-site, so it is a similar build process as you might be undertaking yourself.
Take a look at the video: