Garden office attached to the house

The best answer for this will come from your local planning people. They will give you accurate and honest advice, usually for free and as they are the ones who 'police' this work, it will be they who give the best possible answer. You might even be able to do this by simply emailing them with a hand drawn sketch and detailed explanation of what you want to do and what materials you want to use. So long as it is understandable, you don't need to be an artist, so a hand drawn sketch is fine. If it turns out that you need proper plans, (unlikely, considering the size, but it depends on your local planners) you can often find low cost architects or do a more detailed and to scale design for them. But they will give you all of the information you need.