It is a music studio fully insulated with acoustic treatment. It also has under storage for garden tools, bikes and logs.
I feel its a bit special because its a labour of love. I have no formal training in building apart from a job when I was 16 for 6 months during the holidays working with a carpenter. I learnt everything step by step on Youtube (following most of @ColinFurze guidance but adding more) and did everything apart from the electrics.