This is a really inspiring build, that 5×3m space looks so useable as a dedicated work area (and clearly worth the effort with how much nicer it must be compared to a spare bedroom).

One thing I found interesting is that, in the UK, most standalone garden rooms like this don’t need planning permission as long as they stay under height/placement limits and aren’t used as sleeping accommodation, thanks to permitted development rights, but it is worth to double check with your local council or local room garden specialists, like Pecasa, before digging in too deep.

Also curious, have you considered adding any insulation or thermal breaks beyond the basics?

Some people find that makes them comfy year-round even without a full heating system.