As a resident of a very (and many would say 'overly') regulated country I have to say how lucky you are to just be able to build something like this without constant bureaucratic scrutiny, before, during and after it's built.
Here in New Zealand, regardless of whether it's a non-residential structure or not, full plans, together with engineering detail would need to be submitted and approved before the first sod was even turned.
And, just recently it's become even more draconian, in that, even with a 'permit' to embark on a structure it cannot be built with the active participation of a fully-qualified tradesman.
So, you don't know how lucky you are.
The only downside of your more liberal conditions, that I can see, is if a building like this isn't scrutinised by some authority, what happens if it has a serious structural weakness and, say, the loft collapses and kills somebody - what happens then?
Please don't take this as any criticism. I have my own barn, reasonably similar in design to the one you are building. (Pic attached. Sorry that it's only the rear view).
I did build it myself (about twenty years ago) - and they watched me like hawks every step of the way.
Best of luck.
Ed