Hi Mate,
Congrats on the New pad. I wouldn't use OSB for the floor its like cardboard when it gets wet, I would get a decent 3/4 ply. I would get some damp proof membrane to cover the footings with, this will help protect it a bit from the weather.
Make sure you start out with level ground as it will be a nightmare if not.
you can build to 4M according to this
https://www.mygov.scot/build-shed-garage-greenhouse/
Not too sure of how up to date that is.