I have a pole barn about 40m x 12m and it had a lean to standing about 4' higher up a slope with a retaining block wall below - until the lean to roof collapsed under the snow
Insurance finally came to inspect the damage last Thursday and declared it wouldn't be covered due to "faulty design" and the existence of some damp beams in the original building so I had to withdraw the claim in order not to lose the no claims on my buildings policy
So watch your "design" if you're not going with a commercial company - mine is constructed of tele poles, all of which are still standing, the original area has asbestos sheets and the lean to was metal sheets but not corrugated, more a slight lift to them, not sure if it's box profile or something else
Those sheets are too bent to reuse and I've only a dozen left so am starting the roof from scratch when I can afford materials
Here I believe we only need to inform council of plans for agri buildings up to a certain size, not planning permission. But if it was stables it would need full permission process. Good thing I don't like stables then
I'm thinking of extending the retaining wall to the rest of the original lower barn as there are just cattle gates on at the mo and then possibly yorkshire boarding up the west gable end to improve windbreak but I'm not sure yet, could be one of those things that stays on the to do list a long time