My new shed is 5m wide, 6m long, and I will have to re-weld the steel
trusses to get it under 2.5m high. It's a repurposed concrete prefab garage.

It's going to be 750mm from a boundary.

I think it's OK from Planning's perspective; taking into account size,
position and fraction of ground used, it should be "permitted development".

However, is there anything buildings regulations wise I should look at?
Inside is just less than 30 sq metres, it's going to be on a concrete
slab, and it will have a tin roof over Stirling board.

I wil be running electrickery to it some time.