Man caves and planning permission

[mattvanders]

I’m looking into getting a man cave/outbuilding installed at the end of the garden so have been looking into the planning permission side of this at the same time. I’m looking at building something just smaller than 30m2 and 2.5m high to get around planning permission but then I also saw the clause of requiring to be at least 1m from all fence boundaries or be made of non combustible material. I’m looking at placing the building 0.5m from one of the neighbours fence so my question is what counts as non combustible materials? I mean does it refer to steel sheet cladded or just non wood materials? If anyone has had experience of this or advice that would be very helpful