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Plastic double skinned shed. Lifetime sheds. After years of pondering the problem of a shed surrounded on 3 sides by damp walls I plumped for one and it's been brilliant, doesn't suffer from condensation due to the double walls and easy enough to install. Was going to go the whole self build route with brick walls to a height and a proper roof but it's still difficult to avoid condensation.
You'd need to be careful of the gap filling with debris, and the plants holding moisture against the shed & fence - over time the shed & the fence will rot otherwise.