Post some photos of the proposed installation site - it'll be much easier to advise once we can see what you're dealing with. You'll probably need to go 600mm above the ridge but it depends on where the flue penetrates the roof
Perhaps I should have made it 100% clear that it would have to be a bespoke ( or specialist ) stove and not just hinted at it with this illustration of stoves designe for a descending flue.
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Once you've decided on the wood burner you want, you can check to see how far it needs to be from the rear wall. Some burners are okay at 300mm, but a lot of cheap ones need 450mm, and this would mean a 45degree bend in the flue to move it away from the wall. You'll need to construct a hearth, and a heat proof glass base on top might help.
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