Devonwoody,
I am faced with a similar problem. I recently acquired 12 mahogany boards 12ft long by 12"X2". My wood store is full already.
I am thinking about building a woodstore in the garden as a lean to to my shed. Idea is to mount this on some sleepers to give me 8inches off the soil/gravel. It would be about 5 ft wide, 8 ft tall and 14ft long with a sloped and felted roof. Double doors at front ( 4ft wide) and a double door in the side for access ( 6ft wide).
Materials would be 3 inch frame also forming the racks or rack attachment points. Ply outer cover with vapour barrier and clad somehow..maybe shiplap . Not sure how yet. Roof would initially be felt as this is quite cheap.
I guess my concerns would be damp, but if properly constructed should be dryish. Capacity would be about 330 cu ft with air gaps around the timber.
I even considered a plastic roof to allow some solar gain to dry the wood but am concerned that I hav'nt got a clue as to what I would be doing so its probably a bad idea.
Sorry to ramble on. I got the idea from an old NYW episode where te bearded master has a much larger lean to attached to his workshop.
regards
Alan