Mine would be a pre-existing stone or brick ancient barn, with ventilation slits (arrow slits) and a lovely pantile or slate roof.
Ha - this is exactly the barn we had when I was a child and I loved it. It had a high roof so it fitted a mill and hoppers and you could climb up to sit on the beams. You could also, if you were careful not to dislodge the rooftiles, climb up the outside of the roof and sit on the ridge, where no-one could get you (in the days before vertigo hit).
My brother has since converted the barn to a house
It would need a concrete floor, easily washed down/pressure washed for when animals were in, or sweepable for feed sacks and hay/straw. Double doors for access (ours had double doors on both sides, plus direct access to animal pens built in to one side). Ours also had cute little niches built into the thickness of the walls, maybe for lanterns?
Our current barn cum workshop is stonebuilt but is a row of 2 converted houses and a cottage. The roofs were taken off in the 50s and replaced with corrugated tin, which my husband has lined to prevent condensation. Apart from the roof, it is quite attractive, even though the windows are blocked off and it's only a single storey, but nothing like an old East Anglian barn. We also have a couple of storage barns, one of the roundy-roofed ubiquitous post war corrugated tin sheds for hay and an open-fronted pole barn for implements. There is a larger pole barn under construction for my wool crafts esp weaving at one end, a fleece and vehicle store at the other with what looks horribly like space for a microlight
I wonder when a shed becomes a barn, and when a barn is just a shed?