For most of human history we built without plans. We just put wood together and worked by eye. For the official definition, Vernacular architecture uses locally available resources and traditions to address local needs and circumstances. So, why spend money on sheds that are too small and too expensive? Sheds are basic utility stores that can evolve and fit your whims.
The secret is to think about the large items (typically bicycles or ride on mowers) and ensure there is enough space to house them.
Build a couple of box frames of thick timbers for the walls and join them together to make the front and back. A pitched roof is the hardest part, but looks right and is less prone to leaks than a flat roof. Once a timber frame looks right, clad it in cheap wood. Old palettes are a great free resource, but go with whatever you have to hand. To cover the roof, felt is cheap and easy, ceramic roof tiles are excellent, thatch looks cool, wooden shingles are fantastic and a natural living roof of gets you maximum green points.
So, why spend money on a mass produced shed when you can design and build your own. Since you built it you can change it, so get fancy with fretwork, windows, extra levels and so on. Let your imagination run riot and your shed evolve into something truly amazing.