The definition of a folly (as used in terms of architecture) is a costly ornamental building with no practical purpose. Once, only royalty and the very wealthy could afford such frivolous structures.
Those structures that resemble an ancient ruin, far-away castle, Asian pagoda, or perhaps, even a fantasy structure without a point of architectural reference certainly qualify as follies in today’s garden.
Garden sheds, guest and pool houses, and other utilitarian structures are not really included in this definition but I like to think of the more architecturally advanced designs in these categories as personal follies. A growing number of homeowners are pouring the finest materials and amenities into these structures, creating outbuildings that actually rival their house.
Perhaps storing garden tools is not folly but at what point does adequate become folly—as in, holy cow! You spent how much for that shed?
I love my pets as much as the next person, but chickens in such nice digs? Do their eggs taste better or is owning chickens more palatable when you have a nice structure to look at?
Adaptively using cast off natural materials to create a folly is quite ingenious and artistic.
With vision, and time, your folly may become one with the landscape.
A carefully-crafted and well-executed tree house could be classified as a folly.
A pergola or gazebo could qualify as a folly if it is outrageous enough.
I have a shed in my yard. Unfortunately, no one has ever even whispered the word folly in association with it. But if I close my eyes, I envision so much more and live happily with that. You see, the thing about dreaming and not doing is that you don’t have to commit to one style. Great joy comes in the pastime of considering what something could be.