If I may ...
OMG! Really! It is a shed! An easily transportable shed that is placed on leveling cinder-blocks. Do you get a permit to place a camping trailer on your yard? Even for one of those huge fifth wheel travel trailers? The kind you could live in year round? No, you do not. Same with a shed. Un-permitted structures you are referring to is the kind that is built onsite, attached to the ground via a footer/foundation. Once it becomes "real estate" or a "real estate improvement", then you are kinda asking for trouble.
Real Estate: Land and anything fixed, immovable, or permanently attached to it such as appurtenances, buildings, fences, fixtures, improvements, roads, shrubs and trees (but not growing crops), sewers, structures, utility systems, and walls.