It varies by jurisdiction, and this is one of those gray areas that trips people up. Some townships classify portable buildings on skids as temporary or accessory structures that don't require a permit below a certain size. Others treat anything placed on a property the same as a permanent structure, regardless of whether it has a foundation.
The fact that a building is "portable" doesn't automatically exempt it from permits. Your township's definition is the one that matters, so check with them directly.
