A building is defined as "a structure with 2 or more outside rigid walls and a fully secured roof, that is affixed to a permanent site." So, if your structure meets the definition of a building, then flood rules would apply (including the rules for the detached structure exemption).
Next, you have to ask: is the farm shed used for personal, family or household purposes? If so, you could presumably use the detached structure exemption. If not - such as it being used for primarily agricultural, commercial, industrial, or other business purposes - then you could not use the exemption.
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Adam Witmer, CRCM
All statements are my opinion, not those of my employer, and should not be taken as legal advice.
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