Pretty much every region is going to be different. Here there would be setback requirements, and probably other limits if the interior was enclosed vs. a carport like arrangement.
The tax assessor has quite a bit of latitude in setting the value for a house. Unless one is in a subdivision of cookie cutter buildings, or the assessment states something about the outbuilding, it is hard to determine whether the assessment has 10-20k tacked on for a 'temporary' building.
I have a house with a deck that is not attached at all to the house. Technically, the deck is "temporary" (the reason for detachment is to reduce thermal bridging, not to avoid taxes). The house is full custom and unless the assessment stated something about the deck, I'd be SOL on that argument. Since the town is fairly easy to deal with, and the deck would sell with the house anyway (i.e., increasing the property value) it is reasonable to have some value added to the assessment...