here in Ontario, an unfinished allowance would apply to any unfinished areas of a residential structure, but the allowances aren't left on forever. the assumption is that the area will eventually be finished. if it is above grade, it counts towards the square footage of the home. unfinished basements are assessed as well, just assessed higher when they are finished.
you really need to talk to whoever does the property assessment there. i have no idea how it works in the US. also, i do commercial / industrial now but i did residential and farm for a long time. i could only provide you local regulations and anecdotes which would be of no help to you.
good luck... i own property too and my taxes are rediculous