The California historic code has a specific definition of what a qualifing building must be. Something like "recognized by any state, federal or local agency as historic, or in a historic district, on any inventory of historic buildings," etc.. For anyone doing historic barn reconstructions into residences it might be worth trying to get your barn recognized as historic before deconstruction if your state historic codes are similar to ours. Historic codes hardly give you free reign, in our area I would still have to justify any joinery with engineering and such, but do allow for more reasonable code interpretations than the strict "letter of the law" interpretations that most building dept's give. I could see it being particularly helpful in the old grade stamp difficulty.