If home office remodel is deductible and you add a room to your house and move your existing home office to the new room can you deduct the full cost of addition?

How is it possible that the county destroys blueprints 90 days after the work is done? What’s stopping me from making an unpermitted modification (something that’s not visible from the outside) and claiming that’s how the things were as long as I don’t get caught during construction?