You can't claim something unallowed; that's pretty much the definition of Unallowed = cannot Claim. And if there is no business, there is no business use or business expense. So, there is nothing to report and nothing to claim. Those are two reasons there is nothing to "report" anywhere.

And I doubt the space now is unused; it is likely still part of the house.

If that isn't the answer you want, you will need to provide more details.