In my opinion, a taxpayer is entitled to any deduction
that he/she can prove with credible evidence. IRS
field auditors are mostly interested in documentary and obvious visible evidence. If the auditor thinks something is amiss, then he/she will ask to observe the home office. And, if the taxpayer does not consent, the auditor will disallow the deduction for lack of evidence.
The point is that if an auditor were to come to your home, and it was obvious that 30% of the total square footage of the interior property
is exclusively used as your principal place of business, because it is set up for office rather than personal
use, then you can take a 30% deduction. Otherwise, you cannot, because if you get caught, your deduction will be disallowed -- and, if the same office was used in prior years, you could find yourself being audited for those prior years, as well.
Hope this helps.