I disagree with @AamilD as you can't "deduct" this as an "expense". What you have here is classified as a property improvement. It get's entered as an asset in the Business Assets section and depreciated over 40 years. If this bathroom is only accessible from your home office, then there's no real issue I can see with claiming this property improvement as 100% business use.
Just be aware that while you are required to depreciate business property, depreciation is not a permanent deduction. You will be required to recapture and pay taxes on the total depreciation taken on business assets when you sell or otherwise dispose of the property. Two things to keep in mind about depreciation recapture.
1) Recaptured depreciation is included in your AGI in the year of recapture.
2) The increase of your AGI from that recapture has the potential to bump you into the next higher tax bracket.
One good thing is that if your tax bracket is above the 22% bracket, then recaptured depreciation is recaptured and taxed as ordinary income anywhere from 0% to a maximum of 25%, even if your AGI puts you in a higher bracket.
