The
bottom line is, we use the apartment like an office, and I don't know
if the IRS will look at it differently than an office, simply because
it's typically residential.
No one here could predict how the IRS would look at it but, based on your description, I would present it on your tax return as rent on business property, the same as if you were renting an office from a separate vendor. With a deduction such as that, nothing beats good records that are often other than financial records. I would keep a detailed contemporaneous log of each time the apartment is used and for what purpose.
