fiance and I bought a home in 2020.
Are both of your names on the Mortgage? (I assume yes)
Are both of your names on the deed? (I assume yes)
We are splitting expenses evenly in practice, but all the actual payments to the bank come from her account.
In order to claim the mortgage interest and/or property taxes, there are two basic criteria that must be met by the person claiming those expenses.
1) You must be legally obligated to pay it. (You are if my above assumptions are correct)
2) You must have actually paid it. (and be able to prove it if audited.)
So by all the payments coming from her account, technically she is paying it all and you are not paying a penny. (This would not matter *IF* you two were legally married.) Therefore, each of you individually should pay your equal share of those expenses.
Now for tax purposes if one has significantly higher income than the other, it "might" make more sense to let the one with the higher income be able to show they made all the payments. But it only helps tax-wise if the total of all of the itemized deductions exceed the standard deduction. (mortgage interest and property taxes are only "some" of your possible itemized deductions.)
For a single person under the age of 65, the standard deduction is $12,400 for 2020. So when you add up the total mortgage interest and property taxes actually paid in 2020, and add that total to your other itemized deductions (if you have any) and that total does not exceed $12,400, then it's of absolutely no tax benefit and will not make a penny of difference to your tax liability.
I also make more money than her, and my state/local tax deduction will be close to maxing out from just my wages. We will be filing separately as we are not legally married.
Question - will it be suspicious if she takes the entirety of the property tax deduction from our current home on her return (as I can't take anything), if I also take a portion of the mortgage interest? Or would it be best if she took everything?
