If your dad and mom live together or have lived together at ANY time during the last 6 months of 2020 then he CANNOT file HOH.
A household can only have one head who is the taxpayer that paid n or than half the expenses for the entire household.
Head of Household is for UNMARRIED taxpayers with a related dependent or married and have not lived with their spouse at anytime during the last 6 months of the tax year AND has a child, stepchild or foster child that can be a dependent.
You may be able to file as head of household if you meet all the following requirements.
1. You are unmarried or “considered unmarried” on the last day of the year.
(You could be considered unmarried if your spouse did not live in your home at any time during the last 6 months of the tax year).
If you were considered married for part of the year and lived in a community property state, special rules may apply in determining your income and expenses. See Publication 555 for more information.
2. You paid more than half the cost of keeping up a home for the year.
3. A “qualifying person” lived with you in the home for more than half the year (except for temporary absences such as school) - a parent does not have to live with you to be a qualifying person.
4. If the qualifying person is your qualifying relative, their gross income must have been less than $4,300 (do not include non taxable Social Security) and you provided more than 1/2 of their support
5. You must be able to claim the dependent for the qualifying person except in the case of divorced or separated parents (that lived apart) and the noncustodial parent is claiming the dependent.
A Qualifying person is either:
A qualifying child or a qualifying closely related relative and meets certain other requirements, however if you are considered unmarried it can only be your child, stepchild, or foster child.
See IRS Publication 501 for more information about who is a qualifying person and a worksheet to determine the cost of keeping up a home.
See IRS Pub 501 for more information
are you all renting from a third party?
your own children qualify you and if you are a renter at market rents you can file HOH.
In any case, if you file HOH, you won't be challenged.
My step dad pay mortgage I pay him rent
Do you have a formal rental agreement? Is the rent a fair market amount that a like space would cost on the open market in your area?
Does he declare the rental income as a landlord on a Schedule E and pay tax on it?
If not, then it is not rent no matter what you want to call it - is is a simple sharing of expenses by a family member of the household.
The IRS and tax court has allowed two taxpayers to claim HOH under the same roof as long as they live as two totally separate living units. Separate food, separate living (sleeping) space, etc. I have not seen a case that was approved for closely related persons.
Then it is not sharing the same home - it is a separate home - and would probably be allowed if all the other HOH requirements are met.