Head of Household with nondependent, non-married parents. If mom claims child as dependent and files as single. Can I add child as nondependent and file as HOH?

Question, are you both trying to claim the same child?  If you are, then you cannot claim the same child to justify Head of Household filing status.  Here are the requirements to file as Head of Household:

• Pay for more than half of the household expenses
• Be considered unmarried on the last day of the tax year, and
• You must have a qualifying child or dependent.

This tax filing status includes single parents and divorced or legally separated parents with custody of the child. Further, you can also be an adult who provides support for a parent or other relative under qualifying circumstances.

Here is a link to a TurboTax article that provides additional information regarding Head of Household filing status.

Guide to Filing Taxes as Head of Household

@L_I_P

If I understand your question, no.

Only one person may claim someone else as a dependent.  If you are living together unmarried with the other parent, only one parent can claim the child as a dependent and that parent gets all the tax benefits.  They should answer "no" to the question about a custody agreement because that only relates to a legal agreement between parents who live apart.  The other parent should not even list the child on their return, to prevent problems.  The rules for splitting the benefits of a dependent (which creates the label "nondependent" in the program) only apply to parents who are divorced or separated and living apart and do not apply to unmarried parents living together.

So basically, if the parent who pays more than half the expense claims the child, they probably qualify to also file as HOH, and the other parent files single with no dependents.  If the parent who pays less than half the expense claims the child, they file single, and the parent who pays more than half also files single because they have no dependent to qualify for HOH.