Even if you were legally married as of December 31, you are considered unmarried (and therefore eligible for Head of Household) if all 5 of these conditions apply:
- You won't be filing jointly with your spouse
- Your spouse didn't live in your home after June (temporary absences due to illness, school, vacation, business, or military service don't count)
- Your home was your child's, stepchild's, or foster child's main home for more than half the year (non-child dependents in your home don't qualify)
- You paid more than half the costs of keeping up your home during the tax year
- You meet the qualifications to claim the child as your dependent, even if the other (noncustodial) parent is actually claiming the child as a dependent on their return
You can also be considered unmarried for Head of Household if your spouse was a nonresident alien at any time during the tax year and you're not treating them as a resident alien.