If you are legally married, your choices are to file as married filing jointly, married filing separately, or head of household (if you meet the other qualifications).
The IRS says you can file single only if you are "Legally separated from your spouse under
a divorce or separate maintenance decree."
Unmarried persons. You are considered un-
married for the whole year if, on the last day of
your tax year, you are either:
• Unmarried, or
• Legally separated from your spouse under
a divorce or separate maintenance decree.
State law governs whether you are married
or legally separated under a divorce or separate
maintenance decree.
The problem here is that I have never heard of a separation that actually meets the qualification to file as single. A while ago, I. reviewed a couple of Tax Court cases from New York and New Jersey. The bottom line was that, to qualify as unmarried, the legal separation must say things like "the spouses can never get back together again." New York does have this kind of legal separation on the books, but no one uses it. It's a holdover from earlier times when divorces were not allowed by certain religions. It is a divorce in everything except the name and for the fact that the parties can't remarry. Because of this limitation, no one in New York actually uses this kind of separation any more. "Regular" legal separations, even with court supervision and alimony, allow the couple to reconcile, and my recollection is that was one of the key reasons the Tax Court ruled that the parties were not "unmarried."
I can't address your specific situation or your state laws, or the exact form of your separation. Your tax status is governed by where you live (where your permanent residence is located, not just a temporary home) and your spouse's tax status is governed by the state where your spouse's permanent residence is located. If your state considers you unmarried but your spouse's state considers them married, then that's what each of you is required to do. But the key fact is that the state where you live must recognize you as not merely separated, but separated according to the strict IRS definition. Only a tax specialist or attorney in your state can tell you if your state laws qualify.
