Almost never. The IRS says that you can be “considered unmarried“ if you are separated under a decree of separate maintenance according to state law. The problem is that I have never encountered a state law that actually met the IRS the definition where a separation could be considered unmarried**. For a final determination, you would have to ask a tax attorney in your state. You could file as head of household if you paid more than half the cost of your home for the year and you provided care in that home for a qualifying person, usually a child dependent.
** some time ago, I researched this for New York and New Jersey. In those states, the tax court has ruled that legal separation is not the same as being considered unmarried, because even if a separation is under a judge’s supervision, the couple can get back together at any time, and they don’t have all the rights and freedoms of truly unmarried people. New York does have a form of legal separation on the law books that does meet the IRS requirement, but no one ever uses it anymore. It seems to be a holdover from historic times when divorce was difficult to obtain and was socially and religiously unacceptable. The law specifies that even though the couple is not divorced, they have no legal ties and no legal obligations toward each other, are not allowed to interfere in each others lives, and so on. Because this is the same as a divorce except that you can’t get remarried, nobody chooses to be separated under this particular law any more. And typical legal separation as now used in New York does not qualify for the tax treatment of being “unmarried“ so that you could file single. Your state may be different, of course.
