I have the same question. We are legally separated under a DC court order. My ex-wife is neither US resident nor US taxpayer and has returned to her (non US) country of origin
@funckee - you should ask your attorney if it meets the IRS legal requirements that require a separate maintenance decree (similar to alimony but without a divorce.)
The IRS is not the issue. They recognize what state laws provide for. The question is a matter of NY state and DC law.
...more specifically whether NY state recognizes a DC separation maintenance decree as ground for filing as single. The separate question that does concern the IRS is whether you can file as single, if your legally separated spouse is not subject to US tax laws, because she is a foreign citizen, living in her country of citizenship and paying taxes in that country, not in the US.
That is a legal question best asked to an attorney that practicals in that state. Nobody here can give legal advice over the internet. It would need legal research and knowing exactly what the court order says. No attorney would give advice without first reading the documents you have and the laws of the stated involved.
According to page 6 of Pub 501 under the heading, "marital status", if you are legally separated from your spouse under a divorce or separate maintenance decree as of December 31, 2016, you are considered unmarried for the whole year. It goes on to say that "state law governs whether you are married or legally separated under a divorce or separate maintenance decree".
https://www.irs.gov/pub/irs-pdf/p501.pdf
So if you are considered legally separated under your state law, then you can file as either single or head of household. There are specific requirements that must be met in order to file as head of household. Please refer to the following FAQ to learn more about these requirements. https://ttlc.intuit.com/replies/3288637