If you financially support your partner (heterosexual or same-sex), you may be able to file a tax return as a single person and claim your partner as a dependent. To be able to do this legally, you must meet the following five tests.
Support. The supporting partner must provide at least 50% of the other partner's total support for the year. Support includes food, shelter, clothing, medical and dental care, education, entertainment, and just about any expense you can think of.
Citizen or resident. The supported person must be a U.S. citizen, resident alien, or citizen of Canada or Mexico.
Income. The supported person's taxable income cannot exceed $3,650. Nontaxable money, such as gifts, welfare benefits, and nontaxable Social Security benefits, don't count toward gross income.
Relationship. Under IRS regulations, a person who lived in your home for the entire year can be considered a dependent as long as the relationship does not violate local law. Our advice: If you meet the other four tests but may be violating the law in a state where fornication, cohabitation, or sodomy is still against the law, go ahead and claim your partner as a dependent anyway. Recent court decisions have made those laws questionable at best, and the worst that can happen is that the IRS won't allow your deduction and your tax bill will be recomputed without the deduction.
Unmarried person. If the supported person is married and files a joint tax return with a legal spouse, the supporting partner in this relationship cannot claim the supported person as a dependent. There's one exception: If the married couple did not earn enough to have to file a tax return and did so only to get a refund, the supporting partner can claim the dependent.
To learn more about laws affecting unmarried couples, get Living Together: A Legal Guide for Unmarried Couples , by Ralph Warner, Toni Ihara, and Frederick Hertz (Nolo).