You and your partner live together and do everything together. With your lives so closely intertwined, filing your taxes together would seem logical, but the laws of your state, as well as your own intents and purposes, govern whether you can do so. If you intend to eventually marry, common law marriage laws in your state, if applicable, could allow you to file jointly. Absent any intention to marry, however, even common law won't help and you'll have to file solo.

Common Law Primer

Only a dozen or so states and the District of Columbia recognize common law marriage as a legitimate form of marriage, on equal footing with a real marriage. Common law refers to the body of U.S. law set by precedent and modified by the different states. If you and the person you're living with meet the four criteria for a common law marriage in your state, you'd be able to file jointly on your tax returns: 1) you live together, 2) you intend to marry, 3) you hold yourselves out to be married to the community and 4) you've been together for an unspecified length of time. (Ref 5)

Cohabitation Quagmires

Being in a non-common law environment doesn't automatically disqualify you for common law marriage status. States are required to recognize common law marriages formed in other states. So you could potentially file jointly if at some point you lived where common law prevailed. Cohabitating couples who do not meet common law requirements and/or form cohabitation agreements are not considered married. As life management resource Flying Solo spells out, unless you are married, you won't be able to file jointly on income taxes or partake of the gift and estate tax deductions that married couples have available to them as a matter of course.

Partner Dependent Exemption

For the unwed-living-together who had hoped to file jointly but could not due to ineligibility, there are alternatives to filing jointly that simulate the favorable deductions available to joint filers. By claiming your partner as a dependent, you'll have access to an extra deduction, but you'll have to provide over half the total support of your partner, who must not make more than a certain maximum amount per year, and have lived in your home as the primary residence the entire year and not filed jointly with anyone else. (Ref 1)

Head of Household Status

Unmarried couples, even if they cannot file jointly, may be able to file as head of household to obtain higher levels of deductions. You must meet a four-point test to be eligible for this status, requiring you to demonstrate that you have a qualifying person in your household that entitles you to preferred tax treatment. In some cases, a dependent child living with you takes care of it; in others, if you are able to show that your partner is a qualifying relative -- i.e. an unrelated person who lived in your household -- you could potentially come away with three deductions. (Ref 1)

Same-Sex Partnerships

As of the time of publication, only Washington, D.C., and Rhode Island recognize common law same-sex marriage as a valid form of marriage that's eligible for federal tax treatment the way actual marriage is. Although tax reforms of 2013 accorded same-sex marriages federal tax standing on par with opposite-sex marriages, common law marriage as applied to same-sex couples is not as clear-cut. Unmarried same-sex couples in civil unions or registered as domestic partners do not get to file taxes as married people.