Courts in many states today are reviving the old concept of common law marriage. However, not every state is which creates the need for estate planning for couples who consider themselves to be common law spouses.
In most states, the idea of common law marriage has been abolished for a long time. Only a small number of states recognized the concept until recently.
Why? Probably because more couples are choosing to live together for long periods of time and acting like married couples, despite never making it “official” through the process of formally getting married.
By recognizing the existence of a common law marriage, the courts are then able to treat the couple as they saw themselves when it comes to divorcing or settling an estate.
Not all states will recognize these marriages though as the Wills, Trusts & Estate Prof Blog points out in "Why Common Law Couples Need an Estate Plan (New York)."
What makes a valid common law marriage varies from state to state.
Normally, if the couple holds itself out to the public as married or tells a government agency that they are married, then a common law marriage exists. For example, if the couple files a joint tax return with the IRS, then the existence of a common law marriage will be recognized in those states that allow them and the couple will be in trouble with the IRS in other states.
Since not all states recognize common law marriages, it is important that you understand that you will still need an estate plan to protect your common law spouse's interests in some states.
Even if you act like you are married, you are not actually “married” in most states and you need to account for that in your planning.
Reference: Wills, Trusts & Estates Prof Blog (Oct. 25, 2017) "Why Common Law Couples Need an Estate Plan (New York)."