There is never a legal obligation or requirement for individual persons to form, draft, or enter into an agreement: an agreement entered into between you and your partner, if properly executed, will be legally binding. Note that you may wish to also consider some ancillary agreements that may be applicable: e.g. a power of attorney in favor of each other, and/or a medical care directive (often called a "living will") giving the other some amount of discretion or authority if one of you is disabled. Your relationship may not be yet be a stage where you wish to cede the other such authority as these agreements would give, but if you are or are becoming essentially as close as a married couple, then you may wish to create agreements that will give each of you some of the rights and authority that a married couple gets by dint of being married. For example: say that paying the mortgage on the property requires contributions from both of you, but one of you becomes incapacitated in an accident; without a POA, the other could not access the disabled partner's bank account to make payment, or if necessary, refinance or sell the home, without the approval and help of whomever does have a POA or is appointed legal guardian.