Common law marriage is not as common as you seems to suggest.  In fact, there is NO common law marriage in NY.  Even if they announce to the world, they would not be considered married under the rules of the state of NY. The few states that have common law marriage, I believe some Southern States. Pre-marital property alwats remains pre-marital unless the wife or husband decide to convert it, meaning to change the title to the house.  Moving in together, has no absolutely no legal consequences.