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The house should be your separate property. The determination of what characterization the property has, separate or community, is determined as of the moment it is acquired. Since you were single at the time it was acquired then it would be separate property. However, Washington does have something similar to a common law marriage known as the “committed intimate relationship” doctrine. That is where property is acquired at a time when a couple are living in a situation that is like being married but aren't.
The courts employ a 3-prong analysis for dividing property after the committed intimate relationship ends. First, it must determine that the parties had a “stable, marital-like relationship where both parties cohabit with knowledge that a lawful marriage between them does not exist.” When the court makes this determination it considers the length of the relationship, how long the parties lived together, the purpose of the relationship, whether the parties pooled resources, and the intent of the parties.
If a court finds that a committed intimate relationship existed, it will evaluate the interest each party has in the property acquired during the relationship and make a “just and equitable” division of that property.
The burden of proof is on them and I would think it is unlikely they would be successful in getting this property characterized as separate property.