We do not have Castle but here is what a gun rights attorney writes in his blog:
One of the most common questions asked by responsible gun owners pertains to the "Castle Doctrine." Although this sounds really technical, Washington law does not technically have a "Castle Doctrine," Instead Washington law talks about the duty to retreat or lack thereof. In reality, what the law states is that people who are lawfully in a place, have no duty to retreat. Without a duty to retreat, one might have the right to protect themselves depending on the amount of force they are threatened with.
One reason deadly force is often justified when encountering home invaders, is that it is reasonable to believe that one is in imminent threat of serious bodily injury or death when your home is invaded. While they may only be there to steal property (which cannot be prevented with deadly force under Washington law) one does not have to wait to see what an intruder's true intentions are before taking defensive action.