You should consult with an attorney and/or tax advisor. It is usually beneficial for people who own rental properties to set up an LLC or other entity for many reasons including liability protection and the ability to deduct business expenses. If you bought the property already you also need to consider if forming an entity and transferring the property will result in a transfer tax - this is why we often advise clients to set up the LLC first and purchase via the LLC to avoid double transfer tax but check with an expert on WA real estate transfers.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/