I also have property in WA, and do not have a company. It is not required by state law to form a company only that you register with the state to collect and pay tax. For more info go here: http://dor.wa.gov/docs/pubs/industspecific/homerentals.pdf
and finally here: http://dor.wa.gov/Content/DoingBusiness/RegisterMyBusiness/
The state determines your filing requirements based on your estimated sales tax collected.
Remember to keep all your income and expenses organized as you will be reqired to file form E - Real Estate rental on your personal taxes.