Legal Question in Real Estate Law in Washington

It is less relevant whether you "feel" you are a landlord and whether you have have a "rental agreement" under Washington State law. The applicable definition is "any and all agreements" for possession of real property. Taking care of the place would generally be thought to qualify.

Don't do this on your own. You will, almost certainly, do something wrong. The 20 day notice is right, but it must be in the right form and served in the correct manner (personal or nail/mail if personal not possible). Unlawful detainers (legal term for eviction -- in your case you might want to also proceed as if it is a forcible detainer just in case) are not intellectually difficult. They are very routine, but are also one of the few areas of law with a likelihood of a "technicality" causing a major annoyance.

Just find an attorney in your area that does these and have him or her do it from start to finish. You'll expect to spend $250-$700 on attorney fees and upwards of $500 on costs, but it will be done right and done quickly. You'll want to do this quite quick as it is probably unlikely you'll be able to meet the 8/1/2014 deadline. You might already be looking at 9/1/2014.