There are lots of answers here and mostly from real estate experts, but maybe I can help.
Some assessed values, regardless of where the county got them from, are just plain wrong. Some are low and some are high. If you believe yours to be high, be prepared to have proof.
I could not find a section in King County's website where you can get the petition form. I got a digital copy of the form and actually held a class on how to fill it out properly. It's not easy, but it can be done.
I didn't know how to post a document on the web to be downloaded other than my member profile on Mortgage News Daily. I've referenced a link to my profile where I host several documents including the King County Property Tax Petition form.... more
Steve has some good advice below (quite a lot ; )) The fastest, easiest way is to take advantage of a real estate agent. Whether or not you are planning to sell, many agents will give you an email link with comps for you to sort through to see which most nearly resemble your property. Just be honest with the agent about your intentions and expectations.
You can also use zillow.com which may or may not give you a reasonable idea. But if you are doing this on your own, and you want to be reasonably accurate, it will take some legwork and some research.... more