My parents want to gift their paid off house to me in king county of Washington state. Which WAC exemption code(s) is needed?

That may create a whacking big federal gift tax bill. You need to take this question to a tax professional. An Enrolled Agent ought to be sufficient as a CPA is a bit of overkill.

Your tax pro will have your WAC citation for you to use if you still want to do this very large gift.

I strongly urge every asking person to actually put every bit of proof you have in front of a lawyer, in person, and get a consultation and opinion. These questions are understandably a bit short on detail, therefore a true complete legal opinion is not possible, nor is my response intended to be anything more than general information based on the facts as presented. I don't "farm" this site for clients. I also don't generally respond to emails/messages for lack of time.

This is a far bigger question than just asking for the WAC exemption. You should consult with an attorney that practices estate planning and/or with an accountant to see if this makes sense. While Washington state does not currently have a gift tax, there is a gift tax at the federal government level. US Citizens do have a large federal estate and gift tax exemption which is currently $11.4 million per person, but you need to know if your parents have used any of that exemption up yet. Gifts are reported on Form 709 each year and either reduce your federal exemption or if this has already been used up then gift tax is owed. There is also an annual gift tax exemption of $15,000 per person per year.
Another item to consider, is that the recipient of a lifetime gift takes the carry over basis of the giver. Whereas, in a bequest at death, the recipient receives a stepped up basis to the fair market value at the time of death. For example, if your parents gift the house to you during lifetime and their basis is $150,000 than that is the basis you will have in the property when you go to sell it in the future (absent any additional capital improvements). Therefore, you sell if for $600,000 than you will have a capital gain of $450,000 that is subject to capital gains tax. In the same scenario for values, your parents die and bequeath it to you then you will receive a stepped up basis to the fair market value at the time of death, which for purposes of this illustration is $600,000, then you sell it for $600,000 there is no capital gain and no capital gains tax.
A discussion with an estate planning attorney and/or your accountant is very appropriate so you and your parents can make an informed decision. Depending upon what you are trying to accomplish, there may be other planning opportunities to discuss such as transfer on death deed, gift with reservation of life estate, etc. An estate planning attorney and/or accountant would be able to guide you through this process.