First, you want to establish what your domicile is. Your domicile is the place of your permanent residence, that is, the place you will return to when your sojourn to another state or country is finished. Domicile is more than just where you are sleeping.
Once you establish your domicile, it does not change unless you take positive steps to move your domicile elsewhere.
In your case, it seems that you are a California resident who is temporarily working out of state. That is, some or all of the following is true: your property where you live is in California, your home phone number (if a land line) is in California, your driver's license is in California, your voter's registration is in California, and so on.
And when the Covid-19 situation ends, you intend to return to California all the while working for the same company.
The question of residency is important, because if you are a resident of California, you owe tax on all income earned from any state or country, no matter where you are working or sleeping at the time.
Please see the following on page 4 of CA Pub 1031:
"A resident is any individual who meets any of the following:
- Present in California for other than a temporary or transitory purpose.
- Domiciled in California, but outside California for a temporary or transitory purpose. See Section L, Meaning of Domicile."
Note that CA Pub 1031 does describe on page 4 a "safe harbor" in which employees who are out of state for more than 546 days (about 18 months) can claim to be nonresidents. But it is not clear to me that this would apply to you. In the first case, you don't plan to be out of state for that long, and in the second case, I do not think that your sojourn in Washington is due to a clause in your employment contract (please correct me if I am wrong) but due to the willingness of the company to let you work remotely from out of state (i.e., they did not tell you where to live as a condition of your employment).
Based on what I see so far, you appear to be domiciled in California, have done nothing to change that domicile, and therefore must file as a resident of California, which means that you are taxed "...on ALL income,
including income from sources outside California." (page 6 of CA Pub 1031).
So, to answer your question, based on what I see so far, you will be liable to pay CA tax on all of your CA income, even that earned while out of state, until such point as you take positive steps to change your domicile to another state.
