As a non-resident you will owe taxes on your income derived while in California if it exceeds a certain amount. The details are here: https://www.ftb.ca.gov/forms/201... Your company should do withholding of California income tax for the period you are working there and should pryou with a W2 for both WA and CA.The difficulty will be if the amount of time in CA is long enough that the FTB considers you a resident. If so you would owe CA taxes on you entire income.My advice is, if your CA income is enough to require filing taxes, to contact an Enrolled Agent knowledgeable in WA and CA taxes to file for you.