If I work for a company that has an office in Seattle, but has its HQ in California, do I need to pay income tax to CA?
No, if you’re in Seattle you pay Taxes for Seattle and Washington
It's entirely based on where you live
What if you work in Oregon and live in Washington?
As a side note: in most cases you do pay tax in other states if you are physically present there. I think you can exclude the occasional business trip but if you, for example, work 1 day a week at the HQ in CA, it is likely you owe taxes to CA. Same for OR company. If you live in Vancouver but commute to Portland everyday to work, you will owe OR income tax (they allow you to claim the other state income tax but that doesn’t help you here :))
Thanks everyone for your answers
Nope. You pay taxes based on the state you live in, not where the company is based out of.