From the
Washington State Constitution, Article VII, Section 1:
All taxes shall be uniform upon the same class of property within the territorial limits of the authority levying the tax.... The word "property" as used herein shall mean and include everything, whether tangible or intangible, subject to ownership.Various court rulings have stated that income is a "class of property" and, as such, any income tax must be taxed identically. The effect was to overturn a state income tax proposed in 1932 and prohibit any similarlly graduated income tax.
Further, from Article VII, Section 2:
... the aggregate of all tax levies upon real and personal property by the state and all taxing districts now existing or hereafter created, shall not in any year exceed one percent of the true and fair value of such property in money... Past courts have ruled that this limit holds to the class of property, so that property taxes are not aggregated with other taxes. The effect of this is that, without changing the state constitution, any income tax would be capped at 1% of total income.
The Legislature could implement a flat income tax of up to 1% total income. It might be possible for "income" to be subdivided into different classes, such as employment earnings, investment income, etc. and possibly squeeze out another percent or two. But to implement a graduated income tax would require that the Legislature propose two different constitutional amendments (amendments can only alter one section of the state constitution), that both proposals pass by two-thirds of both houses of the Legislature, and that both proposals are ratified by a majority of the voters. Extremely unlikely.