If you move into or out of Washington during a tax year after the 9.9% income tax takes effect in 2028, you don't owe a full year's tax — but the part-year rules are not the simple day-count proration most people assume. Two provisions of ESSB 6346 do the work: §406 splits the year into a resident period and a nonresident period, and §315 shrinks your $1 million standard deduction by an income ratio — not a day ratio. Getting the mechanics right can change the answer by six or seven figures. In the examples below, identical facts produce a $111,500 swing based solely on income timing.
This post is part of our Complete Guide to Washington's New Income Tax. For the residency tests themselves, see The 30-Day Rule for Washington Income Tax Residency.
Disclaimer: This article is for general informational purposes only and does not constitute legal or tax advice. The rules discussed here are based on ESSB 6346 as enacted; DOR regulations are still pending. Consult a qualified attorney or tax advisor before making planning decisions.
How part-year taxation works under §406
Section 406 splits a mover's year in two. For the portion of the year you were a Washington resident, your entire adjusted gross income for that period is included. For the portion you were a nonresident, only Washington-source income is included — wages for days worked in Washington, Washington business and pass-through income, Washington real estate, and the other source categories in §§401–405. Pass-through income gets its own rule: §406(2) splits your K-1 items between the resident and nonresident periods by the ratio of resident days to total days in the entity's tax year, with the nonresident-period share then limited to the Washington-source portion.
The standard deduction shrinks by income, not days
This is the part the early commentary — including an earlier version of this post — got wrong. The $1 million standard deduction (§314; combined $1 million for spouses and domestic partners) is reduced for anyone who is not a Washington resident for the entire year. But §315 prorates it by a fraction whose numerator is your Washington base income and whose denominator is your federal AGI from all sources — capped at one. Days of residency are not the test.
The difference matters enormously when income is lumpy, which is exactly the fact pattern of someone timing a move:
- Example 1 — even income. You leave Washington on July 1. Your federal AGI is $4 million, earned evenly, so roughly $2 million lands in your resident period and your post-move income has no Washington source. Washington base income: $2 million. Deduction: $1M × ($2M / $4M) = $500,000. Taxable: $1.5 million. Tax: $148,500.
- Example 2 — lumpy income, well-timed. Same move date, same $4 million AGI — but $3.5 million of it is a bonus and short-term gain received in November, after you've left, with no Washington source. Washington base income: $500,000. Deduction: $1M × ($0.5M / $4M) = $125,000. Taxable: $375,000. Tax: roughly $37,000. The day count is identical to Example 1; the income timing did all the work.
- Example 3 — lumpy income, badly timed. Reverse it: the $3.5 million event lands in March, while you're still a resident. Washington base income: $3.5 million plus your other resident-period income. The prorated deduction barely dents it. Moving in July saved you very little.
The planning rule that falls out: your move date matters far less than where large income events fall relative to it.
Long-term stock gains play by different rules entirely
If the income event you're timing is a sale of stock, the part-year machinery above is mostly not the operative law. ESSB 6346 deducts long-term capital gains out of the income tax base (§302(1)) and adds back only "Washington capital gains" as defined in chapter 82.87 RCW (§302(3)) — and that definition allocates gains on stock and other intangibles by your domicile at the time of sale (RCW 82.87.100(1)(b)), with a credit for capital gains tax paid (§205). A mid-year mover selling stock should be asking "where was my domicile on the closing date?" — not "how many days was I a resident?" For the full analysis, see Residency Planning Before You Sell.
The trap that catches movers: the abode rule
Section 101(8)(c) provides that someone who was domiciled in Washington during part of the year is treated as a resident for the portion of the year in which they were domiciled here or maintained a place of abode here. Keep your Seattle condo after your June move to Florida, and your Washington resident period doesn't end in June — it runs as long as the condo does, potentially the entire year. Income received in the fall, while you're living in Florida, lands in your Washington resident period.
And to correct a misstatement that appeared in an earlier version of this post: for someone not domiciled in Washington, statutory residency requires maintaining a place of abode here and more than 183 days of physical presence (§101(8)(a)(ii)) — not 30. The 30-day figure belongs to the safe harbor for Washington domiciliaries, which is a different rule serving a different purpose. A snowbird with a Washington vacation home who visits for six weeks is not made a resident by those visits.
What to do
Model your year before you pick a move date: list expected income events, place each one against your planned resident and nonresident periods, compute Washington base income, and apply the §315 income-ratio proration — not a day-ratio shortcut. If a stock sale is in the picture, run the domicile analysis separately. Sever the abode connection completely, or understand that §101(8)(c) keeps your resident period running. And document everything: the part-year rules will be enforced through DOR audits where contemporaneous records of your move date and income timing decide close cases.
DOR has not yet issued regulations under §§406 and 315, and the interplay between them has open questions — treat the examples above as the statutory baseline, and revisit when guidance issues. Nothing in this article is legal or tax advice.
The rules are new, the stakes are high, and the window to act is narrowing. If any of the scenarios above look like your situation, a focused conversation now can prevent a costly mistake later.
Planning a move around the 2028 income tax? Book a 20-minute call to stress-test your timeline while the facts still support the position you want.
