Washington’s new income tax—enacted under ESSB 6346—does not start from scratch.
It starts with a familiar number: federal adjusted gross income (AGI).
The rest is where the law gets more nuanced—and where planning opportunities (and risks) start to show up.
Step 1: Start with federal AGI
Washington taxable income begins with federal AGI (Form 1040, line 11).
That matters because:
- many exclusions/deductions are already baked in;
- the federal tax code sets the baseline; and
- if something never enters AGI, it may never enter the Washington tax base.
Step 2: Apply Washington modifications
From federal AGI, ESSB 6346 applies Washington-specific modifications that generally fall into three buckets:
1) Subtractions
Certain categories of income may be removed from the base (by statute and future guidance).
2) Add-backs
Some amounts excluded or treated favorably under federal law could be added back—this is where state policy choices show up.
3) Structural adjustments (allocation/sourcing)
The law defines how income is allocated, sourced, and attributed to Washington—especially important for multistate taxpayers, remote workers, and investment income.
Step 3: Arrive at “Washington taxable income”
After modifications, you arrive at Washington taxable income.
From there:
- a standard deduction (currently $1,000,000 per household) is applied; and
- the remaining amount is subject to the 9.9% tax rate.
Why the AGI starting point matters
Because the system starts with federal AGI:
- federal exclusions often carry through,
- federal characterization becomes critical, and
- planning at the federal level directly affects Washington exposure.
This creates both opportunities and traps.
What this likely means for QSBS (Section 1202)
QSBS is one of the most important implications.
If a gain is properly excluded under Section 1202 and never included in federal AGI, it likely never enters the Washington tax base—unless the statute explicitly adds it back.
As currently structured, ESSB 6346 does not appear to add back Section 1202 gains.
Where things get confusing
Expect uncertainty and disputes around:
1) What was included in AGI (and why)
If the starting number is wrong, everything downstream is wrong.
2) Documentation
Practically: if it’s not documented, it didn’t happen.
3) Interaction with other taxes
Washington already imposes a capital gains excise tax. Credits and overlapping concepts will matter.
4) Multistate issues
Expect complexity around residency vs. domicile, sourcing, and partial-year moves.
A simple example
Assume:
- federal AGI: $2,000,000
- no material Washington modifications
- standard deduction: $1,000,000
Then: Washington taxable income = $1,000,000; tax at 9.9% = $99,000.
Key takeaways
- Washington taxable income starts with federal AGI;
- modifications apply, but the federal baseline drives the result;
- if income never enters AGI, it may never be taxed by Washington; and
- documentation/classification matter more than ever.
Tools + next steps
- Washington Income Tax Calculator
- QSBS guide
- DOR documentation case analysis
If you want help planning around Washington’s new income tax, reach out.
