NSO Exercise State Tax for Remote Workers: Where Is the Income Sourced?

A staff engineer worked at a Bay Area tech company from 2021 through 2024, living in San Francisco. She accepted a promotion to a fully-remote role and moved to Austin in January 2025. In October 2025 she exercised 12,000 NSOs. Strike $18, FMV $95. Spread: $924,000. Her Texas residency meant no Texas state tax on compensation. But California sent her a 2025 tax notice: 78% of the NSO spread was sourced to California based on workdays performed there during the vesting period of the underlying options. California’s claim: $720,720 of the spread was California-source income.

California’s supplemental withholding had not been applied by the employer because she was no longer a California resident. The state wanted $73,706 of tax plus interest and potential penalties for under-withholding. Her CPA eventually negotiated the sourcing down to 62% based on a precise workday analysis, reducing the California tax to about $58,600. The lesson: state source rules for equity compensation do not follow current residency. They follow where services were performed during the period the equity was earned.

The sourcing rule

Services-based sourcing

Most states source equity compensation based on where services were performed during the vesting period. For a 4-year vesting grant, the compensation is allocated across the four years based on the ratio of workdays in each state.

California applies this rule aggressively. 18 Cal. Code Regs. §17951-5 and related California FTB guidance provide that income from stock options is sourced based on days worked in California during the vesting period.

New York, Massachusetts, and other high-tax states apply similar rules. Some states use a simpler “grant-to-exercise” allocation; others use grant-to-vest; a few use exercise-year residency only.

The California allocation formula

California’s approach for NSOs:

Sourced California income = Total spread × (CA workdays during vesting period / Total workdays during vesting period)

For a 4-year vest starting in 2021 with exercise in 2025:

• Vesting period: January 2021 to January 2025 (shares fully vested)
• Workdays per year in California: varies by year
• Exercise year California work: zero

If an employee worked 1,000 days total during the vesting period and 800 were in California, the ratio is 80%. Spread sourced to California: 80% of $924,000 = $739,200.

The grant-date-to-exercise period

Some states use grant date through exercise date rather than grant through vest. This extends the period and potentially reduces the California portion if the employee left California before exercise.

California’s rule uses vesting, so the exercise date itself does not end the sourcing window for shares already vested. For unvested shares later vesting outside California, the vesting period would be fully outside California.

The employer’s withholding behavior

Single-state assumption

Most payroll systems withhold state tax based on the employee’s current work state. If the system shows the employee as Texas-based, California withholding is not applied.

This creates a mismatch: California taxes the income (because California sourced it), but no California withholding occurred. The employee owes California tax with the non-resident return, without any offsetting California withholding.

Sophisticated employer handling

Some large employers with sophisticated payroll systems (typically those with significant California workforce historical) track per-employee work-state history during vesting periods. They apply California withholding to the sourced portion of supplemental wages.

This is rare. Most employers apply current-state withholding only. Employees are responsible for filing the California non-resident return and paying tax on the sourced portion.

The 1099-B and the state reporting

The employer’s W-2 typically reports the full spread to the employee’s current state (Texas, in the opening example, which has no income tax). California is not reported on the W-2 because the employer’s payroll system is not tracking the California sourcing.

The employee files a California Form 540NR (non-resident) with the California-sourced portion, computed per California’s formula. The resulting California tax is paid with the filing.

The workday count mechanics

What counts as a workday

California’s rule counts days worked for the employer in California. Business travel to California counts. Vacation days do not count. Sick days generally do not count unless they are in California while on a California work assignment.

Employees who travel to California for meetings, quarterly reviews, or hybrid office days accumulate California workdays even while maintaining residency elsewhere.

The documentation burden

California audits of non-resident equity compensation commonly require detailed workday documentation:

• Calendar records showing location on each work date.
• Travel records (flights, hotels, expense reports).
• Meeting schedules showing where the employee was present.
• Employer attendance records for office-based work.

Employees who moved out of California and do not maintain good records are at a disadvantage. A conservative default of “100% California” often results when records are insufficient.

The trailing nexus doctrine

California’s position

California’s FTB has taken an aggressive position on trailing nexus for former residents. The state asserts tax jurisdiction over compensation that vests or is exercised after departure if the underlying services were performed during California residency.

This applies not just to NSO spread but to:

• RSU vesting (compensation income)
• ISO exercise bargain element (AMT preference allocable to California)
• Bonuses deferred from California work periods
• Pensions attributable to California service (limited by federal law under 4 U.S.C. §114 for qualified plans)

Constitutional limits

Federal statutes and constitutional due process impose some limits on state taxation of former residents. 4 U.S.C. §114 prohibits state taxation of qualified pension distributions to non-residents. Equity compensation is not covered by this statute.

The state’s right to tax compensation for services performed within the state has been upheld repeatedly. Non-residents who performed services in the state during the relevant period are taxable on the related compensation.

The multi-state wrinkle

Mixed workstate history

An employee who worked in California 2021-2022, New York 2022-2023, and Texas 2023-2025 has sourcing across three states. The compensation from a grant vesting during this period must be allocated:

• California portion: days in California during vesting / total vesting days
• New York portion: days in New York during vesting / total vesting days
• Texas portion: not taxable (no state income tax)

Each state’s return reports the sourced portion. The employee may owe tax to both California and New York for their respective portions.

The double-taxation question

Normally, the residence state provides a credit for tax paid to other states on double-taxed income. For non-residents, the source state taxes first, and the residence state (if any) provides a credit.

For an employee in Texas with California and New York sourced income, there is no residence state to provide a credit (Texas has no income tax). The source state taxes apply in full without offset.

For an employee in a state-income-tax residence state, the residence state’s credit for out-of-state tax can reduce the residence state’s tax. The mechanics are specific to each state’s credit rules.

The New York specific issue

New York applies similar sourcing rules with a twist: New York uses a convenience-of-the-employer test for certain remote-work situations. If the employer’s office is in New York and the employee chose to work remotely for personal convenience, New York may still source the compensation to New York regardless of where the employee actually worked.

This is controversial and litigated. New York’s position affects New York-headquartered companies with remote employees working from Florida, Texas, and other non-tax states.

The California alternative: Avoid, not Defer

Unlike federal tax, California tax on NSO spread cannot be deferred or reduced through retirement accounts in the same way. The options to reduce California tax are:

1. Move to California for the exercise year and file as a California resident for all wage income. This rarely helps because California’s rates are among the highest in the country.
2. Exercise while physically outside California and document non-California workdays rigorously.
3. Wait until California’s statute of limitations runs (typically four years) and hope California does not pursue.

Option 3 is risky. California’s aggressive enforcement makes quiet non-compliance a poor strategy for material amounts.

Documentation checklist

Before an exercise by a former California resident, document:

1. Work location by date during the vesting period.
2. Any California visits and their purpose (business vs personal).
3. Departure date from California (physical and tax residency).
4. Evidence of current residency (new state driver’s license, voter registration, home ownership or lease, moving expenses).
5. Employer’s record of work location at relevant periods.

A rigorous file defends against California’s default “100% California” assumption.

Frequently asked

What if I never lived in California but I worked there for meetings?

California taxes the compensation attributable to California workdays even for non-residents who never established California residency. Business travelers who performed material work in California while vesting equity may owe California tax.

Does the sourcing apply to the capital gain on sale?

No. Capital gain on sale is typically sourced to the state of residence at the time of sale. The spread (compensation) is sourced to services states; the post-exercise capital gain is sourced to residency.

Can the employer handle California withholding for former residents?

Some do. Most do not. The employee typically handles California non-resident filing and payment.

What if the vesting period spans a move?

The sourcing ratio reflects the mix. Days in California during vesting divided by total vesting days gives the California portion. Days in Texas during vesting are not taxable.

Is there a California de minimis exception?

California has no general de minimis rule for non-resident compensation sourcing. Even small amounts are technically taxable. Practical enforcement focuses on larger amounts.

Before you exercise as a former California resident, run the sourcing calculation and confirm with a tax advisor who handles California non-resident returns. See the California trailing nexus article for residency and departure specifics.