Tax Tip Tuesday: Shadow Payroll 101 - Why Employers Split Payroll for Global Talent — and Why Your State Return May Not Get the Memo

New York City skyline representing shadow payroll and cross-border tax compliance for global employees

If you have a client who's working across borders — a US employee on assignment in London, a UK national temporarily in a New York office, or an executive splitting time between two countries — chances are “shadow payroll” has come up in conversation. It sounds like something out of a spy movie, but it's actually one of the most common (and most misunderstood) tools in global mobility tax planning.

Here's what it is, why employers use it instead of just moving someone onto local payroll, and the state-level traps — on both the outbound and inbound sides — that catch even sophisticated taxpayers off guard.

What Is Shadow Payroll?

Shadow payroll is a in the host country that exists purely to calculate and report the local income tax and social security withholding an assignee owes — while the employee continues to be paid, in reality, from their home-country payroll.

No duplicate paycheck lands in the employee's bank account. The “shadow” run is a calculation and reporting exercise: it tells the host country's tax authority what should have been withheld had the employee been paid locally, and it generates the filings (and sometimes actual tax remittances) needed to stay compliant.

Why Not Just Move the Employee to Local Payroll?

It seems simpler on paper — put the employee on the host country's payroll system and be done with it. In practice, employers avoid this for several reasons:

1. Benefits and Pension Continuity

Moving someone off home-country payroll can disrupt retirement plan contributions, equity vesting, health coverage, and years-of-service credit. Shadow payroll lets the employee stay tied to the home-country plan while the company still meets host-country withholding obligations.

2. No Local Legal Entity Required

Many host countries require withholding from day one of physical presence, even for a short business trip — but the employer may not have (or want to stand up) a local legal entity just to run payroll. Shadow payroll satisfies the withholding requirement without that overhead.

3. One Employer of Record

Keeping the employee on a single home-country payroll simplifies HR administration, employment law compliance, and avoids inadvertently creating a permanent establishment or local employment relationship the company didn't intend to create.

4. Social Security Totalization

Under a totalization agreement, an assignee may be able to stay in their home country's social security system rather than starting fresh in the host country. Splitting payroll (rather than fully localizing it) helps preserve that treatment.

5. Speed and Flexibility for Short-Term Assignments

For assignments under a year or two, standing up full local payroll — with local tax IDs, local bank accounts, and local compliance calendars — is often more administrative burden than the assignment is worth. Shadow payroll is built for exactly this window.

When Does It Make Sense to Fully Localize Instead?

Shadow payroll isn't the answer for every situation. Employers typically move an employee onto full local payroll when:

•      The assignment becomes long-term or permanent (the employee is relocating, not visiting)

•      Local employment law requires it (some countries mandate local payroll for local employment contracts, regardless of assignment length)

•      The cost and complexity of maintaining shadow payroll outweighs simply localizing the role

The Federal Piece: Foreign Tax Credit

For US citizens and green card holders on outbound assignment, foreign income tax withheld through shadow payroll (or paid directly abroad) generally generates a Foreign Tax Credit (Form 1116) against US federal tax on that same income, preventing federal double taxation. This is the mechanism most clients already know and expect.

The Trap: Not All States Allow the Same Credit

This is where things get expensive, fast — and it's the piece that catches even well-advised clients off guard.

Not every state conforms to the federal Foreign Tax Credit. California is the clearest and most consequential example:

•      California taxes residents on worldwide income, full stop.

•      California does not allow a credit for foreign income taxes paid, and does not allow the Foreign Earned Income Exclusion either — even though both are perfectly usable on the federal return.

•      The Other State Tax Credit (Schedule S) only applies to taxes paid to another US state — not to a foreign country.

•      The result: a California resident on assignment abroad can legitimately owe US federal tax of close to zero (fully offset by FTC) while still owing full California tax on the same income, with no offsetting credit at all.

California isn't unique in having its own conformity rules here — but because so many of our clients are California residents, it's the one that shows up in real returns most often.

The takeaway for every client with foreign-source income: never assume the state return follows the federal result. Each state's treatment has to be checked on its own.

A Simple Illustration

A California-resident employee is assigned abroad for 14 months under a shadow payroll arrangement. Foreign tax withheld is substantial, and the federal return shows only a small residual US tax liability after the FTC. The employee assumes the state return will land somewhere similar.

It doesn't. Because California disallows the credit, the full foreign-source income is taxed again at the state level — with no mechanism to offset it. Absent proactive planning (residency positioning, safe harbor analysis, or comp structuring before the assignment starts), that surprise shows up for the first time on the return itself, when it's too late to do anything about it.

Why This Matters for Planning — Not Just Filing

The point of catching this early isn't just accurate compliance — it's avoiding a six-figure surprise. Before an assignment starts is the time to look at:

•      Residency status — Does the employee qualify for California's safe harbor (generally an 18-month-plus foreign work contract with limited California presence), which would remove California taxation of the foreign income entirely?

•      State of domicile — Is there an opportunity to establish residency in a different state before the assignment begins?

•      Compensation structuring — Should the assignment agreement anticipate the state-level cost and gross up accordingly?

•      Withholding elections — Are state withholdings calibrated to the real expected liability, or copied over from a federal assumption that doesn't hold?

None of this works well as a year-end conversation. It works as a before-the-assignment-starts conversation.

It Works Both Directions: Shadow

Payroll Inbound to the US

Everything above applies just as much in reverse. When a foreign company sends an employee into the US — a UK firm placing someone in a New York office, an Indian tech company assigning an engineer to Texas — US withholding obligations attach almost immediately, even though the employee stays on the foreign entity's payroll and is paid from abroad.

Foreign Employee Coming to the US: How It Actually Works

The situation: A foreign employer sends one of its employees to work physically in the US — a client engagement, a rotational assignment, opening a US office — while keeping them employed and paid by the foreign entity back home.

Why the foreign employer doesn't just put them on a US payroll:

•      No US legal entity exists, or the employer doesn't want to stand one up for one or two people

•      Doing so can risk creating a US permanent establishment, triggering unwanted US corporate tax exposure for the foreign company

•      The employee stays enrolled in home-country benefits, pension, and social security without interruption

The step-by-step process:

1.       Trigger. The moment the employee starts performing services physically in the US, US wage withholding under IRC §3402 attaches to the portion of pay tied to US workdays — even though a foreign company is paying them from abroad.

2.       Employer registers for US payroll tax purposes. The foreign company gets an EIN and registers with the IRS and the relevant state, usually through a shadow payroll provider or PEO rather than building out real US payroll infrastructure.

3.       A parallel, non-cash calculation runs each pay period. US-workday wages are apportioned out of the employee's total compensation, run through federal withholding tables and the relevant state's rules, and FICA is calculated — unless a Certificate of Coverage under a totalization agreement exempts it.

4.       The employer remits real cash to the IRS and state — but the employee's actual paycheck doesn't change. It still comes from the foreign entity, in the foreign currency, on the foreign payroll cycle. No second paycheck exists. This is why shadow arrangements are often paired with a tax equalization policy, since the employer is effectively absorbing the US withholding cost rather than passing it to the employee.

5.       Year-end reporting. The employee gets a W-2 (or Form 1042-S if treaty benefits apply) for US-source wages, and files a US return — Form 1040-NR in most cases, or a full Form 1040 if they meet a US residency test — to reconcile actual liability against what was withheld.

6.       The state layer is where it gets complicated, and it's where the two traps below come in.

Two State-Level Traps, Not Just One

Inbound assignments carry two separate state-level issues, and both vary by state:

1. When does withholding actually start? Arizona doesn't require withholding until a nonresident has been in the state more than 60 days in a calendar year; Georgia's threshold is more than 23 days in a calendar quarter or $5,000/5% of total income; Illinois uses a 30-working-day threshold. By contrast, 22 states have no meaningful nonresident filing threshold at all, requiring most nonresidents to file from day one.

2. Does the state honor the federal tax treaty? This is the direct inbound mirror of the FTC problem above. States aren't bound to honor federal tax treaties, and most do — but Alabama, Arkansas, California, Connecticut, Hawaii, Kansas, Kentucky, Maryland, Mississippi, Montana, New Jersey, North Dakota, and Pennsylvania do not. A foreign national who is fully treaty-exempt on the federal return can still owe full state tax on

that same income in one of those states.

How the Major Inbound-Talent States Compare

State	Talent Hub For	Withholding Trigger	Honors Treaties?
New York	Finance, media	Effectively day one for most nonresidents	Generally yes
California	Tech, entertainment	Wage-threshold based, no meaningful safe harbor	No
Texas	Tech, energy, corporate HQs	No state income tax — no withholding issue at all	N/A
Washington	Tech	No state income tax (aside from a capital gains tax on high earners)	N/A
Massachusetts	Biotech, finance	No meaningful threshold — essentially day one	Generally yes
New Jersey	Pharma, finance	No meaningful threshold — essentially day one	No
Illinois	Finance, corporate HQs	30 working days	Generally yes
Georgia	Corporate HQs, logistics	23 days/quarter or $5,000/5% of income	Generally yes

Texas and Washington are the easy cases — no state income tax means the entire state withholding question disappears. New York, Massachusetts, and Illinois require getting the day-count mechanics right early, but generally don't add a treaty trap on top.

California and New Jersey are where it bites hardest: no treaty conformity, low or no safe harbor, and aggressive enforcement, especially from California's FTB.

Three Illustrations

New Jersey — the treaty trap. A German engineer is assigned to a New Jersey office for eight months under a US-Germany treaty exemption for short-term dependent personal services. The federal return correctly shows the wages as treaty-exempt, no federal withholding, no federal tax due. New Jersey doesn't honor the treaty. The full wages are taxable at the state level from day one, and because there was no state withholding set up (the payroll team assumed “treaty-exempt” meant exempt everywhere), the assignee owes a lump sum — plus underpayment penalties — when the NJ-1040NR is filed.

Texas — the non-event. The same German engineer, on the same assignment terms, is placed in a Dallas office instead. Federal treaty analysis is identical. But Texas has no state income tax, so there's no state withholding question, no state treaty conformity question, and no state return to file. The entire state-level complexity above simply doesn't exist. This is a big part of why Texas and Washington show up so often as landing spots for inbound talent when the assignment structure allows any flexibility in location.

Illinois — the timing trap, not a treaty trap. A Canadian analyst is assigned to a Chicago office for what's planned as a 25-working-day project. Illinois's 30-working-day threshold means no withholding is set up initially. The project runs long — 34 working days by the time it wraps. Once the 30-day threshold is crossed, Illinois requires withholding retroactive to day one of the assignment, not just from day 31 forward. Because the shadow payroll wasn't tracking days in real time, the employer is now catching up on withholding after the fact, with the assignee facing an unexpected balance due.

Shadow payroll solves a real compliance problem — it lets employers meet host-country (or home-country) withholding obligations without disrupting benefits, entity structure, or employment terms, whether the assignment is outbound or inbound. But it doesn't make the tax picture simple, and it definitely doesn't make the state tax picture match the federal one. Whether it's a Foreign Tax Credit disallowed at the state level or a tax treaty a state won't honor, the pattern is the same: federal relief does not automatically travel to the state return.

If you have an employee heading on international assignment, or you're bringing global talent into the US — and especially if the destination is California, New Jersey, or another state that doesn't play by the federal rules — the time to loop in a tax advisor is before the assignment starts, not after the first shadow payroll run hits.

Reach out to MKHS Tax Group at info@mkhstaxgroup.com if you have any questions.