If you are a US citizen living in the UK and married to a British (or other non-American) spouse, your default US filing status is Married Filing Separately — and your non-US spouse generally stays outside the US tax system entirely. But you have choices: you can elect to file jointly and treat your spouse as a US resident, or in some cases file as Head of Household. Each route has real trade-offs — not just on the tax you pay, but on whose UK accounts the IRS gets to see, whether your spouse needs a US tax number, and even how US estate and gift tax treats you as a couple. Here is how the options compare and how to decide which one fits your household.
The starting point: Married Filing Separately
When a US citizen is married to a non-resident alien (a spouse who is neither a US citizen nor US resident), the IRS default is Married Filing Separately (MFS). The big advantage is simplicity and protection: your non-American spouse's income and UK accounts stay out of the US system. You report only your own income, and where your spouse's SSN would go, you write "NRA" (nonresident alien).
MFS does have downsides — a low filing threshold, less favourable tax brackets, and some credits are restricted — but for couples where the non-US spouse has substantial UK income or assets, keeping them out of the US net is usually worth it. For context on the wider return, see how US tax returns work for Americans in the UK.
Option 1: keep it simple with MFS
This is the right answer for most couples, especially where the non-American spouse earns well, has UK investments, or simply wants nothing to do with the IRS.
- You report only your own worldwide income to the IRS.
- Your non-US spouse files nothing with the IRS and needs no ITIN (unless you claim a benefit that requires one).
- Your spouse's UK salary, ISAs, pensions and accounts stay outside US reporting.
- You write "NRA" for the spouse's tax number and can usually still e-file.
Option 2: the joint election (treat your spouse as a US resident)
Under Internal Revenue Code section 6013(g), you can elect to treat your non-resident spouse as a US resident for tax purposes and file Married Filing Jointly. This unlocks the more generous joint brackets and standard deduction, which can lower your US tax — but there is a catch that is decisive for most UK couples.
Once you make the election, your spouse's worldwide income becomes subject to US tax and reporting too. That means their UK salary, their ISAs (with the PFIC problems that ISAs carry), their pensions, and their bank accounts all come into the US system — including FBAR and FATCA reporting. For a spouse with meaningful UK assets, that is usually a poor trade. The election can make sense where the non-US spouse has little or no income, but it should be modelled carefully because it is binding until revoked and can only be made once in this way.
Option 3: Head of Household
There is a lesser-known middle path. If you are married to a non-resident alien and you maintain a home for a qualifying dependent (such as a child), you may be able to file as Head of Household — which has better brackets and a higher standard deduction than MFS — without pulling your spouse into the US system. The qualifying person generally cannot be the non-resident spouse, so this route depends on having another qualifying dependent. Where it applies, it is often the best of both worlds.
Getting an ITIN for your spouse (Form W-7)
If you do choose to file jointly, your non-US spouse needs a US taxpayer number — an Individual Taxpayer Identification Number (ITIN), applied for on Form W-7, since they are not eligible for an SSN. The application is usually attached to the front of the joint return; using an IRS Certifying Acceptance Agent lets you avoid mailing original passports. You can read the basics on the IRS nonresident spouse page and the Form W-7 / ITIN page. If you stay with MFS and are not claiming a benefit that requires it, your spouse generally needs no ITIN at all.
Which option should you choose?
- Spouse has significant UK income or assets → usually MFS (keep them out of the US system).
- Spouse has little or no income → the joint election may lower your US tax — model it first.
- You maintain a home for a qualifying child → check whether Head of Household beats MFS.
- You want to claim the refundable Child Tax Credit → your filing status and the children's SSNs both matter.
Pitfalls to avoid
- Making the joint election without realising it exposes your spouse's UK assets to US tax and reporting.
- Putting your spouse's SSN field blank instead of "NRA" on an MFS return.
- Assuming Head of Household is available when there is no qualifying dependent other than the spouse.
- Getting an ITIN you do not need, or missing one you do.
- Overlooking the knock-on effects on credits, the FEIE-versus-FTC choice, and PFIC exposure.
What each option means for FBAR and FATCA
Filing status does not just change your tax — it changes whose accounts the US gets to see. This is often the deciding factor for couples in the UK.
Under Married Filing Separately, your non-American spouse's solo UK accounts are generally outside US reporting. You still report any accounts you own or have signature authority over, but your spouse's personal current account, ISA and pension stay private. Make the joint election, however, and your spouse becomes a US person for tax purposes — which pulls their accounts into FBAR and FATCA reporting too, and exposes any UK funds or ISAs they hold to PFIC rules. Jointly held accounts already need reporting either way, but voluntarily importing your spouse's entire financial life into the US system is a serious step that the tax saving rarely justifies for a working spouse.
The ITIN process and timing in practice
If you do elect to file jointly, plan the ITIN application carefully because it controls your timeline. You complete Form W-7 for your spouse and, in most cases, attach it to the front of the joint return, which is then filed on paper rather than e-filed. The IRS issues the ITIN and processes the return together.
The friction is documentation: the IRS needs to verify your spouse's identity and foreign status, traditionally by certified copies of a passport. Mailing an original passport abroad is unappealing, so many couples use an IRS Certifying Acceptance Agent, who can verify the documents and submit the application without you surrendering the original. Processing can take several weeks to a few months, so if a joint election is your plan, start the ITIN step early in the filing season rather than at the deadline.
If your spouse later becomes a US person
Circumstances change. Your non-American spouse might move to the US with you, take up a Green Card, or naturalise — at which point they become a US person in their own right and must file regardless of any election. Equally, a section 6013(g) election once made stays in force until it is revoked, and it can generally only be made once, so revoking it casually can close off the option permanently.
Because these decisions have long tails, it is worth thinking a few years ahead: if there is a realistic chance your spouse becomes a US person, or that your family relocates, the right filing choice today may be different from the one that looks cheapest this April. Mapping the likely path with a US tax specialist who understands the UK side avoids decisions you cannot easily unwind.
Estate and gift tax: the non-citizen spouse trap
There is one more cross-border wrinkle that mixed-nationality couples should know about, because it works very differently from what most people assume. Between two US-citizen spouses, transfers are effectively unlimited — you can give your spouse any amount during life or at death with no US gift or estate tax, thanks to the unlimited marital deduction. That unlimited deduction does not apply when the recipient spouse is not a US citizen.
Instead, lifetime gifts to a non-citizen spouse are limited to a special, much higher annual exclusion (well above the ordinary annual gift exclusion, and indexed each year), and transfers at death to a non-citizen spouse generally do not qualify for the marital deduction unless they pass through a special structure known as a Qualified Domestic Trust (QDOT). For couples with significant assets, this can create a US estate-tax exposure that a same-nationality couple would never face.
This is firmly specialist territory and depends on the size of your estate and your domicile position in both countries — but it is worth flagging early, because the planning (wills, trusts, how assets are titled) is far easier to arrange in advance than to fix later.
Get the status right from year one
Filing status is one of the highest-leverage decisions on a cross-border return, and the right answer is genuinely couple-specific. The wrong election can expose a spouse's entire UK financial life to the IRS unnecessarily — or leave money on the table. A US tax specialist in the UK can model MFS, the joint election and Head of Household on your actual numbers.
