ITINs for Non-US Spouses: Explained for Wealthy US-UK Dual Filers
An ITIN for a non-US spouse is a US tax identification number that your UK partner may need to claim them on a US return. Your UK spouse is a nonresident alien with no US filing duty on their own income, but the moment you choose Married Filing Jointly, an ITIN and worldwide-income exposure follow. This guide explains when the number is required and which filing status it applies to.
By the TaxYork Cross-Border Tax Team — reviewed by a US-UK dual-qualified adviser (CPA / Enrolled Agent).
What is an ITIN, and does my UK spouse actually need one?
An ITIN (Individual Taxpayer Identification Number) is a nine-digit number that the IRS issues to people who must appear on a US tax return but cannot get a Social Security Number. Your UK spouse needs one only if a US return has to show them — most often because you elect to file jointly, or because you file separately and want to claim their exemption-linked benefits. Absent that, a UK spouse with no US income and no US status has no reason to hold an ITIN at all.
A UK-only spouse is a nonresident alien (NRA) in the US's eyes. Their UK salary, ISA growth, rental income, and pension sit outside the US net, and they file nothing with the IRS. The ITIN question is really about your return: whose income you pull onto it, and which status you claim.
The difference between an ITIN and an SSN
An SSN is issued to US citizens and to people authorized to work in the US. An ITIN does the opposite job: it lets someone outside that system be identified on a tax filing. An ITIN never confers work rights, immigration status or benefit entitlement. It is purely a tax-processing number, and for many UK spouses, it is the only US number they will ever hold.
How do you apply for an ITIN from the UK?
You apply on Form W-7, and in almost every case,e you file it with the US tax return that creates the need for the number — not on its own. The application has to prove foreign status and identity, which for a UK spouse means the passport, and the passport route is where most families come unstuck.
Three ways to prove identity
The IRS will not accept an ordinary photocopy of a passport. You have three practical options: For high-net-worth couples who travel, the Certifying Acceptance Agent route is usually the only sensible one. It avoids surrendering a passport and reduces the risk of a rejected application, which restarts the whole clock. A rejected W-7 can also delay the tax return to which it is attached.
Timing matters
Because the W-7 is filed with the return, the ITIN application is tied to your US filing deadline. The IRS treats an ITIN as applied for on time if the W-7 is submitted by the return due date, even if the number itself is issued weeks later. Leaving it to the last week of the deadline is where problems start.
MFJ or MFS — which filing status does the ITIN unlock?
This is the decision that drives everything. Once your UK spouse has (or is applying for) an ITIN, you choose between Married Filing Jointly (MFJ) and Married Filing Separately (MFS) — and the two point in opposite directions on tax.
Married Filing Jointly and the 6013(g) election
To file jointly with an NRA spouse, you make a Section 6013(g) election to treat that spouse as a US resident for the whole year. The upside is real: joint brackets are wider, the standard deduction is larger, and one return covers both of you. The catch is that the election pulls your UK spouse's worldwide income — their UK salary, investment gains, rental profit, the lot — onto your US return.
For a UK spouse with a substantial income or large unsheltered gains, MFJ can be a costly trap. Foreign tax credits and the treaty soften it, but US rules on ISAs, offshore funds, and UK pensions do not always align, and PFIC treatment of UK funds can result in US tax with no UK offset. The 6013(g) election is also, in practice, a lasting commitment: once revoked, the couple can never make it again.
Married Filing Separately
MFS keeps your UK spouse's income entirely out of the US net. You report only your own income; their UK earnings stay UK-only. The price is stiffer brackets, a compressed standard deduction, and the loss of several credits. On an MFS return, you generally enter the spouse's ITIN, or write "NRA" in the spouse's identification field if they have no number and you are claiming nothing that requires one.The rule of thumb: if your UK spouse earns little, MFJ often wins on the numbers. If they have a high income, a portfolio, or a UK property empire, MFS usually protects far more than the wider brackets could ever save. Run both before you commit.
Can I file as Head of Household instead?
Sometimes, yes. A US person married to an NRA can be treated as "considered unmarried" for Head of Household (HOH) purposes if they have a qualifying dependent — typically a child — and meet the household-cost tests. HOH gives better brackets and a larger standard deduction than MFS without dragging your UK spouse's income onto the return.
This is a genuinely useful status for a US parent living in London with a non-US spouse and a child. It is not automatic; the dependent tests are strict, and the child's identification number then becomes the next question.
What happens to the Child Tax Credit if my child only has an ITIN?
The Child Tax Credit (CTC) — worth up to $2,200 per qualifying child for 2025 — requires the child to have an SSN. A child with only an ITIN cannot claim it. For 2025 and later, the filing parent must also hold a valid SSN to claim the credit at all.
A child with an ITIN instead of an SSN may still qualify for the smaller Credit for Other Dependents (ODC), worth up to $500, provided the ITIN was issued by the due date of the return. For US-UK families whose children were born in the UK and never obtained an SSN, this is a common and unwelcome surprise: a $2,200 credit shrinks to $500 solely because of the number the child holds.
Getting an SSN for a U.S. citizen child born abroad
A child born to a US-citizen parent is usually a US citizen and is entitled to an SSN, even when born in the UK. Registering the birth and obtaining a Consular Report of Birth Abroad open the door to that SSN — and to the full CTC. Where a child is a US citizen, chasing the SSN rather than settling for an ITIN is almost always worth it.
Do ITINs expire?
Yes. An ITIN expires if it is not used on a US tax return for three consecutive years. An expired ITIN does not vanish, but a return filed with one can be processed with credits and exemptions disallowed until the number is renewed, which delays refunds and can trigger notices.
For a UK spouse whose ITIN was obtained for one MFJ year and then left dormant, there is a real risk of expiry. If you expect to use the number again — even intermittently — keep track of the three-year window and renew before you next need it, not after.
A worked example: the Hampstead couple
Consider an anonymized client. James is a US citizen and a partner at a London firm; his wife, Priya, is a UK citizen with no US ties, earning around £180,000 as a management consultant plus a buy-to-let portfolio. They have one child, born in London.
James first assumed MFJ was the obvious choice for the wider brackets. Modeling both routes told a different story. A 6013(g) election would have pulled Priya's £180,000 salary, her rental profits, and her UK fund gains onto the US return, where PFIC rules on her UK funds produced US tax with no UK credit to offset it. The "saving" from joint brackets was dwarfed by the new US tax on Priya's world income.
The couple filed MFS instead. Priya obtained an ITIN through a Certifying Acceptance Agent so James could claim what he was entitled to without surrendering her passport, and they pursued an SSN for their US-citizen child to secure the full $2,200 Child Tax Credit rather than the $500 ODC. Keeping Priya's income out of the US net saved a five-figure sum a year.
Talk to a US-UK adviser before you elect
The ITIN is the easy part. The filing-status choice behind it — MFJ against MFS, HOH where it fits, the once-in-a-lifetime nature of a 6013(g) election — is where wealthy US-UK families gain or lose the most. TaxYork models both routes on your real numbers before anything is filed.
Speak to our cross-border team: hello@taxyork.com | 020 3488 8606 | taxyork.com. We handle US-UK dual filers, ITIN applications, and the filing-status decisions that follow.