UK Self Assessment Tax Returns for U.S. Expats Explained
If you’re an American living in the UK, you may need to file a Self Assessment tax return with HMRC in addition to your U.S. return. According to HMRC, Self Assessment applies to self-employed individuals, company directors, landlords, and anyone with untaxed income above £2,500, including foreign income.
Common triggers that require self-assessment include:
- Self-employment income over £1,000
- Total taxable income of £100,000 or more
- Untaxed income from tips, rental income, or foreign sources above £2,500
- Capital Gains Tax owed on the sale of assets
Filing UK and U.S. Taxes? Get It Right From the Start
Below, you’ll find how the UK Self Assessment process works, what it costs, and how to coordinate it with your U.S. tax return.
Filing Both U.S. and UK Taxes? We Can Help
At Greenback, we don’t just help Americans file U.S. expat taxes. We also prepare UK Self Assessment tax returns. If you live in the UK, our team can handle both your U.S. and UK filings so you stay fully compliant. Learn more about Greenback’s UK tax services.
What Is Self Assessment in the UK?
Self Assessment is the UK’s system for reporting income that hasn’t been taxed at source. Unlike employees who have tax automatically deducted through PAYE (Pay As You Earn), Self Assessment requires you to report your income, calculate what you owe, and file a tax return directly with HMRC.
The UK tax year runs from April 6 to April 5 (not the calendar year), and Self Assessment returns cover all taxable income earned during that period.
Who Needs to File a Self-Assessment Tax Return?
You’re required to file a UK Self Assessment tax return if any of the following apply:
- You’re self-employed and earned more than £1,000
- You’re a partner in a business partnership
- You earn £100,000 or more in total taxable income
- You or your partner earns £60,000+ and claims Child Benefit (the High Income Child Benefit Charge kicks in at £60,000 and fully withdraws the benefit at £80,000)
- You earn more than £2,500 in untaxed income (including tips, rental income, or foreign income)
- You’re a company director (unless you received no compensation at all)
- You have untaxed income from savings, investments, or dividends over £10,000
- You owe Capital Gains Tax from selling assets
- You earn a UK income while living abroad
- You receive foreign income that needs to be reported to HMRC
The rules can be complex and situation-dependent. If you’re unsure whether you need to file, consult an expat tax professional who is familiar with both the UK and U.S. tax systems.
For Americans in the UK: You may need to file a Self Assessment even if your income is already reported to the IRS. Foreign income thresholds and dual-country filing requirements often trigger Self Assessment obligations.
When Do I Register for Self Assessment?
If you’ve never filed a Self Assessment return before, you must register with HMRC first.
Registration Deadlines
- If you’re self-employed: Register by October 5, following the end of the tax year
- If you need to file for another reason: Register as soon as you know you need to file
Once registered, HMRC will send you a Unique Taxpayer Reference (UTR) number, which you’ll need to file your return. The UTR can take up to 10 business days to arrive by post (or up to 21 days if you’re overseas).
You’ll also need to set up a Government Gateway account to file online, which requires an activation code sent by mail.
Learn more about registering for Self Assessment on the GOV.UK website.
UK Self Assessment Deadlines
The deadline for filing your Self Assessment tax return depends on how you file:
- Paper returns: October 31, following the end of the tax year
- Online returns: January 31, following the end of the tax year
Payment Deadlines
- First payment on account (if applicable): January 31
- Second payment on account: July 31
- Final balance: January 31
UK vs. U.S. Tax Filing Deadlines
If you’re filing both UK and U.S. taxes, here’s how the deadlines compare:
| Country | Tax Year | Filing Deadline | Extension Deadline |
|---|---|---|---|
| UK | April 6 to April 5 | January 31 (online) | None (late filing penalties apply) |
| U.S. | January 1 to December 31 | June 15 (automatic extension for expats; typical deadline is April 15) | October 15 (filing an extension request is required) |
Why this matters for expats: UK Self Assessment is due in January, while U.S. expat tax returns aren’t due until June 15 (with an extension to October 15). Many Americans in the UK file their UK return first, then use that information to complete their U.S. return. Managing overlapping deadlines and exchange rate calculations requires careful planning.
Read more about UK vs. U.S. tax differences.
How Do I File My Self-Assessment Tax Return?
Once you’re registered and have your UTR number, there are three ways to file your Self Assessment return:
1. File Online Through Government Gateway
This is the most common method. After registering, you’ll receive an activation code by mail to complete your Government Gateway setup. You can then file directly through HMRC’s online portal.
2. Use Commercial Tax Software
HMRC-recognized software can simplify the filing process, especially if you have complex income sources. See the official list of approved software providers here.
3. File by Paper
You can download Form SA100, complete it by hand, and mail it to HMRC. Remember, paper returns have an earlier deadline (October 31).
How Much Does It Cost to File a Self-Assessment?
If you’re an American in the UK filing both Self Assessment and U.S. taxes, working with an expat tax service that handles both returns offers significant advantages
At Greenback, we’ve partnered with a UK Chartered Accountant to provide seamless coordination between your U.S. and UK tax returns:
- Single document upload: Submit your tax documents once through your secure Tax Companion account
- Coordinated filing: Your U.S. and UK accountants work together to ensure accurate, tax-efficient reporting across both countries
- Cross-border expertise: Our UK partner has deep experience with cross-border tax matters for Americans living in the UK
- Flat-fee pricing: Transparent costs with no hidden charges
This collaborative approach saves time, reduces errors, and often results in better tax outcomes than working with separate accountants who don’t communicate with each other.
Why Bundled Services Matter
When your U.S. and UK accountants work in silos, you risk:
- Duplicate work explaining your situation twice
- Inconsistent treatment of foreign income
- Missed opportunities for tax optimization
- Exchange rate calculation errors
- Higher overall costs from coordination time
Learn more about our UK tax services and get a quote.
Other Costs to Consider
- Late filing penalties (see below)
- Payment on account: If you owe more than £1,000 in tax, HMRC requires advance payments for the next year
- Amended return fees: If errors are discovered after filing
Living in the UK as an American? Start Here
Self Assessment Penalties and Late Filing Consequences
HMRC imposes automatic penalties if you file or pay late. Here’s what you’ll face:
Late Filing Penalties
| Timing | Penalty |
|---|---|
| 1 day late | £100 fixed penalty (even if no tax is owed) |
| 3 months late | £10 per day for up to 90 days (£900 maximum) |
| 6 months late | £300 or 5% of tax owed (whichever is higher) |
| 12 months late | Additional £300 or 5% of tax owed, plus potential investigation |
Late Payment Penalties
| Timing | Penalty |
|---|---|
| 30 days late | 5% of tax owed |
| 6 months late | An additional 5% of tax owed |
| 12 months late | Additional 5% of tax owed |
Interest charges: HMRC also charges interest on unpaid tax from the due date until payment is made. As of 2025, the rate is set at the Bank of England base rate plus 4%, compounding daily.
Can penalties be appealed? Yes, if you have a reasonable excuse (such as serious illness, bereavement, or technical issues). HMRC reviews appeals on a case-by-case basis.
For expats: If you’re new to the UK tax system and didn’t know you needed to file, HMRC may waive first-time penalties, but only if you proactively register and file as soon as you realize your obligation.
What’s Different for Americans Filing UK Self Assessment?
As a U.S. citizen or green card holder in the UK, you face additional requirements that UK-only filers don’t:
- Dual filing obligations: You must file both UK Self Assessment and U.S. tax returns, reporting worldwide income to both HMRC and the IRS.
- Exchange rate conversions: Income and expenses must be converted using different rates for each country (spot rates for HMRC, annual averages for the IRS), which can create discrepancies between returns.
- Overlapping deadlines: The UK Self Assessment is due on January 31, while U.S. returns aren’t due until June 15 (or October 15 with an extension). Most expats file UK first, then use that data for their U.S. return.
- Treaty benefits to claim: The U.S.-UK tax treaty prevents double taxation through Foreign Tax Credits and the Foreign Earned Income Exclusion (up to $130,000 for the 2025 tax year, or $132,900 for the 2026 tax year), but you must actively claim these benefits.
These complexities make professional help valuable. Read our full guide to U.S. taxes for expats in the UK.
Making Tax Digital (MTD) for Self Assessment
Making Tax Digital is HMRC’s initiative to modernize tax reporting by requiring digital record-keeping and quarterly updates through compatible software.
Current Status
MTD has been mandatory for VAT-registered businesses since 2019. Starting in April 2026, MTD for Income Tax Self Assessment is being rolled out in phases for self-employed individuals and landlords:
| Start Date | Who’s Affected |
|---|---|
| April 2026 | Self-employed individuals and landlords with income over £50,000 |
| April 2027 | Those with income over £30,000 |
| April 2028 | Those with income over £20,000 |
What MTD Means for You
- You’ll need to use MTD-compatible software to keep digital records
- You’ll submit quarterly updates to HMRC instead of one annual return
- All submissions (including year-end declarations) must go through third-party software; HMRC will not provide an online filing portal for those in MTD
- HMRC has confirmed a “soft landing” period, meaning penalty points will not be issued for missing the first four quarterly update deadlines
For Americans in the UK: MTD will add another layer of quarterly reporting alongside your existing U.S. and UK annual filing obligations. Planning ahead and using compliant software will be essential.
Read our full guide to Making Tax Digital.
Need Help Filing Your UK Self Assessment and U.S. Tax Return Together?
Dual filings involve two deadlines, overlapping rules, and complex foreign income reporting requirements. Greenback helps expats prepare both returns (including your UK Self Assessment) accurately and on time.
Our UK Tax Services Include
- UK Self Assessment preparation and filing
- Foreign income and Capital Gains reporting
- Coordination with your U.S. expat tax return
- Exchange rate calculations and compliance
If you’re an American living in the UK, we handle both sides of the Atlantic so you can focus on your life abroad. Learn more about Greenback’s UK tax services.
If you’re ready to be matched with a Greenback accountant, get started today. For general questions on taxes or working with Greenback, contact our Customer Champions.
Simplify Your UK and U.S. Tax Filings
Frequently Asked Questions
No. HMRC imposes automatic penalties starting from just one day late. However, if you have a reasonable excuse, you may be able to appeal the penalty.
Generally, no, unless you earn £100,000+ or meet one of the other criteria listed above. PAYE income is already taxed at source.
HMRC will issue penalties, charge interest on unpaid tax, and may open an investigation. Persistent non-compliance can result in court action.
You’ll still need to file for the portion of the tax year you were a UK resident. You may also need to file a “split year” return depending on your circumstances.
No. Tax preparation fees for personal returns are not allowable expenses in the UK, even if related to your U.S. filing obligations.
This article is for informational purposes only and does not constitute tax advice. Tax situations vary based on individual circumstances. Consult a qualified tax professional for advice specific to your situation.
Related Resources
- U.S. Expat Tax Guide for Living in the UK
- UK vs. U.S. Taxes for Expats: Rates, Key Differences, and How to Avoid Double Tax
- Making Tax Digital for UK Expats
- U.S.-UK Dual Citizen Taxes Explained
- Tax Help for Americans Living in the UK
- UK Tax Services for Expats
- Tax Planning Before Moving to the UK
- Foreign Earned Income Exclusion
- FBAR Filing Requirements
- U.S. Tax Filing Deadlines for Americans Living Abroad
