Filing Your First UK Tax Return: A Guide to Self-Assessment

Ah, the brown envelope. It arrives unassumingly, slips through your letterbox, and lands on the doormat with a soft thud. But for many expats new to the UK, the sight of "HMRC" in the corner can trigger a unique blend of excitement ("I'm officially a UK resident!") and pure, unadulterated panic ("What on earth is a Self Assessment?!").

If you’re nodding along, take a deep breath. You’re not alone. Navigating the UK’s tax system for the first time feels like being handed a complex board game without the instruction manual. But here’s the good news: it’s entirely manageable. I’ve been there, staring at the forms, deciphering the jargon, and wondering if I’ll ever see my weekend again.

This guide is the instruction manual I wish I’d had. We’ll break down everything you need to know about filing your first UK Self Assessment tax return, from figuring out if you even need to file, to understanding the key deadlines and expat-specific rules that can save you a lot of money and stress.

Do I Even Need to File a Self Assessment?

First things first. Not everyone in the UK needs to file a tax return. If you’re a straightforward employee and your only income is your salary, your tax is likely handled automatically through the Pay As You Earn (PAYE) system. You can probably stop reading and go enjoy a well-deserved cup of tea.

However, as an expat, your financial situation is often more complex. You’ll almost certainly need to register for Self Assessment if you meet any of the criteria set by HMRC for the 2024/2025 tax year (the one you'll file by January 2026):

• You were self-employed as a sole trader and earned more than £1,000.
• You are a partner in a business partnership.
• You earned £100,000 or more in taxable income in the tax year.
• You have untaxed income from renting out a property in the UK or abroad.
• You received significant income from savings, investments, or dividends.
• You need to claim certain tax reliefs.
• You have income from overseas that you need to pay UK tax on (this is a big one for expats!).
• You have capital gains to report from selling assets like shares or a second home.
• You or your partner received Child Benefit and your income is over £60,000.

If any of these sound like you, welcome to the club. It's time to get acquainted with Self Assessment.

Key Concepts Every Expat Must Understand

Before you dive into the paperwork, you need to get your head around a few core concepts. These are especially crucial for expats and determine how much tax you’ll pay.

1. The UK Tax Year

Unlike many countries that use the calendar year, the UK tax year runs from April 6th to April 5th. This is fundamental. When you file a return in, say, January 2026, you are reporting on your income earned between April 6th, 2024, and April 5th, 2025.

2. Your Residence Status

This is the big one. Your UK tax liability hinges on whether you are considered a UK resident for tax purposes. This isn't just about having a visa; it's determined by the Statutory Residence Test (SRT). The SRT is a complex set of rules, but it boils down to how many days you spend in the UK and what "ties" you have to the country (like work, family, or accommodation).

You can use the GOV.UK tool to check your status, but in short, it’s a flowchart of tests. If you’re in the UK for 183 days or more in a tax year, you’re automatically a resident. If you’re here for fewer days, your ties to the UK will determine your status. It's vital to get this right.

3. Domicile vs. Residence

These two terms sound similar but are critically different in UK tax law.

• Residence is about where you live and spend your time now.
• Domicile is about where your permanent home is, your "homeland." For most expats, your domicile will remain your country of origin, even if you’ve lived in the UK for years.

Why does this matter? Because your domicile status can unlock a different way of being taxed.

4. The Tax Basis: Arising vs. Remittance

This is arguably the most important—and confusing—tax concept for non-domiciled expats.

For your first few years, choosing the remittance basis can be a huge tax-saving strategy if you have significant income outside the UK that you plan to keep there. However, the rules are complex, and professional advice is highly recommended before making this claim on your tax return.

Getting Started: A Step-by-Step Guide

Ready to tackle the practical side? Here’s your game plan.

Step 1: Register for Self Assessment If you’ve never filed before, you must register with HMRC. You can’t file a return without doing this first. The deadline to register is October 5th after the end of the tax year you need to report on.

• Go to the GOV.UK website and search for "Register for Self Assessment."
• HMRC will create an account for you and send you a Unique Taxpayer Reference (UTR) number in the post. Guard this number with your life—you’ll need it for everything tax-related.

Step 2: Gather Your Documents Don't wait until the last minute to start digging through your files. Create a folder (digital or physical) and start collecting everything you’ll need.

Your UK Information:

• P60: Your end-of-year tax summary from your employer.
• P45: The form you get from an employer when you leave a job.
• P11D: Details of any benefits in kind (like a company car or private health insurance).
• Records of any self-employed income and allowable expenses.
• Statements of interest from UK bank and savings accounts.
• Dividend vouchers from UK investments.
• Records of pension contributions.

Your Overseas Information:

• Details of salary from an overseas job.
• Records of rental income from a property abroad.
• Statements from foreign bank accounts showing interest.
• Dividend statements from foreign investments.
• Proof of any foreign tax you’ve already paid (this is crucial for claiming double-taxation relief).

Step 3: Choose Your Filing Method You have two choices: paper or online. Honestly, in 2025, there’s only one real choice.

• Online (Recommended): The system is user-friendly, it does the calculations for you, it gives you an instant confirmation, and the deadline is later.
• Paper: The forms are long, you have to do the maths yourself, and the deadline is much earlier (October 31st). Avoid this unless you have a very specific reason.

Filling Out the Return: A Quick Tour

The online Self Assessment form is smart; it will only ask you questions relevant to the information you provide. The main sections you'll encounter are:

1. Main Return (SA100): This is the core section covering your personal details, UK employment income, bank interest, and dividends.
2. Supplementary Pages: This is where things get interesting for expats. You’ll likely need to complete additional forms depending on your circumstances. The most common ones are:
   • SA102 (Employment): If you had more than one UK job.
   • SA103 (Self-Employment): For your business income and expenses.
   • SA106 (Foreign): This is where you declare your foreign income and claim Foreign Tax Credit Relief.
   • SA109 (Residence, Remittance Basis, etc.): This is the crucial form for declaring your residence status and, if applicable, claiming the remittance basis.

Take your time, read each question carefully, and use the little question mark icons next to each box—they provide surprisingly helpful explanations.

Deadlines and Penalties: Don't Get Caught Out

HMRC is not flexible when it comes to deadlines. Missing them results in automatic penalties, so get these dates in your calendar now.

What happens if you're late?

• 1 day late: An instant £100 penalty.
• 3 months late: Daily penalties of £10 per day, up to a maximum of £900.
• 6 months late: A further penalty of 5% of the tax due or £300 (whichever is greater).
• 12 months late: Another 5% or £300 penalty. These penalties are in addition to interest charged on the unpaid tax, so the costs can spiral quickly.

Paying Your Tax Bill and Payments on Account

Once you submit your return, the system will calculate your tax liability. You can pay this directly through the HMRC website via bank transfer, debit card, or Direct Debit.

One final surprise for many first-timers is the "Payments on Account." If your Self Assessment tax bill is over £1,000, HMRC will ask you to pay half of your estimated next year's bill in advance.

• First Payment on Account: Due by January 31st (along with your current year's bill).
• Second Payment on Account: Due by July 31st.

This can be a shock to the system, as your first tax bill can feel like it's almost double what you expected. It's essential to budget for this.

Final Takeaway: Plan Ahead and Don't Be Afraid to Ask for Help

Filing your first UK tax return is a rite of passage for every expat. It can feel daunting, but it's a process that millions of people successfully complete every year.

My best advice?

1. Start early. Don't be one of the frantic filers on January 30th. Give yourself weeks, not days.
2. Be organized. Keep your financial documents in order throughout the year. It makes the whole process infinitely less painful.
3. Understand your status. Take the time to work out your residence and domicile status properly. It has huge financial implications.
4. When in doubt, get professional help. If you have a complex situation—multiple income streams, capital gains, or you're considering the remittance basis—paying an accountant who specializes in expat tax is an investment, not an expense. They can often save you far more than their fee.

You’ve successfully navigated an international move; you can definitely handle this. Treat it as a learning process, be methodical, and before you know it, you'll be a Self Assessment pro, confidently helping the next wave of new arrivals decipher their own brown envelopes. Good luck