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How to File a Self Assessment Tax Return UK: Step by Step
If you’ve never filed a tax return before, the words “Self Assessment” can sound intimidating — like something only accountants understand. But here’s the truth: hundreds of thousands of ordinary people in the UK file their own returns every year without any professional help, and you can too.
Whether you’re newly self-employed, renting out a spare room, earning extra income from a side hustle, or you’ve just crossed the threshold where HMRC wants to know about your earnings, this guide breaks the whole process down into plain English. No jargon, no scare tactics — just the practical steps you need to get it done correctly and on time.
By the end of this article, you’ll know exactly who needs to file, what information to gather, how to actually submit your return online, and how to avoid the penalties that catch out so many first-timers.
Who Actually Needs to File a Self Assessment Tax Return?
Not everyone needs to do this — it’s specifically for people whose income isn’t automatically taxed through PAYE (the system employers use). You’ll likely need to file if you:
- Are self-employed and earned more than £1,000 in the tax year
- Are a company director (with some exceptions)
- Earn rental income from property
- Have income from savings, investments, or dividends above certain thresholds
- Earn foreign income
- Claim Child Benefit and you or your partner earn over £60,000
- Have untaxed income HMRC doesn’t already know about
If you’re not sure, the gov.uk online checker will tell you in a couple of minutes based on a short questionnaire.
Step 1: Register for Self Assessment
Before you can file anything, you need to be registered with HMRC. If this is your first time, you’ll need to do this by 5 October following the end of the tax year you need to report (the UK tax year runs 6 April to 5 April).
- If you’re self-employed, you register as a sole trader through the gov.uk website
- If you’re not self-employed but have other income to declare (like rental income), there’s a separate registration form
Once registered, HMRC will post you a Unique Taxpayer Reference (UTR) — a 10-digit number you’ll need every year. This can take up to 10 working days to arrive, sometimes longer, so don’t leave registration until the last minute.
Step 2: Set Up Your Government Gateway Account
To file online, you need a Government Gateway user ID and password. If you’ve dealt with HMRC before (for example, checking your tax code), you might already have one. If not, you’ll create one during the registration process above.
Keep these login details somewhere safe — you’ll need them every single year, and losing access close to the deadline is a common (and stressful) mistake.
Step 3: Gather Your Information
This is where most of the actual work happens, and doing it early saves enormous stress in January. You’ll typically need:
- Your UTR and National Insurance number
- P60 or P45 if you had any employed income during the year
- Records of self-employed income and expenses (invoices, receipts, bank statements)
- Details of rental income and allowable expenses, if applicable
- Bank interest and dividend statements
- Pension contributions
- Details of any benefits claimed (like Child Benefit, if relevant)
If you’re self-employed, keeping a simple spreadsheet throughout the year — logging income and expenses monthly — makes this step ten times easier. Trying to reconstruct twelve months of finances in one sitting is where mistakes creep in.
Step 4: Complete the Online Return
Once you’re logged into your HMRC account:
- Select “Self Assessment” then “File a return”
- Confirm the tax year you’re filing for
- Answer the initial questions about your circumstances (employment, self-employment, property, etc.) — this determines which sections you’ll need to complete
- Fill in each relevant section with your income and expenses
- HMRC’s system automatically calculates your tax bill as you go
- Review the summary carefully before submitting
Take your time on this step. The system saves your progress, so you can log out and come back rather than rushing through it in one sitting.
Step 5: Understand What You Owe (and When to Pay)
Once submitted, HMRC tells you exactly how much tax and National Insurance you owe. This is where many first-timers get caught out — because your bill might include:
- Tax owed for the year just finished
- A “payment on account” — an advance payment towards next year’s tax bill, usually 50% of what you owed this year
This means your first tax bill can be significantly higher than expected. Budgeting for this in advance, rather than discovering it in January, makes a huge difference to your stress levels.
| Payment | What it covers | Typical due date |
|---|---|---|
| Balancing payment | Tax owed for the previous tax year | 31 January |
| First payment on account | Advance towards current tax year | 31 January |
| Second payment on account | Advance towards current tax year | 31 July |
Key Deadlines to Remember
Missing deadlines is the single most common (and costly) mistake people make with Self Assessment. Here’s a simple breakdown:
| Deadline | What it’s for |
|---|---|
| 5 October | Register for Self Assessment (if new) |
| 31 October | Paper tax return deadline |
| 31 January | Online tax return deadline AND payment deadline |
| 31 July | Second payment on account (if applicable) |
Filing even one day late triggers an automatic £100 penalty, even if you don’t owe any tax. Further delays add daily penalties and interest, so it’s genuinely one of the deadlines worth building reminders around. Citizens Advice has helpful guidance if you’re struggling to pay on time — HMRC can sometimes offer payment plans rather than leaving you to face the full bill in one go.
Common Mistakes to Avoid
A few small errors cause a disproportionate amount of stress each year:
- Leaving registration too late — the UTR can take weeks to arrive
- Forgetting the payment on account — leading to a bigger-than-expected bill
- Not keeping receipts — making expense claims harder to justify
- Guessing figures — always work from actual records, not estimates
- Missing the deadline because of a login issue — don’t wait until 31 January to test your Government Gateway access
Conclusion
Filing a Self Assessment tax return isn’t as frightening as it first appears once you break it into clear steps. To recap:
- Check early whether you actually need to file, and register well before the 5 October deadline if it’s your first time
- Keep organised records throughout the year rather than scrambling in January
- Budget for payments on account, not just the tax on the year just finished
- File and pay by 31 January to avoid automatic penalties
- If your finances are complex — multiple income streams, property, or investments — it may be worth getting advice from a qualified accountant
Getting into a simple yearly routine — registering early, tracking income as you go, and filing a few weeks before the deadline rather than the night before — turns Self Assessment from a source of dread into just another item on your annual to-do list.
Next read: Worried about surprise tax bills? Read our guide on budgeting for self-employed income: /budgeting-for-self-employed-income