Self-Assessment Tax Return Guide: Who Files and How
Self Assessment is HMRC's system for collecting tax that isn't deducted at source through PAYE. Miss a deadline and you face automatic penalties, but completing a return is straightforward once you know what's needed.
Who needs to file a Self Assessment return?
You need to register and file a return if any of the following apply to you in a tax year:
- You were self-employed with income over £1,000 (gross, before expenses)
- You earned more than £100,000
- You received untaxed income over £2,500 (e.g. rental income, tips, freelance earnings)
- You had income from savings or investments over £10,000
- You had capital gains above the annual exempt amount (£3,000 in 2026/27)
- You received Child Benefit and either you or your partner had income over £60,000 (High Income Child Benefit Charge)
- You were a company director (unless it was a non-profit and you received no pay)
- You had income from abroad or lived abroad with UK income
- HMRC has asked you to file, even if none of the above apply
Already pay tax through PAYE? You may still need to file
If you have multiple income sources, rental income, or significant investment returns alongside your employment income, PAYE may not collect all the tax you owe. A Self Assessment return ensures everything is declared and the right amount of tax is paid.
Key deadlines
All dates below relate to the 2024/25 tax year (6 April 2024 to 5 April 2025). Miss any of them and penalties apply automatically.
| Deadline | Date | What it covers |
|---|---|---|
| Register for Self Assessment | 5 October 2025 | If you're filing for the first time, register by 5 October after the end of the tax year. |
| Paper return filing | 31 October 2025 | If filing on paper rather than online. |
| Online return filing | 31 January 2026 | The main deadline for online submission. Also the deadline for paying any tax owed. |
| Payment on account (1st) | 31 January 2026 | First instalment of advance tax payment for 2026/27 (50% of previous year's tax bill). |
| Payment on account (2nd) | 31 July 2026 | Second instalment of advance tax payment (remaining 50%). |
What information to gather before you start
Having the right documents before you start makes the process much faster. Collect:
- Your Unique Taxpayer Reference (UTR), found in previous correspondence from HMRC
- P60 from your employer (total pay and tax for the year)
- P11D or P11D(b) if you have taxable benefits in kind (company car, private medical insurance)
- Records of self-employment income and allowable expenses
- Rental income figures and a summary of allowable property expenses
- Bank and building society interest statements (or the totals from your accounts)
- Dividend income (shown on dividend vouchers or your investment platform)
- Details of any assets sold (for capital gains: date of sale, proceeds, and original cost)
- Pension contribution statements (for higher-rate tax relief claims)
- Gift Aid donations (to claim higher-rate relief)
Payments on account: what they are and why they matter
If your Self Assessment tax bill is more than £1,000 and more than 20% of your total tax liability came from sources outside PAYE, HMRC requires you to make two advance payments toward next year's tax bill. Each payment is 50% of your current year's tax liability.
This catches many first-time filers by surprise: in January after your first year of Self Assessment you owe your full year's tax bill plus the first payment on account (50% extra). Budget for a payment of up to 150% of what you expect in your first year.
You can reduce payments on account if your income has fallen
If you know next year's income will be lower than last year's, you can apply to reduce your payments on account. Be careful: if you reduce them too much and owe more than expected, HMRC charges interest on the shortfall from the original due date.
Penalties for late filing and payment
| Delay | Late filing penalty | Late payment interest |
|---|---|---|
| 1 day late | £100 automatic penalty | Interest accrues from 31 January |
| 3 months late | £10/day (up to 90 days = £900) | Interest continues |
| 6 months late | £300 or 5% of tax due (higher applies) | 5% surcharge on unpaid tax |
| 12 months late | A further £300 or 5% of tax | A further 5% surcharge |
The £100 penalty applies even if you owe no tax. Always file on time even if you can't pay immediately. The late filing penalty is separate from the interest and surcharges on unpaid tax.
Common mistakes to avoid
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Forgetting to claim higher-rate pension relief
If you contribute to a relief-at-source pension and pay 40% or 45% tax, your scheme only claims 20% relief automatically. You claim the extra through your tax return. Many higher-rate taxpayers miss this and overpay tax year after year.
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Missing the registration deadline
You must register by 5 October after the end of the tax year. If you register late, HMRC may still issue a penalty for not registering on time even if you file your return before the January deadline.
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Not declaring bank interest and dividends
HMRC receives information directly from banks and investment platforms. They often already know about interest and dividend income. Failing to declare it risks a penalty for inaccuracy, even if it was an honest oversight.
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Forgetting gift aid and charitable donations
If you make Gift Aid donations and pay higher-rate tax, you can claim additional relief through Self Assessment. The charity claims 20% from HMRC; you can claim a further 20% (40% taxpayer) or 25% (45% taxpayer) yourself.