Self Assessment is the system HMRC uses to collect Income Tax from people whose tax cannot be fully collected through PAYE. Around 12 million people in the UK file a Self Assessment return each year. If you need to file and miss the deadline, penalties start from £100 even if you owe nothing.
Who Must File a Self Assessment Return?
You typically need to file if any of the following apply in the tax year:
- You were self-employed and earned more than £1,000
- You had untaxed income over £2,500 (e.g., rental income, tips, commission)
- Your total taxable income exceeded £150,000
- You or your partner earned over £60,000 and claimed Child Benefit (HICBC)
- You received income from dividends or investments above the relevant allowances
- You had foreign income that needs to be declared
- You were a company director (unless of a non-profit organisation with no pay)
- You owed Capital Gains Tax on the sale of assets
- You need to claim certain tax reliefs (e.g., for employment expenses)
HMRC may also issue a notice to file, in which case you must submit a return regardless.
Key Deadlines for 2025/26
| Deadline | Date |
|---|---|
| Tax year ends | 5 April 2026 |
| Register for Self Assessment (if new) | 5 October 2026 |
| Paper return deadline | 31 October 2026 |
| Online return deadline | 31 January 2027 |
| Tax payment due | 31 January 2027 |
| Second payment on account due | 31 July 2027 |
The online deadline of 31 January is the most important date. Both the return and any tax owed are due by this date.
How to File
- Register: If it is your first time, register for Self Assessment on GOV.UK. You will receive a Unique Taxpayer Reference (UTR) by post within 10 working days.
- Gather records: Collect P60s, P45s, bank interest statements, dividend vouchers, invoices, expense receipts, and any other relevant documents.
- Complete the return: Log in to your HMRC online account and fill in the relevant sections. The system calculates your tax automatically.
- Submit and pay: Submit your return and pay any tax owed by 31 January.
Payments on Account
If your Self Assessment bill is over £1,000 and less than 80% was collected at source (e.g., through PAYE), HMRC requires payments on account. These are two advance payments toward next year’s bill, each equal to half of the current year’s liability:
- First payment: 31 January (same day as the balancing payment)
- Second payment: 31 July
You can apply to reduce payments on account if you expect your income to fall.
Penalties for Late Filing
| How Late | Penalty |
|---|---|
| 1 day late | £100 flat penalty |
| 3 months late | £10 per day (up to 90 days, max £900) |
| 6 months late | Additional £300 or 5% of tax due (whichever is greater) |
| 12 months late | Further £300 or 5% of tax due |
Late payment also incurs interest on the outstanding amount.
Key Takeaway
Do not leave your return until January. Filing early does not mean paying early — you still pay by 31 January — but it gives you time to check figures, budget for the bill, and avoid last-minute technical issues on the HMRC website.