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)
- You had other untaxed income, such as tips, commission, savings interest, dividends or foreign income (since 6 April 2024, HMRC abolished the old rule that required PAYE-only taxpayers to file just because their income was high — if all your income is taxed through PAYE, high earnings alone no longer trigger a filing requirement)
- 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 above £2,500)
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, usually within 15 working days (21 if you are abroad). GOV.UK’s “Check when you can expect a reply from HMRC” tool shows how long a reply is taking.
- 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. Filing on paper instead? Our SA100 form guide goes through the paper return section by section.
- Submit and pay: Submit your return and pay any tax owed by 31 January. If a lender asks for proof of income afterwards, see our SA302 tax calculation guide.
Payments on Account
If your Self Assessment bill is £1,000 or more and 80% or less 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.