5 October — register if you have to
If you became self-employed, started receiving rental income or had untaxed income during the previous tax year (6 April to 5 April), you must register for Self Assessment by 5 October following that tax year.
Miss this and HMRC can charge a failure-to-notify penalty.
31 October — paper return deadline
If you file on paper (almost nobody does any more), the return is due by 31 October. After that, you must file online.
31 January — the big one
The 31 January deadline covers three things in one:
- Your online Self Assessment return for the tax year ended the previous 5 April
- Any balancing tax due for that tax year
- Your first payment on account for the current tax year (if applicable)
An automatic £100 penalty applies if you're a day late — even if no tax is owed. More penalties stack up the longer you wait.
Knowing your tax bill months in advance lets you save for it. IGI files clients' returns early and tells you exactly what to pay by 31 January so there are no surprises.
31 July — second payment on account
If you make payments on account, the second one is due by 31 July. These are advance payments towards the current tax year's bill, based on the previous year's liability.
If you know your current year's income will be lower, you can ask HMRC to reduce the payment on account — but be careful: under-claim and you'll pay interest.
The penalty ladder
| How late | Penalty |
|---|---|
| 1 day | £100 automatic penalty |
| 3 months | Additional £10 per day for up to 90 days |
| 6 months | Greater of £300 or 5% of tax due |
| 12 months | Greater of £300 or 5% (more if HMRC believes you withheld information) |
Plus interest on unpaid tax from the original due date.
How to avoid every one
- Register the moment you start — don't wait for 5 October the following year.
- Keep tidy bookkeeping all year — not just in January.
- Get your return filed early — ideally by October — so you know what you owe by Christmas.
- Set up a tax savings pot — transfer a percentage of every payment into a separate account.
- Use an accountant — the fee usually saves you more in claimed expenses than the cost.
Frequently asked questions
You get an automatic £100 penalty the day after, even if no tax is owed. Further penalties and interest stack up the longer you delay.
Yes, if you have a "reasonable excuse" — serious illness, bereavement, technical failure or similar. HMRC won't accept "I forgot" or "I was busy."
Generally yes, if your last tax bill was over £1,000 and less than 80% was collected via PAYE. There are exceptions — your accountant will confirm.