Why deadlines matter so much
HMRC's self assessment system has hard deadlines, and unlike some areas of tax, missing them triggers automatic penalties with no discretion. You can receive a penalty even if you have no tax to pay, and even if your return is only one day late. The penalty system escalates the longer a return remains unfiled, which means leaving it can become very expensive very quickly.
The good news is that the deadlines are predictable, they fall on the same dates every year. Add them to your diary now, and you'll never be caught out.
Key self assessment dates for 2025/26
The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. Here are all the deadlines you need to be aware of:
| Date | What's due |
|---|---|
| 5 April 2026 Tax year end |
End of the 2025/26 tax year. The period your return will cover. |
| 6 April 2026 | Start of the 2026/27 tax year. HMRC opens the 2025/26 self assessment return for filing. |
| 31 July 2026 Payment |
Second payment on account for 2025/26. This is a payment towards your estimated 2025/26 tax bill, based on 50% of your previous year's liability. |
| 5 October 2026 | Deadline to notify HMRC that you need to complete a self assessment return for 2025/26 (if you have not filed before and have not received a notice to file). |
| 31 October 2026 Paper returns |
Deadline for filing your 2025/26 self assessment return by paper (post). Most people file online, in which case this date does not apply. |
| 30 December 2026 | Deadline to file online if you want HMRC to collect tax owed via your PAYE tax code (only if the amount owed is less than £3,000 and you are employed or receive a pension). |
| 31 January 2027 Key deadline |
Deadline to file your 2025/26 self assessment return online. Also the deadline to pay any tax owed (your balancing payment) and your first payment on account for 2026/27. |
The most important date is 31 January. This is when your return must be filed online AND any tax owed must be paid. Missing either triggers immediate penalties.
What happens if you miss the filing deadline?
HMRC's penalty structure for late self assessment returns is automatic and applies regardless of whether you owe any tax:
- 1 day late: £100 fixed penalty
- 3 months late: £10 per day, up to a maximum of £900 (in addition to the £100)
- 6 months late: A further penalty of 5% of the tax due, or £300, whichever is greater
- 12 months late: A further 5% of the tax due, or £300, whichever is greater (and potentially higher in cases of deliberate non-compliance)
If your return is significantly overdue, total penalties can easily exceed £1,600 before any tax, interest or surcharges are added.
What happens if you miss the payment deadline?
Late payment of tax also attracts penalties, separate from late filing penalties:
- 30 days late: 5% of the tax unpaid
- 6 months late: A further 5% of the tax unpaid
- 12 months late: A further 5% of the tax unpaid
Interest also accrues on unpaid tax from the payment deadline. The current HMRC late payment interest rate is significantly above base rate, so large tax bills left unpaid can accrue substantial interest charges quickly.
What are payments on account?
If your self assessment tax bill exceeds £1,000, HMRC requires you to make advance payments towards your next year's tax, called payments on account. These are made in two instalments: 31 January and 31 July. Each instalment is 50% of your previous year's tax liability.
This can catch first-time filers off guard: in your first year of self assessment, you may face a bill of 150% of your actual tax liability, the balancing payment for the year just ended, plus the first payment on account for the year ahead. Planning your cash flow around this is important.
You can apply to reduce payments on account if you expect your income to be lower in the coming year. Claritax manages this as part of our self assessment service, we calculate whether a reduction is appropriate and apply to HMRC on your behalf.
Do I need to file a self assessment return?
You must file a return if, in the 2025/26 tax year, any of the following apply:
- You were self-employed with income over £1,000
- You were a partner in a business partnership
- You were a company director (unless the company is non-profit)
- You earned over £100,000
- You received untaxed income (rental, savings, dividends above allowances)
- You received Child Benefit and either you or your partner earned over £60,000
- You have capital gains to report
- HMRC has sent you a notice to file a return
If you are unsure whether you need to file, it is always better to check, the penalties for failing to notify HMRC are separate from, and in addition to, filing penalties.
How Claritax helps
As part of our self assessment service, we track all deadlines on your behalf, prepare your return well in advance, and notify you of your tax liability as early as possible so you have time to plan. We also review every return for allowances and reliefs that might reduce your bill, many people overpay simply because they don't know what they can claim.
Don't leave it to January. Book a free consultation now and let Claritax handle your self assessment return, filed accurately, on time, with every allowance checked.
Frequently asked questions
What is the self assessment filing deadline?
31 January following the end of the tax year for online returns, this is also the deadline to pay any tax owed. Paper returns are due earlier, by 31 October.
What's the penalty for filing late?
An automatic £100 fixed penalty applies from 1 day late, even if you owe no tax, followed by £10 a day after 3 months up to a 90-day cap, then further penalties of 5% of the tax due (or £300, whichever is greater) at 6 and 12 months.
Is there a separate penalty for paying late?
Yes, late payment penalties are separate from late filing penalties: 5% of the unpaid tax at 30 days late, a further 5% at 6 months, and a further 5% at 12 months, plus interest accruing on the unpaid balance throughout.
What are payments on account and why do they catch people out?
If your tax bill exceeds £1,000, HMRC requires advance payments towards next year's tax, split across 31 January and 31 July. First-time filers can face a bill of 150% of their actual liability in year one, the balancing payment plus the first payment on account.
Can I reduce my payments on account?
Yes, if you expect your income to be lower in the coming year, you can apply to reduce them. We calculate whether a reduction is appropriate and handle the application to HMRC as part of our self assessment service.