Self Assessment 2025/26: every deadline and penalty to know before 31 January 2027
10 October 2026 · 6 min read
January is coming. For accounting firms, the 31 January Self Assessment deadline is still the single busiest point of the year — and for clients, missing it triggers an automatic £100 penalty even if they owe nothing. HMRC issued over 800,000 late-filing penalties for the 2024/25 tax year alone.
This guide sets out every date that matters for the 2025/26 tax year, what the penalties look like, and how Making Tax Digital changes the picture for some clients from this year onwards.
The key dates for 2025/26
| Date | What's due |
|---|---|
| 5 October 2026 | Register for Self Assessment if 2025/26 was the client's first year of self-employment or untaxed income. This date has now passed — if a new client hasn't registered, do it immediately. |
| 31 October 2026 | Paper tax returns. Almost everyone should file online instead — paper filers who miss this date still owe an online return by 31 January, but the paper deadline counts for penalty purposes. |
| 30 December 2026 | File online by this date if the client owes less than £3,000 and wants it collected through their PAYE tax code over the following year. |
| 31 January 2027 | The big one: online return for 2025/26, the balancing payment of any tax owed, and the first payment on account towards 2026/27. |
| 31 July 2027 | Second payment on account for 2026/27, where applicable. |
Payments on account catch clients out
Clients whose last bill was over £1,000 (and who had less than 80% of their tax collected at source, for example through PAYE) must make payments on account — two advance payments towards the current year, each normally 50% of the previous year's bill, due 31 January and 31 July.
The January bill for a newly self-employed client can therefore be 150% of what they expected: the 2025/26 balancing payment plus the first 2026/27 payment on account. Warn first-year clients now, not on 30 January. If their income has dropped, you can apply to reduce payments on account — but over-reducing means interest on the shortfall.
The penalty ladder for late filing
- 1 day late: automatic £100 penalty — even if no tax is owed or the client is due a refund.
- 3 months late: £10 per day, up to 90 days (£900), on top of the £100.
- 6 months late: a further £300 or 5% of the tax due, whichever is higher.
- 12 months late: another £300 or 5% — and in serious cases HMRC can charge up to 100% of the tax due.
Late payment is charged separately: interest from 1 February, plus a 5% surcharge on tax still unpaid at 30 days, 6 months and 12 months. Clients who cannot pay should agree a Time to Pay arrangement with HMRC — it stops the worst of the surcharge clock if set up promptly.
Where MTD fits in
Nothing changes for the 2025/26 return: every Self Assessment client files as normal by 31 January 2027, including those who joined Making Tax Digital for Income Tax in April 2026.
The change comes next year. Clients in MTD (qualifying income over £50,000 for 2026/27) will file a Final Declaration for 2026/27 instead of a Self Assessment return — due by the same 31 January 2028 date. So January 2027 is the last “normal” Self Assessment season for your largest sole trader and landlord clients. See our complete MTD guide for the details — and our guide to the £30,000 cohort joining in April 2027 for the clients who follow them.
Surviving January: a checklist for firms
- Chase outstanding records in November and December, not January. Clients who send documents in the first week of January are the ones who file late.
- Flag first-year clients whose January bill will include payments on account, and warn them of the amount early.
- Identify anyone who should have registered by 5 October and hasn't — get them registered before the return is due.
- File returns as records arrive rather than in deadline order; an early-filed return still has a 31 January payment date, so clients lose nothing.
- Track who has and hasn't sent their records. A client portal with automated reminders does this chasing for you — clients upload documents as they go, and you can see at a glance which returns are ready to prepare.
The bottom line
The 31 January deadline is fixed; the stress is optional. Firms that collect records steadily through the autumn file steadily through the winter — and their clients never meet the £100 penalty.
Frequently asked questions
What is the Self Assessment deadline for the 2025/26 tax year?
The online filing deadline is midnight on 31 January 2027. Paper returns were due by 31 October 2026. Any tax owed for 2025/26, plus the first payment on account for 2026/27, is also due on 31 January 2027.
What happens if I file one day late?
HMRC charges an automatic £100 penalty the day after the deadline — even if you owe no tax or are due a refund. Penalties grow the longer the return is outstanding: £10 a day after 3 months, then further charges at 6 and 12 months.
What are payments on account?
Advance payments towards your current year’s tax bill, required if your last Self Assessment bill was over £1,000 and less than 80% of your tax was collected at source. Each payment is normally half of the previous year’s bill, due on 31 January and 31 July.
I can’t afford to pay by 31 January. What should I do?
File the return on time anyway — filing and payment penalties are separate. Then contact HMRC about a Time to Pay arrangement, which spreads the bill in instalments. Setting it up promptly avoids the 5% late-payment surcharges, though interest still applies.
Does Making Tax Digital change my January 2027 obligations?
No. Even clients who joined MTD for Income Tax in April 2026 still file a normal Self Assessment return for 2025/26 by 31 January 2027. MTD’s Final Declaration replaces the Self Assessment return from the 2026/27 tax year onwards, first due 31 January 2028.