Self Assessment has one deadline everyone’s heard of, 31 January, and several others that catch people out precisely because nobody talks about them. If you’re filing for the 2025/26 tax year (the one running 6 April 2025 to 5 April 2026), here’s the full calendar, and what genuinely happens if you’re late.
The 2025/26 deadlines at a glance
| Deadline | Date | What it’s for |
|---|---|---|
| Register for Self Assessment | 5 October 2026 | If this is your first year needing to file, or you stopped filing and now need to again |
| Paper tax return | 31 October 2026 | Only relevant if you’re filing on paper, not online |
| Online tax return | 31 January 2027 | Filing deadline for the vast majority of people, who file online |
| Balancing payment | 31 January 2027 | Any tax owed for 2025/26, regardless of whether you filed on paper or online |
| Second payment on account | 31 July 2026 | For anyone required to make payments on account (see below) |
Two things worth flagging on the online deadline specifically. It’s 11:59pm on 31 January, not just “some time that day.” And if you want any tax you owe (under £3,000) collected automatically through next year’s PAYE tax code rather than paid as a lump sum, you need to file by 30 December 2026, a full month earlier than the standard deadline.
Registering by 5 October: the deadline people forget
If 2025/26 is the first tax year you’ve needed to file a return (you started self-employment, began renting out a property, or your circumstances otherwise changed), you need to tell HMRC by 5 October 2026, not just file by January. Miss this and HMRC will still expect a return, but you may also be issued a fresh registration deadline (typically three months from the date they write to you), which pushes everything else back and adds unnecessary stress right before the January crunch.
Payments on account: the deadline that isn’t really “the deadline”
If your Self Assessment bill was over £1,000 last year, and less than 80% of your tax was already collected at source (for example through PAYE), HMRC usually asks you to make “payments on account”: advance instalments toward the year you’re currently in, paid in two halves, 31 January and 31 July.
Each instalment is normally half of last year’s total tax bill. So the 31 January deadline often does double duty: you’re settling any balance owed for the year just finished, and paying the first instalment toward the year ahead, in the same payment. That combination is exactly why January bills can look larger than people expect. It’s not a mistake, it’s two different things landing on the same date.
If you’re confident this year’s profits will be noticeably lower than last year’s, payments on account can be reduced. Reduce them too far, though, and HMRC will charge interest on the shortfall once your actual bill is known, so it’s worth getting this right rather than guessing.
What happens if you miss a deadline
For the 2025/26 return, the traditional Self Assessment penalty structure still applies (more on an upcoming change below).
Miss 31 January, even by a day, and you get an automatic £100 penalty. This applies even if you don’t owe any tax, or you’ve already paid what you owe in full. Filing on time is a separate obligation from paying on time. Still not filed after three months? Daily penalties of £10 a day kick in, up to a maximum of £900. Still not filed after six months, and there’s a further penalty of 5% of the tax due, or £300, whichever is higher. Still not filed after twelve months, and it happens again: another 5% of tax due, or £300, whichever is higher, and in serious cases more.
Late payment is penalised separately from late filing: 5% of the unpaid tax at 30 days overdue, again at 6 months, and again at 12 months, with interest charged throughout on top. HMRC’s late payment interest rate is tied to the Bank of England base rate plus a margin and moves when the base rate does, so rather than quote a figure that might already be stale by the time you read this, check the current rate directly on gov.uk before relying on it for a calculation.
A change worth knowing about for next year
The penalty structure above is what applies to the return you’re filing now. From 6 April 2027, HMRC is extending its newer points-based penalty system (until now only used for people who’ve moved onto Making Tax Digital for Income Tax) to everyone filing a personal Self Assessment return. It’s a different mechanism: points accumulate for lateness and convert to a penalty once you cross a threshold, rather than the escalating fixed and percentage penalties above. Late-payment penalty rates are also set to rise from April 2027. It won’t affect the return due this coming January, but it will affect the one after, so don’t assume next year works the same way this year does. If you’re already within Making Tax Digital for Income Tax because your income is over £50,000, our Making Tax Digital guide covers how the points system already works for you.
Keeping ahead of it
The single most effective thing you can do is not treat January as one task. Registering, filing and paying on time are three separate obligations with their own consequences for missing them, and payments on account mean the amount due on 31 January often isn’t just “this year’s tax.” If you’d rather hand the calendar-watching to someone else, our tax services cover how we handle Self Assessment start to finish. Or just get in touch and we’ll make sure nothing sneaks up on you this year.
Last reviewed: 19 August 2026. This is general guidance, not personalised advice. Rules and figures may have changed since publication, so please check with us before acting on it.
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