Self Assessment deadlines 2026/27 — the complete guide

Self Assessment runs on a small set of dates that never move, and almost every horror story starts with one of them being discovered too late. Here is the full calendar for the 2025/26 tax year (filed in the 2026/27 cycle), what each date means, and the penalties attached to it.

5 October 2026 — register

If you became self-employed, started renting out property, or otherwise gained untaxed income during the 2025/26 tax year and haven't filed before, you must tell HMRC by 5 October 2026. Registration gets you a Unique Taxpayer Reference (UTR), which can take a couple of weeks to arrive — and you cannot file without it. Registering late doesn't itself trigger a penalty if the tax still gets paid on time, but late registration compresses everything else dangerously.

31 October 2026 — paper returns

The deadline for filing on paper. Almost nobody should be doing this: the online deadline is three months later, online returns calculate the tax for you, and paper returns are processed slowly. If you're reading a web page about deadlines, file online.

31 January 2027 — the big one

Three things happen at once: the online filing deadline, the balancing payment for 2025/26, and the first payment on account for 2026/27 (if payments on account apply to you). Miss the filing deadline by a single day and HMRC charges £100 automatically — even if you owe nothing or are due a refund.

After three months, daily £10 penalties start (up to £900). At six months, the greater of £300 or 5% of the tax due is added; at twelve months, the same again. Late payment is penalised separately: 5% of unpaid tax at 30 days, another 5% at six months, another at twelve — plus daily interest throughout.

31 July 2027 — second payment on account

If your last bill was over £1,000 and less than 80% of your tax is collected at source, HMRC asks for advance payments on the current year: half on 31 January, half on 31 July. Each is based on last year's bill. If your income has dropped, payments on account can be reduced by election — legitimately and easily — but reduce them too far and interest applies to the shortfall.

The dates people actually miss

• 5 October registration — first-year self-employed people simply don't know it exists.
• 31 January as a payment deadline — filers who file in April still must pay by 31 January.
• 31 July — six months after the January scramble, when nobody is thinking about tax.
• The 60-day CGT deadline when selling a rental property — separate from all of the above.

The fix

File early — filing in May doesn't mean paying in May; the payment deadline stays 31 January regardless, so early filing just means you know the number eight months in advance. And if you're already late: file now. Every penalty tier above is triggered by time, and filing stops the filing clock the moment it lands.