The 31 January 2027 deadline, in one paragraph
By midnight on 31 January 2027 you must (a) have filed your online Self Assessment return for the 2025/26 tax year, (b) paid any balancing tax owed, and (c) paid your first payment-on-account for 2026/27 if your previous bill was over £1,000. Miss any of these and HMRC charges automatic penalties plus daily-compounding interest on unpaid tax — currently around 7.75% per year at Bank of England base rate plus 2.5 percentage points.
The penalty escalation schedule
HMRC's penalty structure is designed to keep getting worse:
- 1 February 2027 — automatic £100 penalty even if no tax is owed
- 1 May 2027 (3 months late) — daily penalties of £10/day, up to £900
- 1 August 2027 (6 months late) — further 5% of tax due, or £300 if greater
- 1 February 2028 (12 months late) — another 5% or £300, plus potential "deliberate" tagging which can push penalties to 100% of tax owed
Interest accrues separately from penalties and applies from the original due date until the full amount is paid.
If you can't pay by 31 January
File the return anyway — the penalty for late filing is separate from the penalty for late payment. Once filed, you can use HMRC's Time to Pay arrangement (online for debts under £30,000, by phone for larger) to spread the bill over up to 12 months. Interest still accrues but the late-payment surcharge can usually be avoided.
Authoritative reference: gov.uk Self Assessment deadlines and gov.uk: if you cannot pay your tax bill on time.
The five checks worth doing before you press submit
- Use Side Hustle and Self Assessment Checker if you are still not sure whether HMRC expects the return at all.
- Re-check Adjusted Net Income if Gift Aid, pension top-ups or family support thresholds are involved.
- Use CGT on shares if disposals outside wrappers are part of the return.
- Use Sole Trader Tax Estimator if the pressure point is profit, reserves or balancing-payment cashflow.
- Check the official deadline page on GOV.UK for the final filing and payment dates.
Official deadline anchor
GOV.UK's Self Assessment deadline page is the source of truth for the online filing deadline, balancing payment and first payment on account date. Use the site tools to sense-check your numbers, then file using HMRC's own system or your accountant's process.
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