Written and reviewed by James Whitfield · Updated for 2026/27 · Editorial standards · Methodology
Missing a Self Assessment deadline means automatic penalties, even if you owe nothing. This guide covers every deadline in the 2026/27 tax year, what the penalties are and how to appeal them.
The 2026/27 tax year runs from 6 April 2026 to 5 April 2027. After it ends, the key deadlines are: 31 October 2027 for paper Self Assessment returns; 31 January 2028 for online returns; 31 January 2028 for payment of all tax owed for 2026/27 (final balancing payment plus any balancing of payments on account); 31 July 2027 for the second payment on account for 2025/26.
If you are filing your Self Assessment for the first time (for example, if you started self-employment in 2026/27), you must register with HMRC by 5 October 2027 — the October after the end of the tax year you first had income to declare. Late registration can itself incur a penalty.
The 31 January deadline applies to both online filing and the payment due. They share the same date. If you file on time but pay late, you still incur late payment interest and potentially penalties.
Filing your return after 31 January (for online) triggers an automatic £100 penalty, even if no tax is due. After three months late (by 30 April): an additional £10 per day for up to 90 days, capped at £900. After six months late (by 31 July): a further 5% of the tax due or £300 (whichever is greater). After 12 months late (by 31 January of the following year): another 5% of the tax due or £300, with higher penalties if withholding information was deliberate.
The minimum penalty for a late online return is £100 even with zero tax owed. This catches many people who believe that filing is not necessary because they do not owe anything — but once you are registered for Self Assessment, you must file every year until HMRC confirms you are deregistered.
If you have not received a notice to file but should have submitted a return, penalties can still apply from when the notice to file would have been issued. Ignorance of the requirement is not a reasonable excuse.
Late payment of tax triggers interest at HMRC's rate (currently around 7.75%) from the due date. After 30 days late: 5% surcharge on tax unpaid. After 6 months: additional 5% surcharge. After 12 months: another 5% surcharge. These surcharges are on top of the daily interest.
HMRC can also collect penalties and interest through a future tax return — they can adjust future tax codes to recover unpaid amounts from employment income if you also have a PAYE job.
If you cannot pay, contact HMRC before the deadline rather than going silent. They offer Time to Pay arrangements for people who genuinely cannot pay in full. Agreeing a payment plan does not remove the interest, but it can avoid the surcharges and typically prevents debt recovery action.
You can appeal a late filing or late payment penalty if you have a reasonable excuse. Reasonable excuses accepted by HMRC include: serious illness or bereavement of a close family member, a serious computer or software failure, postal delays outside your control, and HMRC system failures.
Not acceptable as a reasonable excuse: 'I forgot', 'I didn't know the deadline', 'my accountant didn't tell me', or cash flow problems (for filing penalties — payment difficulties can be a partial excuse for payment penalties in limited circumstances).
You must appeal within 30 days of the penalty notice. Appeals can be made online through your HMRC online account or by post using form SA370. If your appeal is rejected and you still believe it is unreasonable, you can request a review and ultimately appeal to the First-tier Tax Tribunal.
31 January 2028 for online returns and payment. 31 October 2027 for paper returns. Register by 5 October 2027 if newly self-employed in 2026/27.
An automatic £100 for missing the deadline, plus £10 per day after three months (up to £900), plus further 5% surcharges at six and twelve months.
A prepayment toward the following year's tax bill. You pay two instalments of 50% of the prior year's tax: one on 31 January and one on 31 July.
Yes, within 30 days of the penalty notice. You need a reasonable excuse — illness, bereavement or system failure. Forgetting or not knowing the deadline is not accepted.