Self Assessment looks like a single 31 January deadline, but it is really a sequence: register a new client in time, file on the right date for their filing method, and make the right payments on the right days. Miss any of them and the penalties stack quickly — and some, like the failure-to-notify penalty, are easy to trigger without realising. This guide sets out the full Self Assessment calendar, the penalty ladders, and what changes as Making Tax Digital arrives.
The Self Assessment calendar at a glance
All dates below relate to a tax year that ends on 5 April. For the 2025/26 tax year (ended 5 April 2026):
| Deadline | What's due |
|---|---|
| 5 October 2026 | Register for Self Assessment if newly within scope (notify chargeability) |
| 31 October 2026 | Paper tax return |
| 30 December 2026 | Online return if you want tax collected through your PAYE code (under £3,000 owed) |
| 31 January 2027 | Online tax return; balancing payment; first payment on account for 2026/27 |
| 31 July 2027 | Second payment on account for 2026/27 |
Registration: the 5 October trap
Anyone who becomes chargeable to tax for the first time must tell HMRC by 5 October following the end of the tax year. This catches new sole traders, new landlords, and people who have crossed an income threshold. Missing it is a failure to notify, and the penalty is calculated as a percentage of the tax due — higher if HMRC prompts the disclosure than if it is unprompted, and higher again if the failure is deliberate. Registering on time is the cheapest compliance step in the whole cycle.
Paper vs online filing
Paper returns are due 31 October; online returns get three extra months to 31 January. Almost everyone files online, but the 31 October date still matters: if a client insists on paper and misses it, the £100 penalty applies even though the online window is still open. Switching that client to online filing before 31 October avoids the penalty entirely.
Payments on account
Payments on account are advance instalments towards the next year's bill, each 50% of the prior year's liability, due 31 January and 31 July. They surprise clients in their first year of higher income because they effectively pay 150% of one year's tax in a single January. Where income has fallen, a claim to reduce payments on account can be made — but reduce too far and HMRC charges interest on the shortfall. Payments on account are not required where the prior-year liability was under £1,000 or where more than 80% of tax was collected at source.
Late filing penalties
The late filing penalty ladder is fixed and applies regardless of whether any tax is actually owed:
| How late | Penalty |
|---|---|
| 1 day late | £100 fixed (even if no tax is due) |
| 3 months late | £10 per day, up to 90 days (max £900) |
| 6 months late | The greater of £300 or 5% of the tax due |
| 12 months late | A further £300 or 5% — higher still if information is deliberately withheld |
The penalties are cumulative: a return that is 12 months late can attract £1,600 or more before any tax or interest. Filing the return — even with estimated figures, to be corrected later — stops the daily penalties running.
Late payment penalties and interest
Late payment is penalised separately from late filing. A 5% surcharge applies to tax still unpaid 30 days after the due date, again at 6 months, and again at 12 months. On top of the surcharges, late-payment interest runs daily from the due date at the Bank of England base rate plus a statutory margin (7.75% from early 2026), and interest cannot be appealed. The order of operations after a missed deadline matters: file first to stop the filing penalties, then pay or arrange Time-to-Pay to limit the payment surcharges.
Time-to-Pay
A client who cannot pay can spread the bill with a Time-to-Pay arrangement. For Self Assessment debts up to £30,000 this can be set up online without speaking to HMRC, provided returns are up to date and the plan is requested within 60 days of the payment deadline. Time-to-Pay stops further late-payment penalties accruing while the plan is honoured, but interest continues to run.
Appealing a penalty
A penalty can be appealed if the client has a reasonable excuse — something that stopped them meeting an obligation despite taking reasonable care, such as serious illness or a bereavement. Lack of funds or reliance on someone else is generally not accepted. Appeals use form SA370 (or online) and must normally be made within 30 days of the penalty notice; the excuse must have existed at the deadline and the obligation met without unreasonable delay once it ended.
What Making Tax Digital changes
From 6 April 2026, Making Tax Digital for Income Tax (MTD ITSA) applies to self-employed people and landlords with qualifying income above £50,000. Those clients move to quarterly digital updates plus a final declaration that replaces the Self Assessment return. The 31 January payment dates are unchanged — but the filing rhythm changes completely. We cover this in detail in the MTD ITSA complete guide.
Guides in this series
This guide is the hub for our detailed Self Assessment articles:
Detailed guides in this series are publishing through 2026 — check back soon, or use the deadline checker below in the meantime.
PenaltyProof tracks each client's Self Assessment registration, filing and payment dates — and the rest of their obligations — and alerts you in good time. Try Starter (£29/month, up to 50 companies) free for 30 days with advance alerts 30 days, 14 days, and 7 days before each deadline, plus due-date and overdue alerts. Cancel any time during the trial.
Start 30-day free trial →or monitor free for up to 5 companies with Companies House monitoring (no card).
This is general information, not advice — check the current GOV.UK guidance, or a client's specific position with a qualified accountant, before acting.
Sources
GOV.UK — Self Assessment tax returns: deadlines: https://www.gov.uk/self-assessment-tax-returns/deadlines
GOV.UK — Self Assessment: penalties for late filing and payment: https://www.gov.uk/self-assessment-tax-returns/penalties
GOV.UK — Understand your Self Assessment tax bill (payments on account): https://www.gov.uk/understand-self-assessment-bill/payments-on-account
GOV.UK — If you cannot pay your tax bill on time (Time to Pay): https://www.gov.uk/difficulties-paying-hmrc