Self Assessment penalties (and how they escalate)

How late-filing penalties stack

The architecture is automatic and cumulative — and it does not care whether you owe HMRC anything (GOV.UK).

• Immediately after 31 January: a £100 fixed penalty. It applies whether the tax owed is £0, £100, or £10,000.
• Three months late: £10 per day, for up to 90 days, capped at £900. That is on top of the original £100.
• Six months late: a further £300 or 5% of the tax due — whichever is higher. The 5% calculation only beats £300 once the tax owed exceeds £6,000.
• Twelve months late: another £300 or 5%, on the same basis. In serious cases — deliberate withholding — the penalty can go materially higher.

The reason "I owe nothing, so it doesn't matter" is the most common — and most expensive — Self Assessment myth is that the entire late-filing schedule above is computed without reference to your tax bill. £1,300 of penalties is possible on a return that owes £0.

How late-payment penalties stack (separate from filing)

Late payment is a parallel regime. It runs on top of the late-filing schedule, and it only triggers if there is tax to pay (GOV.UK).

• 30 days late: 5% of the unpaid tax.
• Six months late: a further 5% of the still-unpaid tax.
• Twelve months late: another 5%.
• Interest accrues on the unpaid tax from 1 February throughout, at HMRC's published late-payment rate (Bank of England base + 4 percentage points — around 9%+ in 2026).

The interest line is the one that quietly does the most damage on larger bills. A £20,000 unpaid balance at 9% interest is £1,800 in a year of HMRC interest charges alone — separate from the 5% × three surcharges that can sit alongside it.

"Reasonable excuse" appeals — what HMRC will and won't accept

The penalties are automatic, but they are not always final. If something unexpected and serious stopped you meeting the deadline — and you took reasonable care to comply — you can ask HMRC to cancel the penalty under the "reasonable excuse" rules (GOV.UK — appeal a Self Assessment penalty).

Examples HMRC has accepted include serious illness preventing you from filing, the recent bereavement of a close relative, a fire or flood damaging records, postal disruption, and HMRC's own systems being unavailable around the deadline.

Excuses HMRC does not typically accept: not having the money to pay, finding the online system too difficult, relying on someone else to file (e.g. an accountant who missed the deadline), forgetting, or simply being too busy. Reliance on a third party is generally only accepted as a reasonable excuse where you took proper steps to oversee them and the failure was outside your control.

The appeal route is straightforward. You appeal within 30 days of the penalty notice — through your HMRC online account or in writing — setting out the excuse and any evidence. If HMRC rejects it, you can ask for an internal review by an independent officer, and if needed take the case to the First-tier Tribunal (Tax). The penalty does not have to be paid while a valid appeal is open.

Time to Pay arrangements — the route when you can't pay in full

If you can file but can't pay the bill by 31 January, the route is Time to Pay — HMRC's instalment scheme (GOV.UK — if you can't pay your tax bill).

• Up to £30,000 of Self Assessment debt can be arranged online, through your HMRC account, without speaking to anyone. The plan typically spreads the bill over up to 12 months.
• Above £30,000, you call HMRC's Self Assessment Payment Support Service and arrange the plan directly.
• You must have filed the return before HMRC will set up a Time to Pay plan online. That is the single most important reason to file even when you can't pay.
• While the plan is in place and being paid on time, no further late-payment penalties are added. Interest, however, still runs on the outstanding balance.

The cleanest sequence is: file by 31 January, then immediately set up Time to Pay. That avoids the £100 fixed penalty, stops the daily penalties stacking, and pauses the 5% surcharges from biting at 30 days. Only interest keeps running.

Late-payment interest — what it actually costs

HMRC charges interest on unpaid Self Assessment tax from 1 February. The rate is set at the Bank of England base rate plus 4 percentage points — in 2026 that has translated to a rate around 9%+, and it moves whenever the Bank moves base rate. Always verify the current HMRC rate on gov.uk before relying on a specific number.

Interest is not appealable on the same "reasonable excuse" basis as penalties. It is statutory and runs throughout. A Time to Pay arrangement does not stop interest — it stops further penalties being added while you pay. That distinction is the single most-missed point in the regime.

Related reading in the Self Assessment series

• Self Assessment: the 2025/26 guide — the full pillar: deadlines, who files, the MTD transition.
• Self Assessment deadlines and key dates — every date that matters and why filing early is cheaper than filing on time.
• Payments on account: why your first bill can feel double — the 50% advance payments that catch first-time filers.