A Reddit post on r/HMRC recently captured the panic that hits when the penalty letters start arriving: “I filed 4 months late and I’ve got a £100 penalty, a £900 daily penalty, and now a £300 six-month penalty. That’s £1,300 on top of a £2,000 tax bill. I literally can’t afford this. What do I do?”
The comments were a mix of “just pay it” and “appeal everything” — but nobody laid out the full picture: which penalties are negotiable, which aren’t, when interest stops, and when a payment plan is better than an appeal. The poster didn’t know that a Time to Pay arrangement would suspend the late payment surcharges, or that the late filing penalties could be appealed if they had a genuine reason for being late.
This guide is the single reference for the entire Self Assessment penalty system. It covers the full penalty ladder, how interest compounds, what HMRC accepts as a reasonable excuse, and the decision tree for whether to pay, appeal, set up a payment plan, or do nothing.
Self Assessment Penalties: The Complete Penalty Ladder
There are two separate penalty regimes that run independently. You can be hit by one, both, or neither, depending on whether you filed late, paid late, or both.
According to HMRC’s Self Assessment penalties guidance, here’s the full ladder in one place:
| When | Late filing penalty | Late payment penalty |
|---|---|---|
| 1 day late | £100 (automatic, even if no tax owed) | Interest starts at 7.25% |
| 30 days late | — | 5% surcharge on unpaid tax |
| 3 months late | £10/day (up to 90 days = £900 max) | — |
| 6 months late | £300 or 5% of tax due (whichever is higher) | 5% surcharge on unpaid tax |
| 12 months late | £300 or 5% of tax due (whichever is higher) — can rise to 100% for deliberate failure | 5% surcharge on unpaid tax |
To work out your exact penalties and interest based on how late you filed and paid, use our Self Assessment penalty calculator.
The two columns are independent. If you filed on time but paid late, you only face the right-hand column. If you filed late but paid on time, you only face the left-hand column. If both are late, you face both — the worst case.
For the detailed breakdown of each regime with worked examples, see our guide on what happens when you file late.
1. Late Filing Penalties: What Happens When You Miss the Deadline
The late filing penalty is automatic — HMRC doesn’t need to contact you before issuing it. The £100 is charged the day after the 31 January deadline, regardless of whether you owe tax, are due a refund, or have a £0 bill. After 3 months, £10/day is added (up to £900). At 6 and 12 months, a further £300 or 5% of tax due (whichever is higher) is charged.
The key point: the penalty is for failing to file, not for owing tax. You can be penalised £100 even if HMRC owes you money. The only way to stop the clock is to file — do it immediately, even if you can’t pay the tax bill. For the step-by-step on what to do right now, see our missed deadline guide.
2. Late Payment Penalties: Surcharges and Interest
Late payment triggers two separate charges: interest (daily, at 7.25% as of 2025) and surcharges (5% of unpaid tax at 30 days, 6 months, and 12 months). Interest starts on 1 February — the day after the payment deadline — and accrues every day until you pay. Surcharges are additional flat-percentage penalties on top of interest.
On a £6,000 bill paid 12 months late, the surcharges alone total £900 (3 × 5% × £6,000), plus roughly £435 in interest. That’s £1,335 on top of the original bill — more than 22% extra. For the full breakdown of surcharges and interest, see our filed late guide.
3. How Interest Compounds on Unpaid Tax (Separate from Penalties)
This is the piece most people misunderstand. Interest is not a penalty — it’s a charge for the time value of the unpaid tax. It accrues daily, it is not suspended by a payment plan, and it is not appealable (even if your penalties are cancelled, the interest remains).
The current rate is 7.25% (as of early 2025), set at Bank of England base rate plus 2.5%. It changes when the base rate changes, so check HMRC’s current interest rate page for the live figure.
Here’s how interest compounds on a £6,000 bill:
| Month | Starting balance | Interest (7.25%) | Ending balance |
|---|---|---|---|
| 1 | £6,000 | £36.25 | £6,036.25 |
| 3 | £6,108.84 | £36.91 | £6,145.75 |
| 6 | £6,293.64 | £38.03 | £6,331.67 |
| 12 | £6,602.42 | £39.89 | £6,642.31 |
After 12 months, you owe approximately £6,642 — the original £6,000 plus £642 in interest. And that’s before the three 5% surcharges (£900 total). The math is brutal, which is why paying early — or setting up a payment plan early — matters so much. No existing article lays this out as a standalone reference; the surcharges are covered in the spokes, but the interest compounding is unique to this hub.
4. The 4 Categories of Reasonable Excuse HMRC Accepts
According to HMRC’s reasonable excuse guidance, a reasonable excuse must be something that stopped you from meeting your tax obligation, was beyond your control, and that you couldn’t reasonably have foreseen. HMRC groups accepted excuses into four broad categories:
Category 1: Life events beyond your control
- Serious illness or hospitalisation (with medical evidence)
- Bereavement — death of a close relative or partner shortly before the deadline
- Unexpected absence abroad (e.g., a family emergency overseas)
Category 2: External failures
- HMRC system failure — if the online filing system was down on or near the deadline (HMRC usually announces these and may cancel penalties en masse)
- Postal delay — if you filed on paper and posted it before the deadline but Royal Mail delayed it (you need a certificate of posting, not just a receipt)
- Fire, flood, or theft that destroyed your records
Category 3: Reliance on a third party
- You gave your records to an accountant well in advance and they failed to file through no fault of yours (harder to prove — HMRC will want evidence of when you sent records and that you followed up)
Category 4: Accessibility and disability
- Your disability prevented you from filing and HMRC didn’t provide a reasonable alternative
What is NOT accepted: “I forgot”, “I was too busy”, “I didn’t know the deadline”, “I found the form too complicated”, “I didn’t receive a reminder”, “I didn’t have the money” (for late payment — set up a payment plan instead). For the full appeal process, see our appeal HMRC penalty guide.
5. How to Appeal a Self Assessment Penalty
The appeal process is the same whether you’re appealing a late filing penalty, a late payment surcharge, or both. The golden rule: file your return and pay your tax first, then appeal. HMRC will not cancel a penalty for a return that hasn’t been submitted.
You can appeal online through your HMRC Government Gateway account (fastest, 10-15 minutes) or by post using form SA370. You need to explain what happened, when, and why it prevented you from filing or paying on time. If HMRC rejects your appeal, you can request an independent review, then take it to the First-tier Tribunal (free, no lawyer needed).
For the full step-by-step process with example appeal text, see our appeal HMRC penalty guide.
6. Can’t Pay? Time to Pay and Your Other Options
If the issue is affordability rather than a reasonable excuse, appealing won’t help — HMRC’s position is that “I didn’t have the money” is not a reasonable excuse for late payment. The better route is a Time to Pay arrangement.
A Time to Pay arrangement is a formal agreement to spread your tax bill over monthly instalments. If you owe under £30,000, you can set it up online through your HMRC account in minutes. For larger debts, call the Payment Support Service on 0300 200 3835. The key benefit: late payment surcharges are suspended while you’re on the plan. Interest still accrues, but the 5% charges at 30 days, 6 months, and 12 months are paused.
For the full setup guide with worked examples, see our Time to Pay guide. For the broader picture of what to do when you can’t afford your bill — including enforcement risks and what HMRC can do if you ignore them — see our can’t afford tax bill guide.
7. What If You Never Registered for Self Assessment?
A different penalty regime applies if you were never registered for Self Assessment at all. This is called “failure to notify” (Schedule 41, Finance Act 2008) — and it’s fundamentally different from the late filing regime.
Under failure to notify, the penalty is a percentage of the tax that went unpaid because you didn’t tell HMRC in time. If you owed no tax (e.g., your income was below the Personal Allowance after the £1,000 trading allowance), the penalty is mathematically £0 — there’s no unpaid tax for a percentage to apply to. Even if tax was owed, an unprompted, non-deliberate disclosure can be reduced to 0% where HMRC becomes aware within 12 months.
This is why the scary £4,800 figures you see in online penalty calculators are almost always wrong for people who never registered — those calculators use the late filing regime, not the failure to notify regime. For the full breakdown, see our didn’t know you needed to register guide.
8. The Decision Tree: Pay, Appeal, Time to Pay, or Do Nothing
This is the strategy section that no individual spoke covers — the ordering of which lever to pull first.
Did you file your return?
├── NO
│ └── FILE NOW. Don't do anything else until you've filed.
│ Then ask: Do you owe tax?
│ ├── YES → Pay what you can, then choose below
│ └── NO → You still face the £100 late filing penalty, but no late payment charges
│
└── YES
└── Did you pay your tax bill?
├── YES (paid in full)
│ └── You face late filing penalties only. Do you have a reasonable excuse?
│ ├── YES → Appeal (see section 5)
│ └── NO → Pay the penalty or appeal anyway (HMRC sometimes shows first-time leniency)
│
└── NO (can't afford it)
└── Set up a Time to Pay arrangement FIRST (section 6)
This suspends surcharges immediately.
Then ask: Do you also have a reasonable excuse for filing late?
├── YES → Appeal the filing penalties while on the payment plan
└── NO → Pay the filing penalties + the payment planThe ordering matters because:
- Filing stops the late filing clock — the £10/day charges stop the moment you file
- A payment plan stops the surcharge clock — the 5% charges are suspended from the day your plan starts
- Interest never stops — it accrues until the balance is £0, regardless of appeals or payment plans
- Appeals take 45+ days — don’t wait for the appeal outcome before setting up a payment plan; the surcharges keep accruing while you wait
The worst strategy is doing nothing. Every day you delay, the penalties increase. The best strategy is almost always: file immediately, set up a payment plan if you can’t pay in full, then appeal the filing penalties if you have a reasonable excuse.
The Bottom Line
- Two regimes run independently — late filing (£100 → £10/day → £300 or 5%) and late payment (5% surcharges at 30 days, 6 months, 12 months + interest at 7.25%).
- Interest is not a penalty and can’t be appealed — it accrues daily until you pay, even on a payment plan. The 7.25% rate tracks Bank of England base rate + 2.5%.
- A Time to Pay arrangement suspends surcharges — set one up before the 30-day mark to avoid the first 5% charge. Online if under £30,000, phone (0300 200 3835) if over.
- Reasonable excuses fall into 4 categories — life events, external failures, third-party reliance, and accessibility. “I forgot” and “I didn’t have the money” are not accepted.
- File and pay first, then appeal — HMRC won’t cancel a penalty for an unfiled return. Appeal online through your HMRC account or by post with form SA370.
- Never registered? Different regime — failure to notify penalties are based on unpaid tax, not flat fines. If you owed no tax, the penalty is £0.
- The decision tree is: file → pay (or plan) → appeal — in that order. Filing stops the filing clock, a plan stops the surcharge clock, and an appeal may cancel the filing penalties. Interest never stops.
- Doing nothing is the most expensive option — on a £6,000 bill, 12 months of inaction costs £1,335 in surcharges and interest alone.
If you’re facing a large penalty bill or you’re behind on multiple years, an accountant can file outstanding returns, negotiate with HMRC on penalties, and set up a payment plan on your behalf. Find an accountant through our free matching service — mention your penalty situation and we’ll match you with someone who handles HMRC negotiations.
For the full list of Self Assessment deadlines, penalties, and payment options, see our Self Assessment deadlines and payment hub.
For the step-by-step on what to do right now if you’ve missed the deadline, see our missed deadline guide. For the full breakdown of every penalty and interest charge, see what happens when you file late. For how to appeal, see our appeal HMRC penalty guide. For setting up a payment plan, see our Time to Pay guide. For what to do if you can’t afford your bill at all, see our can’t afford tax bill guide. For the failure to notify regime if you never registered, see our didn’t know you needed to register guide. Once you’ve dealt with the penalties, the Relief Files cover fixing mistakes and chasing refunds — or see the complete Self Assessment guide for the full picture.