Missed the Self Assessment Deadline — What Happens and What to Do Now

Quick answer

The moment you missed the 31 January midnight deadline, HMRC applied a £100 fixed penalty to your account. You cannot undo that. What you can do is stop the situation getting worse. The daily penalties of £10 per day kick in at 3 months late and run for up to 90 days — that is another £900 if you do nothing until the end of April. After 6 months the penalty increases again, and at 12 months it increases once more with potentially more severe consequences if HMRC considers the failure deliberate.

The most important thing to do right now is file your return, even if you cannot pay the tax. Filing late is penalised — but the filing penalties and the payment penalties are separate. Filing stops the filing penalty clock. Then you can deal with the tax owed through a payment arrangement.

The full penalty schedule: exactly what HMRC charges

Filing penalties

HMRC's late filing penalty regime for Self Assessment works in escalating stages.

The structure means the total filing penalty alone can reach £1,600 or more before any tax-related charges are considered — and that is before interest starts running on the unpaid tax.

Late payment interest and surcharges

Failing to pay the tax owed by 31 January also triggers separate, parallel charges.

Interest runs from 31 January at Bank of England base rate plus 2.5 percentage points. At current rates (May 2026) this is approximately 6.75% per year. Interest accrues daily on the outstanding balance.

In addition, HMRC applies late payment penalties at percentage points of the outstanding tax:

These surcharges are separate from the filing penalties and are based on the tax that remains unpaid at each trigger date.

A worked example: 8 months late with £5,000 owed

Suppose your 2024/25 Self Assessment return was due 31 January 2026 and you file and pay in September 2026 — 8 months late — with £5,000 of tax outstanding.

Filing penalties:

• Day 1 penalty: £100
• Daily penalties (£10 × 90 days, from 1 May to 29 July): £900
• 6-month penalty (higher of £300 or 5% of £5,000 = £250): £300
• Total filing penalties: £1,300

Late payment interest:

• £5,000 at approximately 6.75% for 8 months (8/12 of a year): £225

Late payment surcharges:

• 5% surcharge at 30 days (1 March): 5% × £5,000 = £250
• 5% surcharge at 6 months (31 July): 5% × £5,000 = £250
• Total surcharges: £500

Total additional cost: £1,300 + £225 + £500 = £2,025 on top of the £5,000 tax owed.

By the 8-month point, the additional charges represent over 40% of the original tax bill. Leaving it to the 12-month mark would add a further £300 filing penalty and another £250 payment surcharge — bringing the total addition to roughly £2,575, or more than half the tax again.

What to do right now — in order

Step 1: File your return immediately

Do not wait until you can pay. Do not wait until you have assembled every document perfectly. A return filed with reasonable best estimates, flagged as approximate, is better than a return filed another month later because you were waiting for a P60 you could not find.

Filing stops the filing penalty clock. The £100 is already charged and cannot be undone, but every day you delay beyond the 3-month stage costs a further £10.

To file online: log into your Government Gateway account at www.gov.uk/log-in-file-self-assessment-tax-return. If you have never filed before or have forgotten your UTR or Gateway credentials, HMRC's helpline is 0300 200 3310.

Step 2: Pay what you can, immediately

Even a partial payment helps. Interest and payment surcharges are calculated on the outstanding balance — if you pay £2,000 of a £5,000 bill, the 5% surcharge at 30 days is 5% of £3,000 (£150), not 5% of £5,000 (£250). Every pound you pay reduces the interest and surcharge base.

Step 3: Set up a Time to Pay arrangement if you cannot pay in full

If you cannot pay the full amount, do not ignore the debt. HMRC's Time to Pay scheme lets you spread payments over up to 12 months. While interest still accrues on the balance, no additional late payment penalties are charged while you are actively in a payment plan and keeping to it.

Online self-serve (simplest route): If you owe less than £30,000 in total and the filing deadline was less than 60 days ago, you can set up a Time to Pay arrangement online at gov.uk without calling HMRC. You will need your Government Gateway credentials and your Self Assessment reference.

Telephone: If you owe more than £30,000, or you are outside the 60-day online window, call HMRC's payment helpline on 0300 200 3822. Have your UTR, the amount owed, and details of your income and outgoings ready. HMRC will want to understand why you cannot pay and may ask for evidence of financial hardship.

What HMRC considers in a payment plan request:

• Your income and essential expenditure
• Any savings or assets that could cover the debt
• Whether you have taken any other action to raise funds (for example, seeking a loan)
• Whether you have a history of late payment

HMRC is generally willing to agree payment plans for taxpayers who engage proactively. Taxpayers who wait until HMRC chases them receive a more difficult response.

Step 4: Consider appealing the £100 penalty (and daily penalties)

You cannot appeal a penalty that has not yet been formally issued. HMRC issues penalty notices by post — you typically receive the £100 notice within a few weeks of the deadline, and the daily penalty notice after the 3-month mark. Once you have the notice, you have 30 days to appeal.

You can appeal via:

• Online: www.gov.uk/tax-appeals
• By letter: write to the HMRC office shown on the penalty notice, addressed to the Self Assessment department

Your appeal must state that you have a reasonable excuse and explain what it is clearly and honestly. You should also include any documentary evidence (medical certificates, death certificates, GP letters, evidence of IT failure).

What counts as a "reasonable excuse"

HMRC's standard is that a reasonable excuse is "something that stopped you meeting a tax obligation that you took reasonable care to meet." In practice, the following are regularly accepted:

Accepted by HMRC:

• Death of a close family member or domestic partner close to the filing deadline
• Serious or life-threatening illness of the taxpayer (hospital admission, incapacitation)
• HMRC system failure on the filing date itself (HMRC maintains records of these events)
• Unexpected and unforeseeable postal disruption preventing a paper return reaching HMRC on time
• Significant IT failure or hardware loss, provided you have evidence you attempted to file
• Serious domestic emergency (fire, flood, burglary) immediately before the deadline

Not generally accepted:

• "I forgot" or "I didn't know about Self Assessment"
• Being too busy with work
• Relying on an accountant who failed to file (though in some cases where you took reasonable steps to engage an accountant and they failed you, this can succeed at appeal)
• Financial difficulty alone — HMRC treats the inability to pay differently from the inability to file
• Not having your P60 or other documents ready
• Being new to Self Assessment (HMRC says you are responsible for understanding your obligations)

What HMRC's statistics show

HMRC does not publish precise appeal acceptance rates, but practical experience and Freedom of Information data suggest that first-stage appeals are accepted in roughly 30 to 40% of cases where a genuine excuse is presented. Second-stage appeals to the Adjudicator or Tax Tribunal succeed less often, but a well-presented case with clear documentation has a reasonable prospect.

Even if you do not think you have a perfect excuse, it is worth appealing. Worst case, HMRC refuses and you pay the penalty. Best case, you save £100 to £1,300 in filing penalties alone.

What if no tax is owed — or HMRC owes you a refund?

The £100 fixed penalty applies regardless. This surprises many people, but HMRC's penalty regime is tied to the obligation to file, not to the existence of a tax liability.

The percentage-based penalties — the 5% of tax surcharges at 6 months and 12 months — only apply where there is an outstanding tax liability. If you owe nothing or are owed a refund, you will not face those larger charges.

The daily penalties of £10 per day after 3 months also apply regardless of tax owed — they are filing penalties, not payment penalties.

In practical terms, if you filed late but owe nothing, your maximum exposure is:

• £100 fixed penalty
• Up to £900 in daily penalties (if you are 3-6 months late)

If you are in a refund position, HMRC will apply the penalty against any refund due. You may receive less than expected from your refund, or be asked to pay the penalty separately.

Non-filers: HMRC's powers to investigate and assess

Simply not filing does not mean the tax disappears or that HMRC will not find out. HMRC has several mechanisms to identify non-filers:

Data matching: HMRC receives data from employers, banks, HMRC's own systems (RTI payroll data), rental income databases, estate agents, land registry transactions, and since 2024, digital platform operators (letting platforms, gig economy platforms, online marketplaces). If your income appears in these data sources and no return is filed, the discrepancy is flagged.

Nudge letters: HMRC routinely issues "nudge letters" to people it suspects should be filing but are not. If you receive one, respond to it — acknowledge it and contact HMRC proactively. Ignoring a nudge letter substantially increases the likelihood of a formal investigation.

Discovery assessments: HMRC can issue a "discovery assessment" to charge tax it believes you owe, even if you never filed. The time limits are:

• 4 years from the end of the tax year for innocent errors
• 6 years for careless errors or failure to take reasonable care
• 20 years for deliberate evasion or fraud

The 20-year window means that a taxpayer who had substantial undeclared income a decade ago is not safe simply because time has passed.

Formal investigation: HMRC can open a compliance check (formerly called an enquiry) at any time while a return is open (or if no return was filed). At the extreme end, HMRC can refer cases for criminal prosecution, though in practice this is reserved for large-scale or deliberate evasion.

What if you're newly self-employed and this is your first return?

If you only recently became self-employed — for example, you left employment in 2024/25 and started working for yourself — you may not have registered for Self Assessment in time to file by 31 January 2026. Or you may have registered but found the process confusing.

HMRC's first response to a new self-employed person who files late is typically a nudge letter or the standard £100 penalty notice, rather than immediate escalation. The important thing is to:

1. Register for Self Assessment at www.gov.uk/register-for-self-assessment if you have not already.
2. File your return as soon as possible after registration.
3. Be honest about your income — HMRC takes a more sympathetic view of first-time filers who come forward voluntarily than those who ignore the system.

If you are newly self-employed and unsure what you owe, use a tax calculator to get an estimate before your return is complete:

Hardship: when HMRC will defer or waive

HMRC has a limited but real discretion to defer or in exceptional cases write off tax debt in cases of genuine financial hardship. The standard is high — HMRC expects you to have exhausted other options (savings, borrowing, sale of assets) before agreeing to hardship relief.

If you are facing serious financial difficulty:

1. Contact HMRC as early as possible — proactive contact is treated more favourably.
2. Be prepared to provide detailed information about your income, expenditure, assets, and liabilities.
3. If HMRC's payment helpline cannot help, consider whether you qualify for free debt advice from Citizens Advice, StepChange, or the Money Advice Service.

HMRC can suspend debt recovery action while it reviews a hardship claim, which stops the 5% surcharges from applying during the review period.

What about payments on account?

Many Self Assessment taxpayers are also required to make payments on account — advance payments towards the following year's tax bill, due on 31 January and 31 July each year.

If you missed the 31 January deadline, you may have missed both the balancing payment for 2024/25 and the first payment on account for 2025/26.

Late payment interest applies to each of these separately. If you cannot pay the full amount, state explicitly to HMRC which element you are paying against — otherwise HMRC may allocate payments in a way that maximises the interest they can charge.

If your 2025/26 income is expected to be significantly lower than 2024/25, you can apply to reduce your payments on account to a lower level. This is done by completing form SA303 or by applying via your online Self Assessment account. If you reduce them too much and it turns out 2025/26 income was similar to the prior year, HMRC will charge interest on the shortfall.

The tax owed: estimate it now

If you are still in the process of gathering your documents and are not sure what you owe, an estimate is better than nothing. Use HMRC's Self Assessment calculator or a reliable third-party tool to get a working figure:

For self-employed income where Class 4 National Insurance also applies:

Having an estimate lets you:

• Make a partial payment now to reduce interest accrual
• Set up a Time to Pay arrangement before you have the final figures
• Appeal a surcharge if it turns out you overstated the liability

Summary of immediate actions

The longer the delay, the higher the total cost. Filing today, even imperfectly, is significantly better than filing next month with perfect accuracy.

Sources

• HMRC: Self Assessment penalties for late filing and payment
• HMRC: Time to Pay Self Assessment
• HMRC: Appeal a Self Assessment penalty
• Finance Act 2009, Schedule 55 (late filing penalties)
• Finance Act 2009, Schedule 56 (late payment penalties)
• HMRC: Interest rates on late payments (May 2026)