How to Appeal an HMRC Late Filing Penalty UK 2026

Every year, HMRC issues over 1.1 million penalties to UK taxpayers for filing their Self Assessment tax return late. If you have received one of these penalties, you are not alone — and you may have grounds to appeal.

This guide covers everything you need to know about appealing an HMRC late filing penalty in 2026, including deadlines, what counts as a reasonable excuse, and how to structure your appeal for the best chance of success.

What is the late filing penalty?

If you miss the Self Assessment deadline (31 January for online filing), HMRC automatically issues a £100 penalty — even if you owe no tax or have already paid what you owe. The penalty structure escalates significantly over time:

Day 1: £100 automatic penalty
After 3 months: £10 per day for up to 90 days (maximum £900)
After 6 months: 5% of the tax due or £300, whichever is greater
After 12 months: Another 5% of the tax due or £300, whichever is greater

This means a taxpayer who files 12 months late could face penalties of £1,600 or more, on top of any tax owed. The financial incentive to appeal is significant.

Your 30-day appeal window

You have 30 days from the date on your penalty notice to appeal. This is not 30 days from when you opened the letter or found out about the penalty — it runs from the date HMRC printed on the notice.

If you miss the 30-day window, you can still appeal, but you will need to explain why your appeal is late. HMRC may accept a late appeal if you have a good reason, such as not receiving the original notice or being seriously ill during the appeal period.

What counts as a reasonable excuse?

HMRC will cancel your penalty if you can demonstrate a reasonable excuse — something that genuinely prevented you from filing on time. HMRC publishes its own list of acceptable reasons:

Serious illness or hospitalisation (you or a close relative)
Bereavement of a partner, close family member, or dependent
Fire, flood, or natural disaster affecting your home or records
HMRC website or software failure (documented IT issues near the deadline)
Postal delays (if you were sending a paper return and it was delayed)
HMRC's own error or delay (e.g., sending you the wrong UTR, failing to issue a notice to file)
Mental health conditions that prevented you from managing your affairs
Disability that was not previously taken into account

Crucially, HMRC does not accept these as reasonable excuses: forgetting, being too busy, not knowing about the deadline, relying on someone else (unless they let you down at the last minute through no fault of yours), or not having the money to pay your tax bill.

How to submit your appeal

There are four ways to appeal an HMRC late filing penalty:

Online via Government Gateway: Log in to your Self Assessment account and select “Appeal a penalty”
Form SA370: Complete the official HMRC appeal form and post it to the address on your penalty notice
Formal letter: Write a structured appeal letter citing your reasonable excuse and the relevant legislation
By phone: Call HMRC Self Assessment helpline on 0300 200 3310

A written appeal — whether by letter or through the SA370 form — is generally the strongest approach. It creates a paper trail, allows you to cite specific legislation, and gives HMRC a clear, structured case to assess.

What legislation should your appeal cite?

A strong appeal letter references the specific legal provisions that apply to your penalty. For late filing penalties, the key legislation is:

Finance Act 2009, Schedule 55 — the statutory framework for late filing penalties
Taxes Management Act 1970, s.93 — the original late filing penalty provision
Perrin v HMRC [2018] UKUT 156 — the leading case establishing a structured test for reasonable excuse

The Perrin v HMRC case is particularly important. It established a four-part test: (1) what was the reasonable excuse? (2) when did it arise? (3) when did it end? (4) did the taxpayer file without unreasonable delay after the excuse ended? Addressing all four parts in your appeal significantly strengthens your case.

What happens after you appeal?

HMRC aims to respond within 45 days. There are three possible outcomes:

Appeal accepted: The penalty is cancelled in full. You receive a letter confirming this.
Appeal rejected: HMRC disagrees that you had a reasonable excuse. You can then request a statutory review.
Statutory review: A different HMRC officer reviews your case independently. If they also reject it, you can appeal to the First-tier Tribunal (Tax Chamber), which is free and completely independent of HMRC.

Tips for a successful appeal

Be specific: Include dates, reference numbers, and concrete details about what happened
Provide evidence: Medical letters, hospital appointment confirmations, death certificates, screenshots of error messages — anything that supports your case
Show you acted quickly: Demonstrate that you filed your return as soon as the obstacle was removed
Cite the law: Referencing the correct legislation shows HMRC your appeal is serious and well-informed
Keep it professional: A calm, structured letter is far more effective than an emotional complaint