How to Appeal an HMRC Fine: A Step-by-Step Guide to Form SA370

The SA370 is HMRC's formal appeal form for Self Assessment penalties. It is available online through your HMRC Government Gateway account or as a downloadable paper form. The form itself is short — but the section that matters most is the one most people complete incorrectly.

Before you touch the form: two non-negotiables

The return must be filed before HMRC will process your appeal. File first, even on estimated figures. Then appeal.

The appeal must be submitted within 30 days of the date on the penalty notice. Count from the notice date — not from when you received it. If you are outside 30 days, you can still submit a late appeal but you will need to explain the delay, and HMRC has discretion to refuse it.

Step 1 — Access the SA370

Online: log into your HMRC Government Gateway account, navigate to Self Assessment, and select "appeal a penalty" from your account summary. The online version is faster and creates an automatic audit trail.

Paper: download form SA370 from HMRC's website, complete it, and post it to the address shown on your penalty notice.

Step 2 — Complete the penalty reference and year

You will need the penalty reference number from your penalty notice, the tax year in question, and your Unique Taxpayer Reference (UTR). These are all on the notice letter.

Step 3 — Select your grounds (the easy part)

The form gives you a dropdown or checklist of standard excuse categories: illness, bereavement, computer failure, HMRC error, and so on. Select the one that most closely matches your situation. This is not the part that determines success or failure.

Step 4 — Write the grounds of appeal (the hard part)

This is where most DIY appeals fail. The written grounds section is where HMRC's caseworkers assess your case. A sentence or two will almost certainly be rejected. What HMRC needs to see:

• The exact dates the preventing circumstance began and ended — not approximate months, but specific dates relative to the 31 January deadline.
• Why specifically it prevented filing — not that it was difficult, but that it was genuinely impossible or unreasonable to expect.
• What evidence you are attaching and what it confirms.
• When the preventing circumstance resolved and how quickly after that you filed.
• An acknowledgement that you understand the return must be filed before the appeal is processed (this signals you know the process).

Step 5 — Attach evidence

For online submissions: scan or photograph documents and upload them. For paper: attach copies (never originals). The evidence must be specific, dated, and directly relevant to the excuse claimed. A generic GP letter noting you had "health issues during 2025" is significantly weaker than a letter confirming you were admitted to hospital on 24 January and discharged on 3 February.

What happens after you submit

HMRC aims to respond within 45 days. During this period, enforcement action on the appealed penalty should be paused. If HMRC rejects your appeal, you can request an internal review (handled by a different caseworker), and then if that also fails, appeal to the independent First-tier Tax Tribunal — which frequently overturns HMRC rejections where the original grounds were sound.