To appeal a Self Assessment late filing or late payment penalty as an agent, use form SA370 by post for an individual, or SA371 for a late partnership return. The online route runs through the client’s own HMRC sign-in. Either way, the appeal should reach HMRC within 30 days of the date on the penalty notice.
SA370 is HMRC’s paper form for appealing Self Assessment late filing and late payment penalties. An agent can complete and sign it for a client, entering the client’s details and stating “agent” as the signing capacity, then post it to HMRC at BX9 1AS within 30 days of the penalty notice date, with a dated reasonable excuse and evidence.
Key points
- GOV.UK offers two routes: appeal online or download SA370 or SA371 to appeal by post.
- The usual time limit is 30 days from the date the penalty was issued. Late appeals need a reason for the extra delay.
- Clients who joined MTD for Income Tax in April 2026 still face the existing penalty rules on the 2025 to 2026 return due 31 January 2027.
SA370: the appeal form and the penalty notice
The reference SA370 is used for two different documents, which confuses clients. SA370 the appeal form is the two-page form, “Self Assessment: Appeal against penalties for late filing and late payment”, that you fill in and post. SA370 the notice is one of HMRC’s “Notice of penalty assessment” letters; the appeal form itself tells you the notice date is at the top of either an SA326D or an SA370 notice. When a client says “I’ve had an SA370”, check which one they mean before you do anything else.
A reasonable excuse is something outside the client’s control that stopped them filing or paying on time. A nominated partner is the partner the partnership names to deal with HMRC on the partnership return.
Which penalties SA370 covers
The amounts below come from GOV.UK’s Self Assessment penalties guidance; SA370 has a box for each.
| Penalty | Amount (GOV.UK) | Where it goes on SA370 |
|---|---|---|
| Return 1 day late | £100 | Tick box in the late filing section |
| Return 3 months late | £10 a day, up to £900 | 3 months late filing amount and notice date |
| Return 6 months late | 5% of the tax due or £300, whichever is greater | 6 months late filing amount and notice date |
| Return 12 months late | A further 5% or £300, whichever is greater | 12 months late filing amount and notice date |
| Tax unpaid at 30 days, 6 months and 12 months | 5% of the tax unpaid at each point | Late payment section, one line per penalty |
For a partnership, GOV.UK notes that every partner is penalised when the partnership return is late. Those penalties are appealed together on SA371.
Self Assessment penalty appeal form or online: which route to use
Start with GOV.UK’s check on when and how to appeal a Self Assessment penalty. It points you to the online appeal or to the paper forms depending on the penalty. In practice, the choice for an agent usually looks like this:
| Route | Use it for | Who submits | Practice notes |
|---|---|---|---|
| Online appeal | Individual late filing or late payment penalties that the GOV.UK checker routes online | The client, signed in to their own HMRC account; GOV.UK’s guidance does not describe an online appeal route for agents | GOV.UK tells taxpayers never to give their sign-in details to an agent, so your role is preparing the wording and evidence for them to submit |
| SA370 by post | Individual penalties, including several penalties for the same tax year on one form | The client, or you as agent, signing and stating your capacity | Post to Self Assessment, HM Revenue and Customs, BX9 1AS, as printed on the form |
| SA371 by post | Penalties for a late partnership return | The nominated partner; HMRC’s manual also refers to an authorised agent | Send to the HMRC office shown on the penalty notice |
| Signed letter | A late appeal, or where the reasons need more space than the form allows | Client or agent | GOV.UK asks for name, UTR and an explanation including dates |
For a practice handling several appeals, the paper route keeps control with you: your wording, your signature, your file copy.
How to appeal a Self Assessment penalty as an agent: step by step
- Confirm you are authorised to act for the client for Self Assessment. GOV.UK explains how to get authorised as a tax agent, including the online request and form 64-8.
- Collect every penalty notice for the tax year and note each notice date and amount.
- Check the return has been filed and the tax paid, or record when they will be. SA370 tells the taxpayer to file and pay now to avoid further penalties.
- Run GOV.UK’s check on when to appeal to confirm the route for each penalty.
- Draft the reasonable excuse with dates, and gather the evidence.
- Complete SA370 (or SA371) with the client’s details, sign it as agent and add your practice address.
- Post it to the address on the form within 30 days of the notice date, and keep a scanned copy with proof of posting.
- Diary a follow-up, and diary the review and tribunal deadlines if HMRC rejects the appeal.
The 30-day time limit and late appeals
SA370 itself says to send the form within 30 days of the date shown on the front of the penalty notice. HMRC’s Self Assessment Manual at SAM10080 notes that it allows a further 7 days for printing and dispatch of the notice. Treat that as a margin for error and keep working to the 30 days.
If you miss the deadline, the appeal can still be accepted. Under section 49 of the Taxes Management Act 1970, a late appeal goes ahead if HMRC agree or, where they do not, the tribunal gives permission. HMRC must agree where the request is made in writing, there was a reasonable excuse for the delay, and the request was made without unreasonable delay once the excuse ended. A late appeal therefore explains two delays: the filing or payment, and the appeal.
Filling in SA370 section by section
The form is two pages. Download the current version each time and read the SA370 notes alongside it.
About you
When you appeal for someone else, the form tells you to enter their details here. That means the client’s tax reference exactly as shown on the penalty notice, their National Insurance number if known, and their full name and address.
The penalty or penalties you want to appeal against
Enter the tax year (ended 5 April), then the amount and notice date for each penalty, taken from the top of the SA326D or SA370 notice. The £100 penalty is a tick box; every other filing and payment penalty has its own amount and date. Appeal every penalty the excuse covers on the one form.
Your reason for making an appeal
Tick boxes cover ill health, postal issues, fire, flood or other natural disasters, not receiving the return, theft or crime, bereavement, IT difficulties, and “other”. Below them is a free-text box for a factual description with dates. For “other”, the form asks for as much detail and evidence as possible. Extra sheets can be attached to the back.
Signature and capacity
The signer dates the form and, when signing for someone else, states their capacity (agent is one of the examples given), with a daytime phone number and their own address. Use your practice address so HMRC’s reply comes to you.
Writing the excuse
GOV.UK’s reasonable excuse guidance rules out not getting a reminder, a payment failing for lack of funds and a mistake on the return, and says the return or payment must be sent as soon as the person is able to. A structure that works for most SA370 descriptions:
- What prevented the client from filing or paying, in one sentence.
- The dates it started and ended.
- Why it stopped the client (or you) from meeting the deadline.
- When the return was filed or the tax paid, showing it was done promptly once the problem ended.
- A list of the evidence attached.
Our longer guide to appealing HMRC late filing and late payment penalties covers evidence and wording in more depth, and our HMRC reasonable excuse examples show which reasons tend to be accepted or rejected.
What happens next: review and tribunal
GOV.UK advises that you consider paying the penalty while you appeal, because if the appeal is rejected, interest runs on the penalty from the date it was due.
If HMRC rejects the appeal, you can ask for a review by an HMRC officer not involved in the original decision, or go to the tax tribunal. According to GOV.UK’s guidance on reviews, reviews usually take 45 days, and if you disagree with the result you must usually appeal to the tribunal within 30 days of the date on the review result letter. Diary both dates on arrival.
A note on MTD for Income Tax clients
GOV.UK’s MTD penalties guidance says the new points-based penalties apply from the tax year a client joins MTD for Income Tax, so the 2025 to 2026 return due by 31 January 2027 stays under the current rules and SA370 still applies. See our MTD for ITSA guide for the new regime and our MTD quarterly deadlines calendar for the update dates.
FAQs
Where do I find HMRC form SA370?
HMRC form SA370 is on GOV.UK under “Self Assessment: appeal against penalties for late filing and late payment (SA370)”. The same page links to SA371 for partnerships and to the online route. Download a fresh copy for each appeal, because HMRC updates its forms and an old version may carry outdated instructions or an old postal address.
Is it SA370 or SA371?
Use SA370 for penalties on an individual’s own Self Assessment return, including sole traders and landlords. Use SA371 when the penalty arises from a late partnership return. Each partner receives a penalty in that case and the nominated partner appeals for the whole partnership on SA371.
Can an accountant sign SA370 on behalf of a client?
Yes. The form has a box for anyone signing on behalf of someone else to state their capacity, and agent is one of the examples given. Make sure you are authorised for the client’s Self Assessment first, enter the client’s details in the “About you” section, and put your practice address in the signer’s address box.
Can I appeal a Self Assessment penalty online as an agent?
GOV.UK offers an online appeal alongside the paper forms, but it is made through the taxpayer’s own HMRC sign-in, and GOV.UK’s guidance does not describe an online route for agents. Clients must never share their sign-in details with you. An agent therefore either posts SA370, signed as agent, or drafts the wording for the client to submit online.
What if the 30-day appeal deadline has passed?
You can still appeal, but you must explain the extra delay as well as the original lateness. HMRC must accept a late appeal where there was a reasonable excuse for missing the deadline and the request was made without unreasonable delay afterwards. If HMRC refuses, the tribunal can give permission for the late appeal.
Should the client pay the penalty while the appeal is open?
Usually, yes. GOV.UK points out that if the client does not pay and the appeal is rejected, interest is charged on the penalty from the date it was due until the date it is paid. Paying while the appeal is considered stops that interest building up during what can be a long wait.
Need help clearing a backlog of penalty appeals?
After 31 January, appeals arrive in batches, and each one needs the notices matched, dates checked, evidence gathered and the form drafted. Our penalty appeals support for UK practices handles that preparation and tracking inside your own systems, while you review, approve and submit. Book a consultation to plan it before the January peak.
