How do I appeal a Self Assessment late filing penalty?

You usually have 30 days from the date the penalty was issued to appeal, online or on form SA370, with a reasonable excuse. For 2025-26, an online return filed after 31 January 2027 gets a £100 penalty.

A late filing penalty is not the end of the matter. If something genuinely stopped you filing on time, HMRC can cancel it. The rules below follow GOV.UK and HMRC's own manuals as checked on 16 October 2026.

The penalties you might be appealing

These are the Self Assessment late filing penalties for a return such as 2025-26:

If you have joined Making Tax Digital (MTD), a missed tax return deadline in an MTD year earns a penalty point instead. That penalty or point comes with a letter, and the letter tells you how to appeal. Your 2025-26 return stays under the table above even if you started MTD in April 2026.

What counts as a reasonable excuse

GOV.UK describes a reasonable excuse as something that stopped you meeting a tax obligation for a valid reason. Its examples are:

• your partner or another close relative died shortly before the deadline
• an unexpected stay in hospital
• a serious or life-threatening illness
• your computer or software failed while you were preparing your online return
• problems with HMRC online services
• a fire, flood or theft stopped you completing your return
• postal delays you could not have predicted
• delays related to a disability or mental illness you have
• you were unaware of or misunderstood your legal obligation
• you relied on someone else to send your return, and they did not

HMRC's manual adds that the excuse must have actually happened, ideally with documents to show it. It must exist by the deadline and last for the whole time you were late. Once the problem is over, you must file without unreasonable delay.

What HMRC does not accept

GOV.UK says these will not be accepted:

• your cheque bounced or a payment failed because you did not have enough money
• you found the HMRC online system too difficult to use
• you did not get a reminder from HMRC
• you made a mistake on your tax return

The manual also says pressure of work and lack of information are not normally enough on their own. Relying on someone else only works if you took reasonable care yourself: HMRC expects you to have explained what was needed, set deadlines and checked on progress.

How this plays out for trades and landlords

Whether your excuse works depends on your own facts. HMRC's manual stresses that every case is judged on its circumstances.

What this means for you

If you got a penalty for a late 2025-26 return, check the date on the notice and count 30 days. Gather the evidence first: dates, letters, hospital papers, police references or screenshots of error messages. If you did not need to send a return at all, GOV.UK says you can ask HMRC to cancel the penalty.

Think about paying the penalty while you wait. If the appeal fails and you have not paid, interest is added from the due date. If it succeeds, HMRC repays you with interest, unless you owe other tax.

Steps to appeal your penalty

1. Find the penalty notice and note the date it was issued. You usually have 30 days from then.
2. Have your Unique Taxpayer Reference (UTR) ready, along with the date you filed and the date you paid, if you have.
3. Write down your reasonable excuse in date order: what happened, when, and when you filed once it was over.
4. Use the GOV.UK appeal checker. It sends you to the online appeal, or to a form to appeal by post: SA370 for your own return, or SA371 if a partnership return was late.
5. If you have no appeal form, send a signed letter to the HMRC office related to your return, with your name, UTR, the reason and the dates. For a computer problem, include the date you tried to file and any error message.
6. If HMRC says no, ask for a review, or take the appeal to the tax tribunal.

Filing on time removes the problem altogether. The free MTD check shows whether Making Tax Digital starts for you in April 2027, and there is more in the trades section and the landlord section.