Last updated: 16 October 2026
What should you do if you missed some income off your tax return?
Amend the return. You can change a Self Assessment return online within 12 months of the 31 January filing deadline, so a 2025/26 return can be amended until 31 January 2028. After that, write to HMRC or use the Digital Disclosure Service to tell them about the missing income. Pay any extra tax as soon as you can, as interest runs from the original due date. Telling HMRC before they find the error usually means no penalty for genuine mistakes and much lower penalties for carelessness.
How to correct the return
| When | How |
| Within 12 months of the deadline | Amend online in your HMRC account or software |
| Paper return in the same period | Send corrected pages |
| After that, up to 4 years | Write to HMRC with the corrections |
| Deliberate omission or older years | Digital Disclosure Service |
Penalties for errors
- Reasonable care taken: no penalty
- Careless: 0% to 30% (0% if unprompted)
- Deliberate: 20% to 70%
- Deliberate and concealed: 30% to 100%
Unprompted disclosures get the lowest rates.
Common types of missed income
- Bank and building society interest above the allowance
- Rental income from a room or property
- Dividends from shares
- Side income from online selling or freelance work
- Foreign income
- Capital gains on shares or crypto
Example
Sarah files her 2025/26 return in January 2027. In April 2027 she realises she forgot £3,000 of rent from a lodger above the £7,500 rent-a-room limit. She amends the return online, adding £3,000 of rental income. The extra tax is £600 at basic rate. She pays it with interest, and there’s no penalty because she corrected it herself before HMRC asked.
If HMRC contacts you first
If HMRC opens a check and finds the missing income, the penalty is higher. Cooperate fully and provide information quickly to get the maximum reduction.
Payments on account
Amending your return may change your payments on account for the next year. Your HMRC account will recalculate them.
Common mistakes
- Waiting for HMRC to notice
- Amending without paying
- Forgetting payments on account change
- Not keeping notes of why the error happened
Records to keep
Keep the original and amended returns, evidence of the missed income and records of your payment.
Keeping records of side income
Keep a simple record of all income from your side business, including dates, amounts and which platform or customer paid you, plus receipts for related costs. Good records make it easy to decide whether the £1,000 trading allowance or actual expenses gives the better result, and they are essential if HMRC asks questions after receiving platform data.
Getting help
If you are unsure whether you need to register, get advice before the 5 October deadline.
Frequently asked questions
Will HMRC know I amended it?
Yes, it shows in their records.
Can I amend more than once?
Yes, within the time limit.
What if I overpaid by mistake?
You can amend to claim a refund too.
Does it trigger an inquiry?
Not usually, though HMRC can open one within 12 months of an amendment.
Preventing errors
Keep a checklist of income sources each year and gather documents before filing.
Related: disclosing undeclared income, freelance income while employed and tax on website earnings.
Talk to Accotax
Want this handled for you? Our team at Accotax can help with tax return amendment help. See our self assessment accountants service, or contact us for a free, no-obligation quote.
This page is general guidance based on HMRC rules for the 2026/27 tax year, checked against gov.uk on 16 October 2026. It is not personal advice, so speak to an accountant about your own situation.
