How to amend a Self Assessment tax return
Finding an error after submitting Self Assessment is uncomfortable, but the UK system provides a correction route. The right process depends on the tax year, filing method and time elapsed. This guide explains the normal 12-month amendment window, what to do after it closes, how an amendment can change tax and payments on account, and how to leave a clear record showing why the original return changed.
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Who this guide applies to
- taxpayers who found an error after filing
- sole traders and freelancers
- landlords
- people reporting dividends or other income
- taxpayers using commercial filing software
- people expecting extra tax or a repayment
Information to review before amending
- the originally submitted return and calculation
- new or corrected income documents
- expense receipts and working papers
- PAYE, dividend and property statements
- payments on account and tax already paid
- HMRC notices and submission acknowledgements
Start by identifying the exact error
Do not reopen a return and change numbers until the problem is understood. Compare the filed return and calculation with source documents. Identify the tax year, return section, original amount, corrected amount, evidence and reason. Errors can include omitted freelance income, duplicated platform sales, an expense in the wrong category, a missing P60, incorrect tax deducted, overlooked dividends, property costs treated incorrectly, or a student loan answer selected in error. Some differences are not errors: a calculator estimate may use assumptions that differ from the official return, and a bank payout may be net of fees while gross turnover is correctly reported elsewhere. Review linked fields because changing profit can alter National Insurance, student loan repayments, payments on account and other charges. If several years or taxes are involved, list each issue separately. A precise correction is easier to support than replacing the entire return from memory. Preserve the original version before editing so the before-and-after position remains visible.
The normal 12-month amendment window
HMRC generally allows a Self Assessment return to be amended within 12 months of the normal filing deadline. For the 2024/25 return, whose normal online deadline was 31 January 2026, the ordinary amendment deadline is 31 January 2027. This deadline is linked to the statutory filing date, not a year after the date on which an early return was submitted. A person who filed in June 2025 still usually has the same amendment end date. An amendment can be made even if the original return was on time and fully paid. Do not wait until the last day if the error is already known, particularly where more tax is due and interest may continue. Confirm the date for the specific tax year using current HMRC guidance. Returns involving special deadlines, notices issued late or unusual circumstances can require closer checking. An amendment changes the filed information; it does not erase the original submission or make an inaccurate statement something that never occurred.
How to amend an online HMRC return
For a return filed through HMRC’s online service, sign in through GOV.UK, open the Self Assessment account, choose the relevant tax year and use the option to amend the return. HMRC says to wait 72 hours after filing before trying to make an online correction. Work through the affected pages and check the full return rather than altering only the final tax figure. The tax calculation should be generated from corrected income, expenses and answers; it should not be overwritten to match a preferred result. Review the declaration, submit the amendment and save the new calculation and acknowledgement. Note the submission date and what changed. If the system does not show the expected year or amendment option, do not create a duplicate return. Check whether the original was filed using commercial software, whether the amendment period has ended, or whether account access is linked to the correct UTR. Use official support if the online route is unavailable.
Paper returns and commercial software
The correction route should follow the original filing method and software capability. For a paper return within the amendment period, send corrected pages or a corrected return as directed by HMRC, mark it as an amendment, identify the tax year and explain the changes. Keep a complete copy and proof of posting. If commercial software filed the return, use that provider’s amendment function where supported so the submission reaches HMRC in the required format. HMRC’s online account may not allow a return created by third-party software to be amended directly. Contact the software provider or HMRC if the product cannot submit the necessary correction. Do not assume changing a figure inside local software has amended HMRC’s record; obtain a successful transmission acknowledgement. Where an accountant filed the return, tell them promptly and provide the evidence rather than submitting a competing version without coordination. Two uncoordinated amendments can create confusion over which figures are intended to be final.
What to do after the amendment deadline
Once the normal online amendment window has closed, HMRC says to write to them about the correction. The letter should identify the taxpayer, UTR, tax year, the figures being changed, why they were wrong, the corrected tax effect and supporting calculation. Do not send original documents unless requested; retain copies and evidence of delivery. A claim for overpaid tax after the amendment period may need to meet the rules for overpayment relief, including its own time limit and required declarations. A late disclosure of income can involve interest and penalties and may need a different HMRC disclosure route depending on the facts. The appropriate treatment depends on whether the mistake was careless, deliberate, prompted, unprompted and how quickly it was corrected. Do not disguise the timing by entering the amount in a later return. Where the value is material, several years are affected, offshore matters are involved or the original return was knowingly wrong, obtain professional advice on the correct disclosure process.
Worked example: omitted freelance invoice
Maya files her 2025/26 return in November 2026. In February 2027 she discovers that a £2,400 client payment received in March 2026 was excluded from the cash-basis records because it went to an older payment account. She downloads the processor statement, checks the fee was also omitted and reconciles the bank transfer. Because the normal amendment deadline for that return is 31 January 2028, she can use the online amendment process. She adds the gross income, records the eligible fee in the appropriate expense category, regenerates the calculation and submits. The corrected profit increases tax and may alter the payments on account calculated for 2026/27. Maya saves the original return, new return, both calculations, processor statement and a note explaining the discovery. She pays the additional amount shown using the correct reference rather than waiting for the next annual return. The example demonstrates why the amendment should follow the transaction evidence, not simply add the net bank deposit to profit.
When the amendment produces more tax
A correction that increases income, reduces expenses or changes another answer can increase the balancing payment and may recalculate payments on account. Review the updated Self Assessment statement after processing; the submitted calculation and account balance may not update at exactly the same moment. Pay additional tax promptly using the correct reference because interest may apply from the original due date, not the amendment date. If the full amount cannot be paid, filing the accurate amendment remains important. Pay what is affordable and contact HMRC through its payment-support route rather than leaving the return wrong to delay the bill. Check whether a payment already made is correctly allocated. An amendment can also reduce an earlier payment-on-account reduction, creating a larger shortfall. Keep payment confirmations and monitor the account until the correction, interest and payments appear consistently. A public calculator can help sense-check the direction of change, but the HMRC calculation and statement determine what is formally due.
When the amendment produces a repayment
A correction may reduce tax because income was duplicated, tax deducted was omitted, an eligible expense was missed or another entry was wrong. Submit the correction with evidence and review the resulting calculation. HMRC may use a credit against other liabilities before issuing a repayment, and security checks can delay payment. Verify bank details only through the official account and ignore unsolicited messages asking for a fee to release a refund. Do not treat an expected repayment as certain cash until the amended return has processed and the account shows the position. If the correction also lowers current payments on account, distinguish the repayment for the completed year from the revised advance payments. An overpayment claim made after the amendment window has additional conditions and should not be assumed to follow the same online route. Keep the original and amended workings because a large refund or unusual claim may be queried. The goal is an accurate return, not merely the largest repayment a software prompt permits.
Records, explanations and reasonable care
Create an amendment note with the tax year, discovery date, original entry, corrected entry, source documents, tax difference, submission route and payment or repayment action. Keep it with both versions of the return. This shows that the taxpayer investigated the issue and corrected it rather than silently rewriting history. Review whether the same error appears in another tax year or business process. If a platform report was omitted, check all years using that account. If an expense category was misunderstood, update bookkeeping guidance and earlier returns still within time. Penalties can depend on behaviour and disclosure circumstances, so acting promptly and giving complete information matters. An honest mistake is not repaired by deleting evidence or inventing a new explanation. Where professional advice was followed, keep the advice and the facts supplied to the adviser. The taxpayer signs the return and should still review it. A short annual comparison against bank, PAYE and platform totals can prevent repeat amendments.
Common amendment mistakes to avoid
Common mistakes include changing the wrong tax year, editing local software without transmitting, missing the 12-month deadline, paying the extra tax under an incorrect reference, expecting the account to update instantly, and correcting one number without reviewing connected fields. Some taxpayers enter omitted income in the next year, which distorts both returns. Others delete the original file, leaving no clear audit trail. Do not submit a second registration or return when an amendment is required, and do not wait for HMRC to discover an obvious error. If the difference arises only because an unofficial calculator used another tax-year assumption, check the official return before changing anything. A good sequence is: verify evidence, calculate the correction, select the proper route, submit once, save acknowledgement, pay or monitor repayment, and fix the record process. For complex disclosures, professional advice is part of completing the correction properly, not a sign that the amendment has failed.
Official references
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