Discovering a mistake after submitting a tax return is uncomfortable, but the useful response is to establish the facts and choose the correct correction route. Do not put the difference into next year’s figures simply because that seems easier. The original period, return type and filing deadline determine what needs to happen.
In this article
- Identify the incorrect return details
- Gather evidence for the correction
- Confirm the amendment route and timing
- Keep the revised figures and confirmation
Amending a tax return in the UK can mean changing an individual’s Self Assessment return or a company’s Corporation Tax return. These are separate processes. An error in company accounts may also need a separate accounts correction, even where the tax calculation changes at the same time.
First, confirm there really is an error
Compare the submitted return with the source documents and its supporting calculation. A bank receipt may belong to a different accounting period or already be included elsewhere. A missing invoice may be unpaid, duplicated or outside the business. Establishing the treatment matters as much as finding the amount.
Create a short correction record before editing anything:
| Record | What to capture |
|---|---|
| Return affected | Taxpayer, return type and year or accounting period |
| Original submission | Filing date, saved return and submission receipt |
| Suspected error | Box or schedule affected and original figure |
| Supporting evidence | Invoice, statement, payroll certificate or calculation |
| Proposed correction | Revised figure and explanation of the treatment |
| Other effects | Related accounts, tax payments or later periods to review |
Tell the original preparer promptly if someone filed on your behalf. They may have working papers explaining an apparent omission. Agree who will make the amendment so two people do not submit competing versions.
Self Assessment: check the amendment window
Self Assessment returns can generally be corrected within 12 months of the filing deadline. For 2024–25, GOV.UK gives the usual amendment deadline as 31 January 2027. Online filers must wait 72 hours after submission before updating the return. See HMRC’s Self Assessment corrections guidance.
Within the window, use the appropriate filing route. If you filed directly online, select the correct tax year, make the change and submit the amended return. If you used commercial software, check the provider’s amendment process. Paper amendments require corrected pages marked as an amendment, with your identifying details. The same GOV.UK guidance explains these routes.
Save the new return, computation and submission acknowledgement. Updating a spreadsheet or draft within software does not prove that an amendment has reached HMRC.
Company Tax Returns: a different procedure
Company Tax Return amendments normally need to be made within 12 months of the filing deadline. GOV.UK identifies commercial software and written or paper routes. Beyond that window, underpaid Corporation Tax should be disclosed through HMRC’s online disclosure service promptly; overpaid tax may require an overpayment-relief claim. Follow GOV.UK’s Company Tax Return amendment guidance.
Before resubmitting, reconcile the revised accounts, tax computation and return. An adjustment to an expense may change taxable profit without changing turnover, while another error may affect several schedules. Ask the preparer to explain which documents change and why.
If filed company accounts also contain an error, treat that as a separate task. Companies House’s accounts correction guidance explains filing corrected accounts. Correcting HMRC’s return does not itself replace accounts held at Companies House.
What if the normal window has closed?
For older Self Assessment errors, HMRC directs taxpayers to write to it. Overpayment relief can generally be claimed up to four years after the end of the relevant tax year, subject to the required conditions and declaration. Check HMRC’s corrections instructions before preparing the correspondence.
Do not treat this as an automatic extension for every claim or election. Explain the error, identify the period and obtain advice on the available route. If several years are affected, organise them separately with a summary showing how the mistake carries through. An ongoing HMRC enquiry also needs careful coordination with the adviser handling it.
An illustrative error and correction
Imagine a self-employed consultant finds that a customer’s payment was omitted from the records used for a submitted return. They locate the invoice, bank receipt and original sales schedule. Checking the schedule confirms the income was not recorded under another customer or date.
The consultant sends the evidence to the preparer, who reviews the accounting basis and calculates the effect for the original tax year. They agree the appropriate amendment route, obtain approval of the revised return and retain the acknowledgement. The consultant then compares the updated tax position with payments already made and adjusts their cash plan.
The amount and outcome are deliberately unspecified: this is an illustrative workflow, not a prediction of tax, interest or penalties for a particular case.
Check payments after the correction
An accepted amendment and a settled account are different milestones. Compare the revised tax calculation with HMRC’s statement, including payments already credited and any revised payments on account. Ask for explanations where the statement and calculation do not match. Do not spend an anticipated refund before confirming its status or assume an existing payment instruction covers an increased balance.
Keep a dated note of enquiries, responses and any agreed next steps. If tax is owed and you cannot pay it, raise that promptly rather than letting the amendment become a reason to postpone discussing payment.
Reduce the chance of the same mistake recurring
Identify the cause: a missing bank feed, incomplete information request, duplicate spreadsheet or unreviewed software category. Fix that point in the process. A final pre-filing reconciliation between records, supporting statements and the draft return is more useful than simply promising to be more careful.
For help organising a suspected error, contact EPOS Accountancy’s tax service with the return type, period and evidence available. Confirm whether amendment work, disclosures or specialist representation can be accepted within the agreed scope. Refer to the pricing page for fee information.