BRITVEX ADVISORY LTD  |  Personal tax & Self Assessment  |  Reviewed 10 November 2026

BRITVEX ADVISORY LTD | UK Self Assessment correction and refund guidance | Reviewed 10 November 2026

Quick answer: Yes, you can usually change a Self Assessment tax return after submitting it. HMRC normally allows an amendment up to 12 months after the statutory filing deadline. An online filer generally needs to wait 72 hours after the original submission before making a correction. You can update missing income, correct an expense, add an eligible mileage claim or fix a calculation, but you must submit the amended return—not simply edit your saved copy. A correction can produce extra tax, a reduced liability or a refund, depending on the full calculation. If the amendment window has passed, a different written process may be needed.

Already submitted a return and found something wrong? Start by preserving the original tax calculation, identifying the precise tax year, and gathering evidence of the item that needs correcting. Get a quote for a Self Assessment amendment review, or read about Britvex Self Assessment services. Existing clients can upload documents securely through the client portal.

Can you amend a Self Assessment return after it has been filed?

A filed return is not necessarily final. A taxpayer may discover that an accountant used an outdated figure, a mileage log was missing, a rental expense was overlooked, a dividend statement arrived after filing, or a second source of income was left off. HMRC provides an amendment process for correcting the return and its self-assessment calculation within the statutory window. The point of an amendment is to replace the affected figures with a complete, accurate position—not to treat the tax return as a negotiation over which costs can be claimed.

There are three practical stages. First, establish what was sent to HMRC, including the submitted return and tax computation. Second, determine what the correct position should be using supporting records and the rules for that particular tax year. Third, submit the correction through the appropriate channel and check how HMRC recalculates the tax account. Do not assume that a lower profit always creates a cheque: payments already made, unpaid liabilities, payments on account and other adjustments all affect the final balance.

This is distinct from the separate question of Making Tax Digital digital-record obligations. MTD quarterly updates and annual Self Assessment returns are different workflows, and the correct correction route depends on the system and tax year involved.

At a glance: what to do when you discover an error

A single figure may interact with another: an increase in taxable profit can change the income-tax calculation, payments on account, a student-loan repayment, certain income-tested charges or reliefs. The figures below are illustrations, not an individualized tax calculation.

The normal amendment deadline: 12 months after the statutory filing date

HMRC’s published guidance allows a taxpayer to correct a Self Assessment return within 12 months after the statutory filing deadline, usually 31 January following the end of the tax year. This is not 12 months after the day you chose to file. An individual who files early normally has the same amendment deadline as someone who files close to the usual 31 January cut-off. A notice to file issued late can produce a different statutory deadline, so confirm the original notice when a date looks unusual.

Here are two examples from the current filing cycles; the article title and slug deliberately contain no years so this guidance can be refreshed without changing its permanent URL:

A paper return ordinarily has an earlier filing deadline, but HMRC uses the statutory 31 January date for the normal amendment-window calculation. Unusual notice-to-file dates and an enquiry can affect the practical route. Official source: HMRC – change a tax return after submitting it.

Do I get another 12 months if I amend the return once?

No. Sending an amendment does not create a fresh 12-month correction window. It remains linked to the statutory deadline for the return, subject to the relevant legislation and special circumstances. Keep a record of every version, acknowledgement and amendment date so that an accountant can reconcile the final tax computation without confusing it with an earlier version.

Should I amend if HMRC has already issued a refund?

Yes, if the submitted return is materially wrong and the correction route remains available. An earlier repayment does not make the underlying figures correct. Where a correction creates additional liability, HMRC may adjust the account and seek recovery, including applicable interest. Conversely, a further justified reduction could create another repayment or be credited against outstanding amounts. Reconcile the statement rather than treating the refund received as the amount of relief legally due.

How to amend a filed Self Assessment return online

HMRC says to wait at least 72 hours after filing an online return before trying to change it. Then follow its account navigation. The exact screen labels may change, so use the current official route rather than an old screenshot:

1. Sign in at GOV.UK – Self Assessment using the appropriate account credentials.
2. Open your Self Assessment account and use the More Self Assessment details route.
3. Under the account overview, select Tax return options.
4. Select the relevant tax year; do not assume the latest return is the one needing correction.
5. Open the submitted return, revise the affected figures and the relevant supplementary sections.
6. Review the updated calculation, declarations and any changes to payments on account.
7. Submit the amended return and retain HMRC’s acknowledgement, your revised return and calculation.
8. After processing, check your statement to understand the revised tax, interest, payments and refund position.

HMRC says the revised calculation is displayed when you update the return online and the statement generally shows the difference within three days. That is a statement of the normal service process, not a guarantee that every repayment arrives within three days. Bank-detail checks, existing liabilities or security reviews can affect repayments.

The difference between an amendment and an unsubmitted draft

A common problem is believing that changing the figures in accounting software or downloading a revised PDF changes HMRC’s copy. It does not. If commercial software was used originally, contact the software provider about the supported amendment procedure. Keep evidence that HMRC actually received the replacement return. The client should approve the final corrected calculation before Britvex submits anything under appropriate agent authority.

Paper returns, agent-filed returns and software limitations

If the original return was paper, HMRC explains that the taxpayer may send corrected SA100 pages and any amended supplementary pages, marking the changed pages as amendments and including identifying details such as the Unique Taxpayer Reference. For a software-filed return, the best method depends on the provider’s capabilities. If the software cannot create a valid amendment, follow HMRC’s guidance or obtain professional assistance before sending inconsistent figures.

When an accountant prepared the original return, ask for the submitted return, calculation, filing receipt, schedules and relevant bookkeeping ledgers. This makes it easier to isolate the error and avoid replacing correct figures. If you are moving from another accountant, clean bookkeeping and a reconciliation of the source records may be needed before a reliable amendment can be made. An amendment is not a substitute for rebuilding an inaccurate year’s accounts if the underlying records are incomplete.

Missed mileage after filing: can you add it?

Often yes, but only where the journeys qualify as business travel, the correct tax treatment is available, and the evidence supports the claim. This is a frequent real-world question: a sole trader submits a tax return and later finds an app export or mileage log covering business appointments. The mileage may be relevant, but the claimant must not automatically apply the employee reimbursement rules, claim ordinary commuting or use the wrong tax-year rate.

Self-employed simplified mileage and the change in rates

For eligible sole traders and certain partnerships using simplified expenses, HMRC permits a flat-rate deduction instead of the actual vehicle running costs. For cars and goods vehicles, the rate for the first 10,000 qualifying business miles increased from 45p to 55p from 6 April 2026, while the 25p rate beyond 10,000 miles remained unchanged. The motorcycle rate is 24p. The amount that may be claimed depends on the tax year being corrected, not the date when the amendment is submitted. See HMRC simplified vehicle expenses.

These are self-employed flat-rate deductions; employee mileage-relief claims are a different tax regime, although HMRC has also updated the main car/van approved mileage rates. Importantly, someone who has already claimed capital allowances or particular actual vehicle costs for the vehicle may be unable to switch to simplified mileage for that same vehicle. You cannot add mileage on top of petrol, insurance, servicing and depreciation already deducted under an incompatible method.

Worked example: adding 6,000 missed business miles

A self-employed consultant prepared a return for 2025–26 but omitted 6,000 qualifying business miles. Assume the consultant was entitled to use simplified mileage and had not already deducted incompatible vehicle costs. At 45p, the additional potential deduction is 6,000 × £0.45 = £2,700. If all that deduction reduces income otherwise taxed at an assumed 20% marginal income-tax rate, the income-tax reduction might be £540 before considering National Insurance, allowances, interactions or other complications. The claim is £2,700 of deductible business expense, not an automatic £2,700 tax refund.

For a 2026–27 return on otherwise identical facts, using the later 55p rate gives a deduction of £3,300 for 6,000 miles. At a purely illustrative 20% marginal rate, that could mean £660 of income-tax reduction. Do not substitute this later figure into a 2025–26 return just because the amendment happens in 2026 or 2027.

For each journey, retain date, route, destination or purpose, distance, vehicle and evidence that it was for business rather than normal commuting. A calendar entry, customer invoice and mapping record can reinforce a mileage log, but reconstructed mileage should be clearly explained and supportable.

Other expenses commonly missed from filed Self Assessment returns

Errors often arise because invoices arrive late, business and personal expenditure were mixed, or a previous accountant did not receive the full records. Depending on the taxpayer and accounting basis, relevant items might include business subscriptions, qualifying professional fees, software, business telephone usage, specific travel and accommodation costs, office expenses and qualifying home-working costs. The fact that a receipt exists does not automatically make it deductible. The law and HMRC guidance distinguish business purpose, revenue and capital expenditure, private use, record keeping and timing.

Sole traders: Review eligible operating costs and the choice between actual costs and simplified expenses where permitted. Landlords: Review rental expenditure separately from capital improvements and mortgage finance costs, since residential finance-cost rules are not the same as ordinary deductible trading expenses. Employees: Review whether expenses belong in Self Assessment and whether a separate relief claim is more appropriate. Company directors: Do not include a limited company’s own expenses as if they were sole-trader expenses; company payroll, reimbursement and corporation-tax rules are different.

A stronger review cross-checks bank and card statements, VAT treatment where relevant, supplier receipts and the original bookkeeping categories. Britvex’s Landlord Tax and Tax Planning services can assist where a correction forms part of a wider property or personal-tax issue.

What if income was left out rather than an expense?

Not every amendment is a refund claim. Forgotten self-employment receipts, employment benefits, foreign income, bank interest, dividends, taxable rental income and certain gains may produce more tax due. The correct approach is to identify the source, verify the amount and relevant tax year, and revise the full calculation rather than make a partial correction that leaves a second omission unaddressed.

Example: suppose an omitted £4,000 of taxable income increases the relevant chargeable income and the marginal rate is 40%. A simplified illustration suggests £1,600 additional income tax, before allowances, National Insurance, reliefs, payments already made and any interest or penalties. It would be misleading to present that as a definitive bill. If the omission was due to carelessness, the person should assess disclosure obligations promptly; if HMRC has already opened an enquiry, professional advice is particularly important.

Does voluntarily correcting an error prevent penalties?

Not automatically. Tax-geared inaccuracy penalties depend on the circumstances, including whether reasonable care was taken, the behaviour involved and disclosure. There can also be interest on underpaid tax. A prompt, accurate voluntary correction may matter when HMRC considers the case, but nobody should promise that an amendment erases every penalty or all interest. Check HMRC penalty and appeal guidance for the relevant facts.

How an amendment affects your tax bill, refunds and payments on account

When the corrected return is accepted, HMRC recalculates the Self Assessment position. A reduction in the year’s tax liability does not always equal an immediate bank repayment. The balance may be offset against overdue tax, an existing amount due or adjustments in payments on account. A rise in taxable profit can increase the overall tax balance and may influence the following year’s payments on account. Interest may appear when additional tax was due earlier than the date of the amendment.

Worked reconciliation: why a £540 tax reduction is not necessarily a £540 refund

Suppose the corrected mileage claim creates an illustrative £540 income-tax reduction, but the individual’s HMRC account has £260 of unrelated unpaid tax. HMRC may apply the available credit to the unpaid amount, potentially leaving £280 to be repaid or otherwise allocated. This simplified example assumes no other account movements, tax charges or interest; the actual statement controls. Confirm both the amended return calculation and the account statement before promising a refund amount.

Practical step: Keep the original tax computation, revised computation, current Self Assessment statement and repayment details in one folder. Where figures disagree, check whether the difference is due to payments, allocation or an older return adjustment rather than repeatedly filing another amendment.

Can you claim a refund after the amendment deadline has passed?

Sometimes. Once the normal statutory amendment period has passed, a taxpayer generally cannot treat an older return as open for normal online correction. HMRC says a person who has overpaid tax may instead be eligible to make a written overpayment-relief claim, ordinarily within four years after the end of the relevant tax year. The relief has statutory conditions and exclusions and is not a catch-all extension of the amendment deadline.

HMRC says a claim should state clearly that overpayment relief is requested; identify the relevant tax year; explain the mistake; provide the amount believed overpaid; state whether a previous appeal has been made for the same payment; and include the required signed declaration that the information is correct and complete to the best of the claimant’s information and belief. The claimant must personally sign the overpayment-relief declaration; an agent cannot simply sign it on the client’s behalf. Keep supporting evidence and copies of correspondence. See HMRC correction and overpayment guidance and its claims time-limit manual.

What if you owe more tax for a closed year?

Do not describe overpayment relief as the route for disclosing omitted taxable income. HMRC instructs people outside the ordinary amendment window who have underreported tax to contact it in writing, explaining the tax year, why too little tax was paid and the likely underpayment, with appropriate authentication. Complex or potentially deliberate omissions require individualized advice. HMRC may make further enquiries or update the amount due under its legal powers.

Differences for landlords, directors and people with overseas income

An ordinary tax-return amendment may be simple when one expense receipt was omitted, but multiple tax regimes can interact. A landlord might need to reconcile property rents, allowable expenses, residential finance costs and the existing property section. A company director may have dividends, benefits or director-loan items that belong in different parts of the return and are supported by company records. An overseas taxpayer might need tax-residence analysis, double-tax relief, foreign tax paid or different supplementary pages.

A return that includes UK property disposals may also sit alongside a separate UK property CGT reporting obligation that has its own timeline. Do not assume that amending Self Assessment replaces the need to correct a separate property disposal account where required. If the original problem arises from inaccurate source bookkeeping, work through bookkeeping reconciliation first and then tax-return review.

What evidence should you give an accountant to review the amendment?

Provide a complete and usable audit trail rather than a collection of unexplained screenshots. A practical amendment package includes the submitted tax return and tax computation, HMRC filing acknowledgement, HMRC account statement, the specific tax year, the new or corrected documents, the original bookkeeping or expense schedule, any HMRC letters and an explanation of when and why the error was found. For mileage, give the trip log and vehicle basis; for property, give the rent/expense ledger; for employment, give the relevant P60, P11D or expense records. If an earlier adviser prepared the return, authorise the necessary record handover where appropriate.

Britvex can ask follow-up questions securely through the client portal. Do not send passwords or two-factor codes by ordinary email.

A practical seven-step correction plan

1. Identify the return and tax year. Do not begin with the assumed refund figure. Obtain the actual HMRC-filed return.
2. Confirm whether the legal amendment window remains open. Look at the statutory filing date, any late issue of a notice and HMRC correspondence.
3. Classify the error. Missing deductible expense, omitted income, wrong amount, wrong year, incorrect relief or an entirely different tax filing.
4. Rebuild the evidence. Reconcile the correct income and qualifying costs against receipts, statements and ledgers.
5. Calculate the full effect. Consider allowances, the relevant year’s rates, repayments and any payments on account; do not use simplified illustrative percentages as a final calculation.
6. Submit using an authorized route. Use HMRC online, appropriate commercial software, paper correspondence or the applicable written claim process.
7. Check the result. Save the acknowledgement, revised HMRC statement and any decision or repayment notice. Review whether future records should change.

In practice, the best amendment is not just the correction of a number. It also fixes the workflow that produced the error. A sole trader missing mileage year after year needs a simple digital mileage log. A landlord missing invoices needs a regular property-expense reconciliation. An employee whose benefits keep being missed needs a reliable annual document checklist. These are recurring record issues, not reasons to submit a new correction every year.

How Britvex can help with a filed tax return

Britvex can review the original return and evidence, assess whether the proposed change is legally supportable, prepare a corrected calculation and coordinate the appropriate HMRC submission or written claim within the agreed engagement scope. The service is especially relevant when a return was filed by another accountant, expenses were omitted, property income has been misstated, or the taxpayer’s HMRC statement does not match the expected repayment.

For a new enquiry, use Get a Quote and select Self Assessment tax return, explaining in the message that the return is already filed and needs an amendment. Review our Self Assessment service, Tax Services hub, or contact Britvex. Existing clients should sign in to the secure client portal or use portal support. Call +44 20 3758 7500 or email info@britvex.co.uk for new engagements.

BRITVEX ADVISORY LTD (Company No. 16133503) is an approved Companies House Authorised Corporate Service Provider, HMRC AML-supervised and ICO registered. These credentials relate to the practice’s regulatory status; they are not an endorsement of any particular tax position or promise of an HMRC refund. Each claim depends on the facts and applicable law.

Frequently asked questions

Can I change a Self Assessment tax return after I have submitted it?

Yes, usually within 12 months after the statutory filing deadline. Online filers generally wait 72 hours after submitting before making corrections. You must submit the amended return and retain confirmation.

How long do I have to amend my Self Assessment return?

The normal amendment period is 12 months after the statutory filing date, generally 31 January following the tax year. A late-issued notice or enquiry may change the practical position, so check your actual documents.

Can I add mileage to a tax return I already filed?

Potentially, if the business journeys qualify, the records support them and you use the right tax-year rate and vehicle expense basis. The mileage amount reduces taxable profits when deductible; it is not automatically paid to you as a refund.

Can I use the new 55p mileage rate when amending an older return?

Not for business miles from a tax year before the increase. The first 10,000-mile car/goods-vehicle flat rate was 45p for 2025–26 and became 55p from 6 April 2026. Apply the relevant tax-year rules.

Does amending my return automatically create a refund?

No. HMRC recalculates the overall tax position. Any credit can be affected by payments already made, outstanding debts, interest or payments on account. A missed expense does not guarantee cash repayment.

Can I amend a return that another accountant filed?

Usually yes if you have the necessary records, access and authority, and the amendment window remains open. Obtain the original submitted return, computation and supporting documents first to avoid replacing correct figures.

What if I forgot to declare income rather than an expense?

Correct it promptly using the applicable route. It may increase the tax due, and interest or penalties could apply depending on the facts. Do not hide a known omission while pursuing an unrelated refund.

What if my return was filed using accounting software?

Use the provider’s supported amendment process. HMRC says to contact the software provider for help, or HMRC if the software cannot make corrections. Confirm that the amendment actually reaches HMRC.

Can I amend more than once?

Further corrections can generally be made while the statutory amendment period remains open. A previous amendment does not restart that period. Keep every filing acknowledgement and amended computation.

Can I claim back overpaid tax after the amendment deadline?

You may qualify for written overpayment relief, normally within four years after the end of the relevant tax year, subject to the rules and exclusions. The claim needs prescribed details and the claimant’s own signed declaration.

Will HMRC charge a penalty because I corrected an error?

Not automatically. Whether an inaccuracy penalty applies depends on the behaviour, reasonable care, disclosure and other circumstances. Interest can still be due on tax paid late. Corrections should be truthful and timely.

Can Britvex review and amend a return filed elsewhere?

Britvex can assess the original return and documents, explain the appropriate amendment or claim route and prepare work within an agreed engagement. Start through Get a Quote with the service set to Self Assessment and identify the tax year and issue.

Official sources and editorial notes

• HMRC — change a Self Assessment return after filing.
• HMRC Self Assessment Manual — amendment time limits.
• HMRC Claims Manual — overpayment relief limits.
• HMRC — simplified vehicle expenses.
• HMRC — allowable travel costs.
• HMRC — employee vehicle and mileage relief.

This guide explains UK tax procedures as reviewed on 10 November 2026. Eligibility, amendment deadlines and reliefs depend on the relevant tax year and actual facts. The examples simplify tax and should not be used as a filed calculation or a guarantee of refund.