How do I fix errors on a submitted self-assessment tax return?

Understanding what “fixing an error” really means

Submitting a Self Assessment tax return can feel like crossing a finish line. You’ve gathered your figures, navigated the forms, and clicked submit. Then it happens: you notice a number doesn’t look right, you’ve forgotten a source of income, you’ve claimed the wrong expense, or you’ve entered something in the wrong box. The good news is that an error on a submitted return is usually fixable. The key is identifying what kind of change you need to make, acting within the relevant time limits, and choosing the correct method to correct it.

In the UK Self Assessment system, there are a few different pathways depending on when you spot the mistake and how big the problem is. Sometimes you can amend your return online quickly. Sometimes you need to write to HMRC. In more complex cases, you may need to disclose the error proactively and pay any additional tax and interest. It’s also important to understand that fixing an error isn’t only about correcting numbers: it can involve updating your personal details, changing your accounting method, correcting a claim, or removing information that shouldn’t have been included.

This article walks you through the practical steps to fix errors on a submitted Self Assessment tax return, explains typical time limits, and helps you avoid common pitfalls. It also covers what to do if the error means you owe more tax, what happens if you’ve overpaid, and how to approach the situation calmly and clearly.

First steps: confirm the mistake and gather evidence

Before you change anything, take a moment to confirm the error. It’s surprisingly easy to assume something is wrong when it’s actually just presented in an unfamiliar way. For example, the tax calculation you see after submitting can differ from your expectations if payments on account apply, if your personal allowance is reduced due to high income, or if a relief has been applied differently than you anticipated. Double-check the figures and the relevant section of your submitted return.

Once you’re sure there is a mistake, gather the information you’ll use to correct it. Keep it simple and organised:

1) Your Unique Taxpayer Reference (UTR) and National Insurance number.

2) The tax year of the return you submitted (for example, 2024 to 2025).

3) A copy (or PDF download) of the submitted return if you have it.

4) The correct figures and how you calculated them (invoices, bank statements, P60/P45, dividend vouchers, interest statements, pension statements, CIS statements, mileage logs, receipts, etc.).

5) Notes on what went wrong (e.g., “Entered gross instead of net,” “Forgot second employment,” “Duplicated an expense line,” “Missed rent income for two months”).

This preparation helps you amend accurately and will also be valuable if HMRC asks questions later. The goal is to ensure your correction is consistent, evidenced, and easy to explain.

Check the clock: deadlines and time limits for correcting a return

The method you use depends heavily on timing. In many situations, you can simply amend your submitted return within an amendment window. Outside that window, you’ll usually need another approach, such as writing to HMRC or making a separate claim for overpayment relief. Because time limits can vary and because missing a deadline can restrict your options, it’s worth checking the applicable time frame as soon as you spot the issue.

As a practical rule of thumb, the sooner you act, the easier it tends to be. If you discover an error shortly after submitting, you’ll often be able to correct it online with minimal fuss. If you discover it a year or more later, you may still be able to resolve it, but the route becomes more formal and you’ll want to be careful about how you present the change and what supporting evidence you include.

If your mistake affects the amount of tax due, also consider the payment deadlines. Even while you are correcting a return, payment obligations may still apply. If you owe more tax than you originally calculated, paying the extra as soon as you can reduces interest charges. If you are due a refund, you can still amend, but you may need to wait for HMRC to process the change before the repayment is issued.

Amending your tax return online: the most common fix

For many taxpayers, the simplest route is amending the return online. If you submitted the return through HMRC’s online service, you can usually go back in, make changes, and resubmit an amended version. This is often the fastest and clearest way to correct figures, add missing income sources, adjust expense claims, or correct reliefs.

Here’s a straightforward approach that works well in practice:

Step 1: Sign in and locate the relevant tax year. Log into your HMRC online account and choose Self Assessment. Select the tax year you need to correct. Make sure you are working on the right year, especially if you have submitted more than one return over time.

Step 2: Review the sections you completed. Instead of immediately jumping to the box you think is wrong, scan the key sections that could be related. Some errors ripple across the return. For example, a trading profit error can affect Class 4 National Insurance, payments on account, and certain reliefs.

Step 3: Make only the necessary changes. Change the figure or answer that was incorrect, and ensure any related entries are updated. If you’re adding missing income, check whether you also need to add associated tax deducted or expenses.

Step 4: Recheck the tax calculation. Once you’ve updated the return, view the calculation again. Compare it to your expectations. If the new figure still looks strange, trace it: look at allowances, reliefs, higher rate thresholds, student loan plan type, High Income Child Benefit Charge, and payments on account. These can all materially change the result.

Step 5: Resubmit the amended return and save confirmation. You should receive a submission receipt or confirmation. Save it. Download or print the amended return for your records. Keep a short note of what you changed and why.

If you used commercial software or an accountant, the steps differ slightly: you would amend via the same software or through your agent. The concept is the same: an amended return replaces the earlier version for that tax year (subject to any enquiry rules).

What if you filed on paper instead of online?

If you filed a paper return, correcting an error typically involves writing to HMRC rather than editing an online form. The correction should be clear, specific, and complete. You want HMRC to understand exactly what is changing, for which tax year, and why. Vague or partial letters can lead to delays or misunderstandings.

A practical structure for a paper-return correction letter includes:

1) Your details: full name, address, UTR, and National Insurance number.

2) The tax year: explicitly state the tax year you are correcting.

3) What you originally submitted: specify the figure/entry that was wrong and where it appears (for example, “Employment income box,” “Self-employment turnover,” “Property income expenses,” “Gift Aid payments”).

4) The corrected information: provide the corrected figure and, if helpful, a brief calculation.

5) Why it changed: a short explanation (for example, “I omitted a P60,” “I duplicated an expense,” “I used the wrong exchange rate,” “I misunderstood allowable expenses”).

6) Supporting evidence: mention what documents you have and include copies if appropriate. Keep originals.

When posting anything to HMRC, consider using a tracked service and keep a copy of the letter and enclosures. This helps if there is any dispute about what was sent and when.

Fixing errors after the amendment window: what to do when it’s “too late” to amend

Sometimes you discover the issue after the normal amendment period has passed. That doesn’t automatically mean you’re stuck. It does mean you need to use a different mechanism and be more careful about how you present the correction.

There are a few scenarios that commonly arise:

Scenario A: You underpaid tax because of the error. You should still tell HMRC and settle the difference. Even if you can’t amend online, you can usually disclose the error, explain what happened, provide corrected figures, and arrange payment. Acting voluntarily is generally better than waiting for HMRC to find the discrepancy through checks or third-party data.

Scenario B: You overpaid tax. You may be able to claim back overpaid tax using a formal claim process. This might involve an “overpayment relief” claim. The process typically requires you to show that you made an error in the return and that the correction results in tax being overpaid. Your claim must be accurate, supported, and within the relevant time limit.

Scenario C: The error is not about tax due but about information. For example, you may have ticked the wrong box, included a wrong reference, or omitted details that don’t change the tax calculation. HMRC may still want the record corrected. Writing with a clear explanation is usually the way forward.

In all cases, write clearly and keep copies. If the sums are significant or the situation is complex (for example, multiple years affected, offshore income, complex reliefs, or uncertainty about the correct treatment), consider getting professional advice so the correction is technically sound.

Common errors and how to correct them

Knowing the type of mistake you’ve made can help you correct it efficiently. Here are some frequent error categories and what to watch out for when fixing them.

1) Missing employment income or wrong PAYE figures

A classic error is omitting a job, using the wrong pay and tax deducted figures, or confusing taxable pay with total pay. Fixing this often means updating the employment section with the correct figures from your P60 or final payslip, including the tax deducted and any benefits reported on P11D if relevant.

When correcting PAYE entries, ensure you include:

- Employer name and PAYE reference (if required).

- Taxable pay for the year.

- Tax deducted.

- Benefits or expenses if applicable.

After the correction, recheck whether the change affects your overall liability, payments on account, and any student loan repayment calculations.

2) Forgetting self-employment income or mixing up turnover and profit

Self-employment sections can be tricky because you may be entering turnover, allowable expenses, and then profit. Some taxpayers accidentally enter profit in the turnover box or forget to include an expense category they’re entitled to claim.

When correcting self-employment figures, verify your bookkeeping totals and ensure your expenses are legitimate and allowable. If you use simplified expenses (for example, mileage or working from home flat rates), be consistent and avoid double-claiming by also claiming actual costs in a way that overlaps.

If the error changes your profit, it can affect:

- Income tax bands.

- Class 2 and Class 4 National Insurance.

- Payments on account for the next year.

- Eligibility for certain reliefs and benefits.

3) Property income mistakes: rent, expenses, and the difference between capital and revenue

Rental income mistakes often involve missing months, incorrect agent statements, or claiming capital improvements as expenses. Repairs that maintain the property are typically treated differently from improvements that enhance it. If you misclassified costs, correct the relevant boxes and keep clear notes and receipts to show the nature of the work.

Also check whether you have multiple properties, overseas property income, or jointly owned properties. Ownership share matters. If you split income incorrectly between you and a spouse or co-owner, you’ll need to correct both the income and associated expenses proportionally.

4) Interest, dividends, and savings income

Interest and dividends are increasingly reported to HMRC by banks and platforms, which can make omissions more likely to be noticed. If you forgot to include savings interest or dividends, correct it promptly. Make sure you use the right tax year statements, not calendar-year summaries.

Also consider whether any of the income is within an ISA (which is not taxable) or whether you included ISA income by mistake. Correcting this can reduce your tax due, but you should be able to show how you determined what was taxable and what was sheltered.

5) Pension contributions and Gift Aid

Reliefs such as pension contributions (especially personal pension contributions where you contribute net and the provider claims basic rate relief) and Gift Aid donations can change your tax position materially. A common mistake is entering the net amount instead of the grossed-up amount (or vice versa) depending on the box and the guidance.

When fixing these entries, check the exact amount paid and how relief is applied. Also check whether you are claiming higher-rate relief properly. Correcting these figures may increase your basic rate band and can affect your personal allowance taper and the High Income Child Benefit Charge in some cases.

6) Student loan and postgraduate loan plan selection

Some errors are not about income but about the loan plan selected. Choosing the wrong plan (or forgetting to indicate you have a loan) can change the calculation. If this is wrong, correct the selection and ensure your employment deductions are properly reflected. Student loan repayments are sensitive to thresholds and plan type, so verify the plan you are on.

7) High Income Child Benefit Charge (HICBC)

If you or your partner received Child Benefit and your adjusted net income exceeds the relevant threshold, you may need to pay the charge. Errors include forgetting to include the charge, incorrectly assuming it doesn’t apply, or miscalculating adjusted net income by ignoring pension contributions or Gift Aid.

Correcting this area can be stressful because it can feel like a surprise liability. If the charge should apply, include it and pay as soon as possible to reduce interest. If you believe it shouldn’t apply, ensure your calculations for adjusted net income are accurate and supported.

What if the mistake means you owe more tax?

If the correction increases your tax bill, the most important thing is to act quickly and transparently. HMRC generally expects taxpayers to take reasonable care and to correct errors once discovered. Waiting can lead to more interest and may increase the risk of penalties, especially if HMRC believes the error was careless or deliberate.

Here’s a practical approach if you now owe additional tax:

1) Amend or disclose the error immediately. If you can amend online, do it. If you can’t, write to HMRC with the corrected figures and explanation.

2) Calculate the additional tax as accurately as possible. The amended calculation may show it, but if you are writing in, provide your best estimate. HMRC will confirm the revised position.

3) Pay the additional amount as soon as you can. Even if the exact figure is pending, paying a reasonable estimate can reduce interest. If you later overpay, the excess can be refunded or credited.

4) If you can’t pay in full, engage early. HMRC has processes for payment arrangements. The key is not to ignore the situation.

5) Keep records of what you did and when. If a penalty question arises, documentation that you corrected promptly and cooperated can matter.

Try not to panic. Many people make genuine mistakes, especially when their tax affairs are changing (new job, side business, investments, property, benefits, or family changes). What tends to cause bigger problems is not the mistake itself but failing to address it.

What if you overpaid and you’re due a refund?

If the correction reduces your tax liability, you may be due a refund. The route depends on whether you can amend the return. If you can amend online, the recalculated position should eventually lead to a repayment or an adjustment against other liabilities. If you’re outside the amendment window, you may need to make a formal claim for repayment, explaining the error and providing the correct figures and evidence.

When seeking a refund, be prepared for HMRC to request more detail. That’s not necessarily a sign of trouble; it’s often routine. Make your claim easy to follow:

- Clearly identify the tax year.

- Explain the original entry and the corrected entry.

- Show how the difference affects tax.

- Attach or offer supporting documents.

If the overpayment is linked to payments on account, a correction can also change what you owe for the next year. That can reduce future payments or create a repayment depending on your circumstances.

Fixing errors related to payments on account

Payments on account can confuse even experienced taxpayers. They are advance payments toward your next tax bill, usually due in two instalments. If your amended return changes your previous year’s liability, it may also change the payments on account calculated for the following year.

Common issues include:

- Not realising that the “amount due” includes a payment on account, not just the balancing payment.

- Expecting a refund but seeing a charge because the system is calculating next year’s advance payments.

- Amending profit downwards and forgetting to adjust payments on account accordingly.

When you amend, review the updated statement of account and the breakdown between balancing payments and payments on account. If your income is significantly lower in the following year, there may be a legitimate reason to reduce payments on account, but reducing them too far can lead to interest if you underpay. Be cautious and base reductions on realistic estimates.

What happens if HMRC has already started checking your return?

Sometimes you find a mistake and then realise HMRC has opened an enquiry or asked questions. The presence of a check doesn’t mean you can’t correct an error, but it can change how the process unfolds. If HMRC is actively reviewing your return, they may prefer the correction to be handled through the enquiry process rather than by a routine amendment.

If you have received a letter or message indicating HMRC is checking your return:

- Respond promptly.

- Be honest about the mistake and explain it clearly.

- Provide the corrected figures and supporting evidence.

- Keep copies of everything you send.

In this scenario, it can be especially helpful to be organised and concise. If the error is substantial or the enquiry is detailed, professional support may be worthwhile to ensure the technical position is correct and your communications are effective.

How to write a clear disclosure to HMRC when you can’t amend online

When you can’t fix the issue by amending online, your written disclosure becomes your main tool. The aim is to make it easy for HMRC to process. A useful disclosure is structured, fact-based, and includes the information HMRC needs without unnecessary narrative.

Consider the following outline:

Paragraph 1: Identify the return. “I am writing to correct an error on my Self Assessment tax return for the tax year [year]. My details are [name, address, UTR, NI number].”

Paragraph 2: State the error. “In the submitted return, I reported [original figure] in [section/box/description]. This was incorrect because [short reason].”

Paragraph 3: Provide the correction. “The correct figure is [correct figure]. The difference arises from [brief calculation or explanation].”

Paragraph 4: Tax impact and payment. “This change means I believe additional tax of approximately [amount] is due / I believe I have overpaid tax of approximately [amount]. I have paid [amount] on [date] / I request the overpayment be repaid or credited.”

Paragraph 5: Evidence and cooperation. “I enclose copies of [documents]. I will provide any further information you need.”

This format shows reasonable care and makes it clear you are actively correcting the record.

Penalties, interest, and how to reduce the risk

People often worry that any correction will automatically trigger penalties. In reality, penalties depend on the nature of the error and your behaviour. The system generally distinguishes between innocent mistakes despite reasonable care, careless errors, and deliberate inaccuracies. Your best risk-reducer is to correct errors promptly, be transparent, and keep evidence that supports your position.

Interest is different from penalties. Interest is usually charged when tax is paid late, even if the error was innocent. Paying as soon as you can minimises interest. If you can’t pay immediately, engaging early and arranging a payment plan can prevent the situation from worsening.

Practical ways to reduce risk include:

- Keeping good records and a clear audit trail.

- Using accurate source documents (P60s, statements, invoices).

- Avoiding “guessing” when a figure can be determined with reasonable effort.

- Correcting mistakes as soon as you discover them.

- Taking advice for complex areas (property, capital gains, foreign income, reliefs).

Capital gains and other complex areas: tread carefully

Some errors are straightforward, like entering the wrong number from a P60. Others are more technical, such as capital gains calculations, share disposals, cryptoasset transactions, foreign income, remittance basis issues, and reliefs with specific conditions. If your error falls into a complex category, don’t rush. Gather full transaction records and ensure you understand the correct tax treatment before submitting an amendment or disclosure.

For example, capital gains errors can arise from:

- Using the wrong acquisition cost.

- Forgetting pooled cost rules for shares.

- Missing allowable costs (such as transaction fees).

- Misapplying reliefs or exemptions.

- Reporting disposals in the wrong tax year.

Correcting a complex error may require recalculating multiple transactions and ensuring the revised figures reconcile to your records. If you’re unsure, professional advice can help you avoid replacing one mistake with another.

Keeping your records: what to retain after you correct a return

When you amend or otherwise correct a return, keep a tidy record pack. This protects you if HMRC asks follow-up questions and helps you if you need to revisit the issue later. A sensible record pack includes:

- The original submitted return (or a copy of it).

- The amended return and submission confirmation (if amended online).

- A brief change log describing what you corrected and why.

- Supporting documents: statements, receipts, invoices, vouchers, P60/P45, pension statements, donation records.

- Any correspondence with HMRC, including dates and copies of letters.

- Proof of payments made, including reference numbers.

Clear records also make next year’s return easier because you can carry forward accurate information and avoid repeating the same confusion.

Preventing repeat errors: a simple pre-submission checklist for next time

Fixing an error can be a useful prompt to improve your process for future returns. You don’t need an elaborate system. A short checklist can reduce mistakes dramatically:

1) Income sources check: employment, self-employment, property, dividends, interest, freelance gigs, CIS, benefits that are taxable, overseas income.

2) Tax deducted check: PAYE tax, CIS deductions, bank tax deductions (if any), withheld amounts, any tax credits.

3) Reliefs check: pension contributions, Gift Aid, marriage allowance transfer (if relevant), professional subscriptions, allowable expenses.

4) Big-ticket flags: capital gains, sale of property, large one-off income, redundancy payments, share options, foreign income.

5) Personal details: address, name, marital status where relevant to claims, student loan plan, child benefit status.

6) Reconciliation: compare totals to your bookkeeping or annual summaries. If the numbers are wildly different from the prior year, ask why before submitting.

Doing these checks before submission can save time and stress later.

When to consider professional help

Many amendments are easy. But some situations justify expert support, particularly where the cost of getting it wrong is high or where you are unsure of the correct treatment. Consider professional help if:

- The error involves large amounts of tax.

- Multiple tax years are affected.

- You have complex income streams (foreign income, multiple properties, investment disposals).

- HMRC is already enquiring or you have received compliance correspondence.

- You are unsure whether something is an allowable expense or a capital item.

- You suspect you may have made the same mistake in previous years.

An accountant or tax adviser can help you correct the return properly, communicate with HMRC effectively, and reduce the risk of compounding errors.

Final thoughts: act promptly and keep it clear

An error on a submitted Self Assessment tax return is not the end of the world, and in most cases it’s fixable. The best approach is to confirm the mistake, gather your evidence, correct it using the appropriate method for the timing, and keep strong records of what changed. If the correction increases your tax, pay as soon as possible and communicate openly. If it reduces your tax, make the amendment or claim clearly so the repayment process is smooth.

Most importantly, don’t let embarrassment or anxiety delay you. Tax returns are complex, life is busy, and mistakes happen. What matters is how you respond once you discover the problem: promptly, accurately, and transparently.