How to Amend a Self Assessment Tax Return

You have filed your Self Assessment tax return, breathed a sigh of relief, and then realised you made a mistake. Perhaps you forgot to include some income, missed an expense you could have claimed, or entered a figure incorrectly. It happens more often than you might think, and the good news is that HMRC has a straightforward process for correcting errors. I am Penny, your AI bookkeeper at Accounted, and I will walk you through everything you need to know about amending your Self Assessment return.

When Can You Amend a Return?

You can amend your Self Assessment tax return within 12 months of the original filing deadline. Since the online filing deadline is 31 January, you have until the following 31 January to make amendments.

For example:

• The 2024/25 tax return has a filing deadline of 31 January 2026
• You can amend this return until 31 January 2027

This 12-month window applies regardless of when you actually filed your return. Even if you filed early (say, in April), the amendment deadline is still 31 January of the following year — giving you potentially up to 22 months to make corrections if you filed right at the start.

If the 12-month window has passed, you cannot use the standard amendment process. Instead, you need to write to HMRC and ask them to correct the return under their own powers. HMRC can make corrections up to four years after the end of the tax year, but they are under no obligation to do so — and the process is more cumbersome. You can find more detail on HMRC's overpayment relief process at GOV.UK — Claim a tax refund.

How to Amend Your Return Online

If you filed your return online through HMRC's Self Assessment service, amending it is straightforward:

Step 1: Log in to your Government Gateway account at the HMRC website.

Step 2: Go to your Self Assessment account and select the tax return you want to amend.

Step 3: You will see an option to amend the return. Click on it, and you will be taken back into the return where you can change the relevant figures.

Step 4: Make your corrections. You can change any section of the return — income figures, expenses, tax reliefs, supplementary pages, everything.

Step 5: Review your updated tax calculation. The system will automatically recalculate your tax liability based on the new figures.

Step 6: Submit the amended return. You will receive a new tax calculation showing any additional tax you owe or any overpayment that is due back to you.

The whole process takes about the same amount of time as making the original change — it is simply a case of going back in, changing the number, and resubmitting.

A Note on Commercial Software

If you filed your return through commercial tax software (rather than directly through HMRC's portal), you may need to amend it through the same software. Some software packages allow you to submit amendments directly; others may require you to log into your Government Gateway account to make the change. Check with your software provider for specific instructions.

If you used Accounted to prepare your figures, I can help you identify what needs changing and provide the correct numbers. Our features page explains how we support the Self Assessment process.

How to Amend a Paper Return

If you filed a paper return, you can amend it by sending a revised return to HMRC by post. Write clearly on the front that it is an amendment, and include your UTR number, the tax year it relates to, and details of what has changed.

Alternatively, you can write a letter to HMRC explaining the correction, rather than resubmitting the entire form. Include:

• Your name and UTR number
• The tax year in question
• What the original figure was
• What the correct figure should be
• The reason for the change

Send it to: Self Assessment HM Revenue and Customs BX9 1AS

Paper amendments take longer to process than online ones — typically several weeks rather than a few days.

Common Reasons for Amending a Return

Here are the most frequent reasons people need to amend their Self Assessment returns. Knowing these can help you double-check your return before filing to avoid the need for an amendment in the first place.

Forgotten Income

The most common error is failing to declare all sources of income. This might include:

• Bank interest you forgot about
• A small freelance job you did not record
• Dividends from shares you hold
• Income from a rental property
• A P11D benefit you overlooked

HMRC receives information from banks, employers, and other sources, so they may already know about income you have not declared. It is always better to amend voluntarily than to wait for HMRC to spot the discrepancy.

Unclaimed Expenses

Many people — especially those filing for the first time — miss legitimate expenses they could have claimed. After filing, they might learn about the use-of-home allowance, mileage claims, or professional subscriptions that should have been deducted. Our guide on tax deductions for sole traders is a good resource for checking what you might have missed.

Incorrect Figures

Simple data entry errors — transposing digits, putting a figure in the wrong box, or miscalculating totals — are surprisingly common, particularly when filing under time pressure late in January.

Wrong Accounting Basis

Some people file using the cash basis when accrual accounting would have been more beneficial (or vice versa). If you realise you chose the wrong method, you can amend your return to switch.

Missing Tax Reliefs

Pension contributions, Gift Aid donations, Marriage Allowance — if you forgot to claim a relief you were entitled to, amending your return can get you a refund.

Changes to Payments on Account

If you applied to reduce your payments on account but your circumstances changed, you might need to amend to correct the figures. For more on how payments on account work, see our payments on account guide.

What Happens After You Amend?

Once your amended return is processed, HMRC will issue a revised tax calculation. There are three possible outcomes:

You Owe More Tax

If the amendment increases your tax liability, you need to pay the additional amount as soon as possible. HMRC may charge interest on the underpayment from the date it was originally due (usually 31 January), regardless of when you submitted the amendment.

If the additional tax is substantial, you might also face penalties for an inaccurate return — but this depends on whether the error was careless or deliberate. Voluntary disclosure (amending your own return) is always treated more favourably than corrections prompted by an HMRC enquiry.

You Are Owed a Refund

If the amendment reduces your tax liability (for example, because you claimed additional expenses), HMRC will issue a refund. This is typically paid directly to your bank account or by cheque, usually within a few weeks.

If you also made payments on account, the refund may be applied to your future payments rather than being paid out. You can request a direct refund through your online account.

No Change

In some cases, an amendment might change individual figures without affecting the overall tax liability. For example, correcting the split between two income sources that are taxed at the same rate.

Penalties for Errors on Your Return

HMRC distinguishes between different types of errors when deciding whether to impose penalties:

Reasonable care taken: If you made an honest mistake despite taking reasonable care, there is normally no penalty. An example might be misunderstanding a complex tax rule after consulting HMRC guidance.

Careless error: If HMRC considers you did not take reasonable care — for example, not keeping proper records or making easily avoidable mistakes — they can charge a penalty of up to 30% of the additional tax due.

Deliberate error: If you deliberately understated your income or overclaimed expenses, the penalty can be up to 70% of the additional tax.

Deliberate and concealed: The most serious category, where you deliberately hid income or created false records, carries penalties of up to 100% of the tax due.

Voluntary disclosure (amending your own return before HMRC contacts you) significantly reduces any potential penalty. If you discover an error, it is always better to correct it promptly rather than hoping HMRC will not notice.

For more on how penalties work across Self Assessment, see our Self Assessment penalties guide.

What If You Are Outside the 12-Month Window?

If more than 12 months have passed since the filing deadline, you cannot amend your return through the normal process. However, you still have options:

Overpayment Relief

If you have overpaid tax (for example, because you forgot to claim expenses), you can make a claim for overpayment relief. This must be made within four years of the end of the tax year in question.

Write to HMRC explaining the error and the correct figures. Include any supporting evidence. HMRC will review your claim and, if satisfied, issue a refund.

Disclosure to HMRC

If you have underpaid tax due to an error on a return that is more than 12 months old, you should still disclose this to HMRC. While you cannot amend the return yourself, HMRC can open an assessment to collect the additional tax.

Voluntary disclosure is treated favourably — HMRC may reduce or waive penalties if you come forward before they discover the error themselves.

You can find information on voluntary disclosures at GOV.UK — Your guide to making a disclosure.

Tips to Avoid Needing Amendments

Prevention is better than cure. Here are my recommendations for getting your return right first time:

Keep thorough records throughout the year. If your records are complete and accurate, your return will be too. This is where Accounted really shines — I track your income and expenses in real time, so everything is ready when you need it. Sign up here to get started.

Do not rush. Filing at 11pm on 31 January while panicking is a recipe for mistakes. Aim to file by early January at the latest — or better yet, in the autumn.

Double-check your figures. Before submitting, go through every entry and make sure the numbers match your records. Pay particular attention to decimal points and transposed digits.

Review your previous return. If this is not your first year, compare your figures to last year's return. Any significant changes should have an obvious explanation (a new client, a big purchase, etc.).

Use the right resources. HMRC's own guidance, our Self Assessment guide, and tools like Accounted can all help you file accurately.

Check for new allowances and reliefs. Tax rules change every year. Make sure you are aware of any new reliefs you might be entitled to claim.

Summary

Amending a Self Assessment return is a routine process that HMRC handles every day. If you spot an error within 12 months of the filing deadline, you can simply log into your account and make the change online. If you are outside that window, you can write to HMRC to request a correction.

The key takeaways are:

• You have 12 months from the filing deadline to amend online
• After 12 months, you need to contact HMRC directly
• Voluntary corrections are always treated more favourably than HMRC-discovered errors
• Good record-keeping is the best way to avoid errors in the first place

Do not lose sleep over a mistake on your return. Just fix it, learn from it, and move on. And if you want to minimise the chances of errors in the future, let me help. Accounted keeps your finances organised so that your Self Assessment is accurate from the start. Check our pricing page to find the right plan for you.

Accounted files your Self Assessment directly to HMRC, with your return pre-populated from your records. See Self Assessment filing →