How to Amend a Self Assessment Tax Return

When can you amend a Self Assessment tax return?

The normal rule is straightforward: you have 12 months after the statutory Self Assessment filing deadline to amend your return.

For most taxpayers, that means:

The position can be different if HMRC issued your notice to file unusually late. HMRC’s legislation and guidance provide special rules in those circumstances.

Importantly, filing your original return late does not necessarily give you another full year to amend it. The amendment period is generally linked to the statutory filing deadline rather than the date you eventually submit the return.

You can check the wider filing timetable in our guide to Self Assessment tax return deadlines.

What mistakes can you correct on a Self Assessment return?

You can amend a return when information you originally submitted was incomplete or incorrect.

Common reasons include:

• forgetting self-employed income;
• entering turnover or business expenses incorrectly;
• missing an allowable business expense;
• reporting income twice;
• using a provisional figure that is now final;
• forgetting bank or savings interest;
• omitting pension information;
• entering the wrong property income or expenses;
• missing Construction Industry Scheme deductions;
• entering figures in the wrong section of the return; or
• discovering that bookkeeping records were incomplete.

For sole traders, amendments often arise because the underlying bookkeeping changes after the return has been submitted.

For example, you may find an invoice that was accidentally left out of your sales records. Alternatively, you might discover legitimate business expenses that were not included.

If you are unsure whether a business cost is deductible, our guide to allowable expenses for sole traders explains the main principles.

How to amend a Self Assessment tax return online

If you submitted directly through HMRC’s online Self Assessment service, HMRC says you must first wait three days, or 72 hours, after filing.

You can then:

1. Sign in to your HMRC online account.
2. Open your Self Assessment account.
3. Select More Self Assessment details.
4. Choose At a glance.
5. Select Tax return options.
6. Choose the tax year you need to correct.
7. Open the return and amend the relevant figures.
8. Review the updated calculation.
9. Submit the amended return.

HMRC provides the current process in its guidance on changing a Self Assessment return.

Do not simply change one figure without checking the rest of the return. One adjustment can affect several parts of the tax calculation.

Example: a freelancer finds missing income

A freelance designer submits their 2025/26 return showing self-employed turnover of £42,000.

Two weeks later, they discover a £1,500 customer payment that was accidentally excluded from their bookkeeping.

The correct turnover is therefore £43,500.

The freelancer should update the relevant self-employment figures and submit an amended return. They should then review HMRC’s revised calculation rather than assuming the additional tax equals a fixed percentage of the missing £1,500.

The final effect depends on their complete tax position.

What if you filed through accounting or tax software?

If you submitted the original return using commercial Self Assessment software, HMRC advises contacting the software provider for instructions on correcting it.

Normally, you should amend the return through the same software rather than separately changing individual figures through another system.

This helps maintain a consistent audit trail between your bookkeeping, submitted return and revised figures.

The position is particularly relevant for people using Making Tax Digital for Income Tax. HMRC says taxpayers already within MTD for Income Tax should make changes to their tax return using compatible software.

You can read more about digital records and reporting in our Making Tax Digital for Income Tax guide.

How do you amend a paper Self Assessment return?

If you originally filed on paper, you can still make an amendment within the permitted period.

For the main tax return, HMRC allows you to obtain another SA100 form. You should send the corrected pages rather than simply writing an informal note with replacement figures.

HMRC says you should:

• write “amendment” on each corrected page;
• include your name;
• include your Unique Taxpayer Reference (UTR); and
• send the pages to the address shown on your Self Assessment paperwork.

If you cannot find the address, HMRC’s current guidance provides a central Self Assessment postal address.

Keep a copy of everything you send.

What happens to your tax bill after an amendment?

Your Self Assessment calculation will be updated using the revised information.

That can result in three broad outcomes:

• you owe more tax;
• you owe less tax; or
• the amendment does not change the amount payable.

If the amendment increases your liability, check the updated calculation and payment position promptly.

Interest can apply where additional tax should have been paid by an earlier statutory payment date. HMRC confirms that late-payment interest can arise on tax becoming payable following an amendment to a Self Assessment.

An amendment may also affect other parts of your Self Assessment calculation, including payments on account where they apply.

Example: an overlooked business expense

A self-employed consultant submits a return and later discovers £800 of properly supported allowable business costs that were omitted.

They should not simply assume HMRC will refund £800.

Instead, the £800 may reduce taxable business profit. The actual tax effect depends on the consultant’s overall income, allowances, tax rates and other circumstances.

Accurate bookkeeping makes this kind of correction much easier. Real Key Accountancy’s small business accountancy services include support with bookkeeping and Self Assessment records.

What if the Self Assessment amendment deadline has passed?

You cannot normally make a standard amendment indefinitely.

If the normal 12-month amendment period has expired, the correct next step depends on whether the mistake means you paid too little or too much tax.

If you did not report enough income or tax

HMRC says you should write to them and explain:

• which tax year needs correcting;
• why you believe too little tax was paid; and
• how much you believe was underpaid.

The notification must also be signed. HMRC will review the information and may revise the amount due.

Do not ignore a known underpayment simply because the online amendment option has disappeared.

If you believe you paid too much tax

You may be able to make an overpayment relief claim.

The normal time limit is four years after the end of the relevant tax year. However, overpayment relief has detailed conditions and exclusions, so it should not be treated as an automatic four-year extension to every type of tax claim.

HMRC requires an overpayment relief claim to contain specific information and a signed declaration. Its Self Assessment correction guidance explains the current requirements.

Will HMRC charge a penalty for amending a tax return?

Making an amendment does not automatically mean you will receive a penalty.

HMRC’s guidance states that an inaccuracy made despite taking reasonable care is not normally penalised. Penalties can arise where an inaccurate return understates tax and the behaviour that caused the problem was careless or deliberate.

Therefore, there is an important difference between correcting a genuine mistake and deliberately providing information you know is wrong.

Once you discover a material error, correcting it promptly is sensible. HMRC also considers the quality of a taxpayer’s disclosure when determining applicable penalties.

What records should you check before amending your return?

Before submitting an amended return, establish why the original figure was wrong.

Depending on your circumstances, check:

• sales invoices and customer payments;
• business bank statements;
• payment-platform statements;
• purchase invoices and receipts;
• mileage records;
• CIS statements;
• employment documents;
• pension statements;
• savings and investment information;
• rental income and property expenses; and
• the original tax return and calculation.

Then keep evidence showing how you arrived at the amended figure.

Good records matter because changing a number without understanding its source can create a second error.

For ongoing support, Real Key Accountancy provides bookkeeping packages for sole traders and small businesses designed to keep income and expenses organised throughout the year.

Common mistakes when amending Self Assessment

One common mistake is correcting the return too quickly. Check whether the error affects other boxes before resubmitting.

Another is entering an adjustment without retaining evidence. Your bookkeeping should explain both the original figure and the correction.

Also avoid these problems:

• assuming the deadline runs from the date you filed;
• forgetting to submit the amended return after editing it;
• overlooking the revised tax calculation;
• ignoring additional tax because the original payment deadline has passed;
• trying to amend online after the statutory amendment period has expired;
• claiming a refund without checking whether overpayment relief rules apply; or
• using different figures in your accounting software and tax return without reconciling them.

If the amendment involves several income sources, capital gains, older tax years or a significant underpayment, professional advice may be worthwhile before anything is resubmitted.

Need help correcting your Self Assessment return?

A straightforward amendment may only involve correcting one or two figures. However, the underlying records should still support the revised return.

Real Key Accountancy provides practical Self Assessment, bookkeeping and financial administration support for sole traders, freelancers and small businesses.

If you have discovered missing transactions, incomplete bookkeeping or figures that no longer look right, speak to Real Key Accountancy about organising the records and establishing the next step.