How to Amend a Self Assessment Tax Return — A Trade Owner's Guide (2026)

Quick Reference: Amending vs Correcting Later

The Key Deadline: 12 Months From the Filing Deadline

The single most important rule is this: you normally have 12 months from the Self Assessment filing deadline to amend a return. Note that the clock runs from the filing deadline, not the date you actually filed.

Self Assessment runs on tax years that end on 5 April. The online filing deadline is 31 January following the end of the tax year. So the relationship looks like this:

• The 2024/25 tax year runs from 6 April 2024 to 5 April 2025.
• The online filing deadline for that year is 31 January 2026.
• You can therefore amend the 2024/25 return until 31 January 2027 — 12 months after the filing deadline.

The same pattern repeats every year. For the 2025/26 tax year (ending 5 April 2026), the filing deadline is 31 January 2027, so you would have until 31 January 2028 to amend. As long as you are inside that 12-month window, amending is straightforward and there is no need to ask HMRC for permission — it is your right to correct the return.

How to Amend Your Return

How you amend depends on how you filed in the first place. There are three routes, and you should use the one that matches your original submission.

1. Online via your HMRC account

If you filed online through HMRC's own service, log in to your Government Gateway account, go to your Self Assessment, choose the tax year you want to change and select the option to amend. Work through the sections, update the figures that were wrong — for example your turnover, allowable expenses or capital allowances — and submit. Your tax calculation updates automatically once the change is processed.

2. Through your commercial software

If you filed using commercial accounting or tax software, you generally cannot amend through the HMRC website — you amend through the same software and resubmit. Open the relevant return in your software, correct the figures and file the amended version. Check your provider's help notes, as the exact steps vary by package.

3. By writing to HMRC

If you filed a paper return, or you are outside the period where the online amendment option is available, you can write to HMRC instead. Send a letter to the Self Assessment address explaining clearly which tax year you are amending, what the original figures were, what they should be and why. Quote your Unique Taxpayer Reference (UTR) on every page so it is matched to your record.

Whichever route you use, an accountant or tax agent can file the amendment on your behalf if you would rather not handle it yourself.

What Happens to Your Tax Bill

When you amend, HMRC recalculates the tax due on the corrected figures. There are two possible outcomes:

• You owe more tax. If your amendment increases your taxable profit — say you forgot to include some cash jobs — the recalculation produces an extra amount to pay. Pay it promptly, because interest runs on tax paid late (more on that below).
• You are due a refund. If your amendment reduces your profit — for example you missed a chunk of allowable expenses — the recalculation shows you overpaid, and HMRC repays the difference or sets it against other tax you owe.

Your payments on account may also change as a result. If your corrected liability is higher or lower, the on-account instalments toward next year's bill are adjusted to match.

Worked Example: A Tradesperson Who Forgot Expenses

Liam is a self-employed electrician. He rushed his 2024/25 return in January 2026 and declared £48,000 of profit. In March he finds a folder of receipts he never entered: £2,400 of tools and test equipment, plus £1,600 of van running costs he had forgotten to claim — £4,000 of allowable expenses in total.

Because he is well inside the window (he can amend the 2024/25 return until 31 January 2027), Liam logs into his HMRC account, opens the 2024/25 return and adds the missed expenses. His profit drops from £48,000 to £44,000. HMRC recalculates the tax and Class 4 National Insurance on the lower figure, and because he had already paid tax on the higher profit, he is due a refund of the difference. He keeps the receipts and a short note of what he changed in case HMRC ever asks him to support the amendment.

The lesson for trade owners: missed expenses are real money. Keeping every receipt and reconciling your records before you file — and being ready to amend if something slips through — keeps your tax bill honest in both directions.

After the Amendment Window Has Closed

Once the 12-month window has passed you can no longer simply "amend" the return. What you do next depends on whether you paid too much or too little.

If you overpaid: claim overpayment relief

If you paid more tax than you should have, you can make a claim for overpayment relief. This must generally be made within 4 years of the end of the tax year the claim relates to. The claim has to be in writing, state that you are claiming overpayment relief, identify the tax year and amount, explain why you believe you overpaid, and confirm you have not previously claimed for the same matter. Include evidence — such as the receipts or records that support the corrected figures.

If you under-declared: tell HMRC

If you realise you under-declared income or over-claimed expenses, you have a duty to tell HMRC. You cannot just leave it because the amendment window has closed. Notify HMRC as soon as you become aware — you can make a disclosure online or in writing. Coming forward voluntarily, before HMRC contacts you, almost always leads to a better outcome than waiting to be found.

Innocent Errors vs Careless or Deliberate Behaviour

HMRC treats mistakes very differently depending on the behaviour behind them. This is central to how any penalty is worked out.

• Innocent error: a genuine mistake made despite taking reasonable care — for example a typo or an honest miscalculation. Where you genuinely took reasonable care, there may be no penalty for an inaccuracy at all.
• Careless: you did not take reasonable care — for example you failed to keep proper records or guessed at figures. This attracts a penalty, but a lower one than deliberate behaviour.
• Deliberate: you knowingly submitted wrong figures. This carries the highest penalties, and if you also took steps to conceal it, the penalty is higher still.

Penalties for inaccuracies are charged as a percentage of the extra tax due, and the percentage depends both on the behaviour (careless vs deliberate) and on whether your disclosure was prompted (you came forward only after HMRC started asking questions) or unprompted (you told HMRC before they had any reason to suspect a problem). Unprompted disclosures attract lower penalties, which is a strong reason to put errors right the moment you spot them.

Interest on Late-Paid Tax

Separately from any penalty, HMRC charges interest on tax paid late. If your amendment or disclosure increases the tax due for an earlier year, interest runs from the date that tax was originally due until the date you pay it. Interest is not a penalty — it applies regardless of whether your error was innocent or careless — so the longer an underpayment sits unpaid, the more it costs you. Paying any additional tax as soon as you can limits the interest that builds up.

Keep Records to Support the Change

Whether you are amending inside the window or claiming overpayment relief afterwards, keep the records that back up your new figures: receipts, invoices, bank statements, mileage logs and a short note of exactly what you changed and why. HMRC can ask you to support an amendment, and well-organised records turn a potential query into a five-minute reply. As a self-employed trader you should keep business records for at least five years after the 31 January filing deadline for the relevant tax year in any case.

If the change is significant — large sums, several years affected, or anything touching deliberate behaviour — it is worth having an accountant file the amendment or disclosure for you. They can present the figures correctly, handle the correspondence and help you reach the best penalty position.

The Bottom Line

Filing the wrong figures is not the disaster it feels like in the moment. Inside the 12-month window, amending is simple and entirely within your control. Outside it, overpayment relief gets your money back if you paid too much, and a prompt, unprompted disclosure keeps penalties to a minimum if you paid too little. The two habits that protect every trade owner are the same: keep good records, and act the moment you spot a mistake. This guide is general information and not a substitute for advice on your specific circumstances.