How to Amend a Self Assessment Tax Return (and Check Your Figures First)

If you have already filed your Self Assessment return and realised something was missing, you can usually fix it online yourself. HMRC lets you amend a return for 12 months after the 31 January filing deadline, so the 2024/25 return can be amended until 31 January 2027 and the 2025/26 return until 31 January 2028. The harder part is getting the corrected figures right first, so you only amend once.

Can I amend my Self Assessment return after I have submitted it?

Yes, you can amend your Self Assessment return after submitting it, for up to 12 months after the 31 January filing deadline for that tax year.

How do I amend my Self Assessment return?

You amend your Self Assessment return the same way you originally filed it: through your HMRC online account if you filed it there, or through the same commercial software if you used software.

A return filed one way cannot be corrected another way. HMRC's online amendment screen only works for returns filed through HMRC's own online service, so a return sent through software will not appear there for you to change. If you cannot remember how you filed, look for your submission receipt: HMRC's online service shows your returns under Tax return options, and software providers usually email a confirmation when they submit.

If you filed through your HMRC online account

1. Wait 72 hours after filing. HMRC's guidance says you must wait 3 days after submitting before you can update a return.

2. Sign in to your HMRC online account.

3. Go to your Self Assessment account, then More Self Assessment details, then At a glance, then Tax return options.

4. Choose the tax year you want to correct and open the return.

5. Change only the figures that were wrong. Everything else stays as you filed it.

6. Submit the return again. Your amended bill shows straight away, and within 3 days your statement shows the difference and any interest.

If you filed using commercial software

Open the return for that tax year in the same software, correct the figures and submit it again as an amended return. Most software has an option to amend a return it has already filed. If yours does not, or you no longer have a subscription, contact the software provider first. HMRC's guidance says to contact HMRC if your software cannot make the correction.

If you no longer have the software, or your accountant filed it

If you can no longer use the software, or the accountant who filed your return has stopped trading or will not reply, you can still amend the return by writing to HMRC. The law allows you to amend your return by notice to HMRC, and HMRC's own manual for its staff (SAM124060) says an amendment can be made by letter, as long as it is in writing.

Your letter should give your name, address and UTR, the tax year, each box you want to change with the old and new figures, and a short explanation of why. Sign and date it. The same 12-month deadline applies, so a letter for 2024/25 has to reach HMRC by 31 January 2027. You do not need your former accountant's permission to correct your own return, because the return is yours.

Keep a copy of every return you file

Get into the habit of keeping a full copy of every return you file, together with HMRC's tax calculation and the submission receipt. If you file through your HMRC online account, save the return as a PDF after you submit it. If you use software, download the return and the receipt before your subscription ends. If an accountant files for you, ask for a copy of the return as filed and the tax calculation every year, and keep them yourself. Without the original figures in front of you, you cannot see what needs correcting, and amending in a hurry from memory is how a second mistake happens. Keep these copies with your records for at least 5 years after the 31 January filing deadline.

If you filed on paper

Send the corrected pages to HMRC with "amendment" written on each page, plus your name and Unique Taxpayer Reference (UTR).

What are the most common reasons to amend a return?

The most common reasons I see for amending a return are income that was left out, expenses that were never claimed, and costs from before the business started that nobody knew could be claimed.

Missed income usually happens when money came in through more than one route. A client paid through PayPal instead of the bank, a few cash jobs were never written down, or sales on a second platform were forgotten. If HMRC later receives information from a platform or a bank that does not match your return, it can write to you. Correcting it yourself first is far better.

Unclaimed expenses are often the business share of something that also has personal use, such as your phone, broadband, car or a room at home. Others are small and frequent, like software subscriptions, bank charges and postage, and they add up over a year. My list of commonly missed sole trader expenses is a good place to check.

Pre-trading expenses catch out a lot of new sole traders. They have their own section below because the rule is less well known.

Can I claim business expenses from before I started trading?

Yes. You can claim business costs you paid up to 7 years before your business started, as long as they would have been allowable if you had already been trading.

HMRC's Business Income Manual (BIM46351) sets out the rule, which comes from section 57 of the Income Tax (Trading and Other Income) Act 2005. The costs are claimed in the tax year your business started, as if you had paid them on your first day of trading.

Pre-trading costs that often qualify include:

• equipment you bought for the business before you started, such as a laptop, tools or a sewing machine

• setting up a website, domain name and email

• business insurance

• advertising or printed flyers before your launch

• travel to meet suppliers or early customers

The costs still have to be wholly and exclusively for the business, so a laptop you also use for family life only qualifies for its business share. If your first year's return is already filed and you left these out, that is the return to amend.

Will I get a penalty for amending my return?

Amending your return does not, on its own, lead to a penalty, and correcting a careless mistake before HMRC contacts you usually means no penalty at all.

HMRC's penalties for mistakes depend on why the mistake happened and whether you told HMRC before it found out. For a careless mistake that you correct without being prompted, the penalty can be reduced to nothing. The same mistake corrected only after HMRC has started asking questions carries a minimum penalty of 15% of the extra tax.

Interest is different. If you owe extra tax, HMRC charges interest from the date the tax was originally due until you pay it, whether or not there is a penalty. HMRC's late payment interest rate is 8% a year from 24 November 2026 [VERIFY: rate on the day you publish], which works out at about £4 a month on £600 of extra tax. If you amend your 2025/26 return and pay by 31 January 2027, there is no interest at all, because the tax was not due yet.

My advice is to amend as soon as you spot the mistake. Waiting does not make the tax go away. It only adds interest, and it risks HMRC finding the mistake first.

What if the deadline to amend has passed?

If the 12-month window to amend has passed, you can no longer change the return online, but you can still put things right in writing.

If you paid too little tax, write to HMRC with the tax year, what was missing, how much you think you underpaid and your signature. Interest runs from the original due date.

If you paid too much tax, for example because you missed expenses, you can make an overpayment relief claim in writing for up to 4 years after the end of the tax year. That means the 2022/23 deadline is 5 April 2027 and the 2023/24 deadline is 5 April 2028. The claim needs the tax year, why you overpaid, the amount, how you want to be repaid and a signed declaration that the details are correct and complete. HMRC can refuse a claim if you knew, or should have known, about the mistake while you could still have amended the return, so make the claim as soon as you find the error.

How can the template help you check your figures before you amend?

The 2025/26 Bookkeeping and Accounting Template helps you check your figures before you amend, because it rebuilds your whole year from your records and works out what your tax should have been.

You enter every payment you received and every cost you paid for 6 April 2025 to 5 April 2026, then your P60 figures if you have a job. The UK Tax Planner works out your profit, income tax, Class 4 National Insurance and any student loan repayment, and compares the £1,000 Trading Allowance with your actual costs. Put its figures next to the tax calculation HMRC showed you when you filed. If the two are different, you know there is something to amend, and you know the corrected figures before you open your return.

Doing it this way means you change the return once, with figures you can back up, instead of amending it piecemeal as you remember things.

The template is built for the 2025/26 tax year, so it is the right tool for 2025/26 returns. If you are correcting an earlier year, the same method works: list every payment and cost for that year from your bank statements, then compare your totals with what you filed. My guide to filing with no records shows how to rebuild a year from your statements.

Worked example: an early filer who left out income and costs

Priya has a job paying £35,000 and started a freelance design business alongside it on 1 August 2025. She filed her 2025/26 return early, in June 2026. When she later entered a full year of records into the template, its figures did not match her return.

Priya had left out £1,400 of income that a client paid through PayPal. On its own, that would have added £280 of tax. But the template also showed £350 of software subscriptions she never claimed, and £900 she spent on a laptop and website in June and July 2025, before she started trading. Those costs reduce her profit by £1,250.

Her corrected profit is £5,250, only £150 more than she filed, so her tax goes up by £30, from £1,020 to £1,050. Her salary already uses her £12,570 Personal Allowance and her total income of £40,250 is under the £50,270 higher rate threshold, so all of her profit is taxed at 20%. Her profit is under £12,570, so there is no Class 4 National Insurance. Her bill is just over £1,000, but more than 80% of her total income tax is collected through PAYE, so she has no payments on account either.

Priya amends her return online in November 2026. Her 2025/26 tax is not due until 31 January 2027, so if she pays the £1,050 by then she pays no interest and no penalty. The laptop is used only for her business. If it were shared with family, she could only claim the business share.

What happens after I amend my return?

After you amend your return online, HMRC updates your bill straight away, and you either pay the difference or request a refund.

If you owe more, your updated bill shows how much and when to pay it. If you are due money back, HMRC does not always send it automatically: request the refund from your online account, and HMRC may set it against anything else you owe first. If your bill changes enough to affect your payments on account, the updated bill shows the new amounts too.

If HMRC has already written to you about your return, read the letter before you change anything. My guide to HMRC letters about online sales explains what the most common ones mean.

Frequently asked questions

Will amending my return trigger an HMRC investigation?

Not automatically. Amending does give HMRC a separate window to check the changes you made, until the quarter day after the first anniversary of your amendment (quarter days are 31 January, 30 April, 31 July and 31 October). Only amend with figures you can back up from your records.

Can I amend my return if HMRC has already opened a check into it?

Yes, but HMRC's guidance says an amendment made while a check (an enquiry) is open only takes effect when the check ends. Send your correction to the HMRC officer named in the letter rather than changing the return online.

How long does HMRC take to pay a refund after I amend?

HMRC does not publish a fixed time for the public. It has told tax professionals it aims to issue automated repayments within 10 working days, but some are held for security checks, and a repayment that follows a recent payment can be held for 14 days.

HMRC sent me an estimated bill because I did not file. Can I amend it?

No. An estimated bill, which HMRC calls a determination, cannot be amended or appealed. You replace it by filing your actual return within 3 years of the original filing deadline, or within 12 months of the determination if that is later.

This article is for general information only and isn't tax or legal advice. Your circumstances may be different. For personalised advice, speak to a qualified accountant.

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