Amending a return or claiming later

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Within 12 months: amend the return

GOV.UK says you can correct a tax return within 12 months of the Self Assessment deadline, online or by sending another paper return. Section 9ZA of the Taxes Management Act 1970 is the legal basis: an amendment can’t be made more than 12 months after the filing date. GOV.UK’s example: for the 2024 to 2025 tax year, you’ll usually need to change your return by 31 January 2027.

Online: GOV.UK says to wait 3 days (72 hours) after filing, then sign in to your HMRC account, go to your Self Assessment account and choose to amend the return for that year. If you used commercial software, contact the software provider first.

On paper: send the corrected pages with "amendment" written on each, with your name and Unique Taxpayer Reference, to the Self Assessment address on GOV.UK.

If a loss claim reduces a bill you’ve already paid, any tax due back can be repaid — but GOV.UK notes that a refund may be set against tax due within the next 45 days, such as a payment on account.

After 12 months: stand-alone claims

GOV.UK says that if you miss the amendment deadline, you’ll need to write to HMRC. For loss reliefs, that doesn’t necessarily mean you’ve lost out:

• HMRC’s claims manual says claims that can no longer go in a return can be made outside a return if their own time limit hasn’t passed (Schedule 1A).
• HMRC’s Business Income Manual says claims that carry a loss back to an earlier year don’t have to be made in a return at all.
• HMRC’s helpsheet HS227 gives the example that a claim to carry losses forward may be made up to 4 years after the end of the year of loss. It lists what a stand-alone claim letter should contain: your name, address and UTR, the name of your trade, how much the loss is and for what period, and how you want to use it.

The time limits for each relief are in deadlines for loss claims. A capital loss, for example, can be notified up to 4 years after the end of the tax year.

Overpayment relief

If you’ve overpaid tax and it’s too late to amend, GOV.UK says you can claim overpayment relief up to 4 years after the end of the tax year. Your letter must include:

• that you’re making a claim for overpayment relief;
• the tax year you think you overpaid for;
• why you think you overpaid;
• how much you think you overpaid;
• whether or not you have previously appealed about the same payment;

plus a declaration that the details are correct and complete to the best of your information and belief. GOV.UK says no one else — not even a tax agent — can sign it for you, and HMRC will reject a claim missing any of this. Keep evidence of the overpayment.

Making Tax Digital

If you use Making Tax Digital for Income Tax, GOV.UK says you change a submitted tax return through your software, within 12 months of the submission deadline — and that refunds are requested through your HMRC online account rather than in the return.

Questions people ask

I forgot to claim a loss on last year’s return. What can I do?

You can usually amend a return within 12 months of the 31 January deadline. After that, many loss claims can still be made as stand-alone claims in writing, as long as their own time limit hasn’t passed — for example a capital loss can be notified up to 4 years after the end of the tax year.

Sources

Checked 4 October 2026 for the 2026/27 tax year. We read these primary sources for this guide:

1. GOV.UK: Self Assessment tax returns — If you need to change your return (gov.uk)
2. Taxes Management Act 1970, section 9ZA (legislation.gov.uk)
3. Taxes Management Act 1970, Schedule 1A (claims outside a return) (legislation.gov.uk)
4. HMRC Self Assessment Claims Manual SACM3030: Claims outside a return (gov.uk)
5. HMRC Self Assessment Claims Manual SACM12155: Overpayment relief time limits (gov.uk)
6. HMRC Business Income Manual BIM85070: Claims involving more than one year (gov.uk)
7. HMRC helpsheet HS227: Relief for trading losses (2026) (gov.uk)
8. GOV.UK: Use Making Tax Digital for Income Tax — Submit your tax return (gov.uk)

Contains public sector information licensed under the Open Government Licence v3.0. This guide adapts and summarises that information in our own words. taxloss.co.uk is an independent guide. It is not affiliated with, endorsed by or connected to HM Revenue & Customs (HMRC) or the UK Government. Spotted something out of date? Tell us.

More on returns, deadlines & records

• 19Deadlines for loss claimsEvery loss relief has a time limit, and they aren’t all the same. Some run 4 years from the end of the tax year; some only to the first anniversary of the 31 January filing date. Here are the main ones in one place, with the 2026/27 dates.
• 20Where losses go on your Self Assessment returnA plain-English map of the loss boxes on the Self Assessment supplementary pages — capital gains (SA108), self-employment (SA103), partnership (SA104), UK property (SA105) and additional information (SA101). Box numbers are from the 2025–26 forms.
• 22Making Tax Digital for Income Tax from April 2026Since 6 April 2026, sole traders and landlords with qualifying income over £50,000 must keep digital records, send quarterly updates and file their tax return through compatible software. Here’s who is affected, when, and what GOV.UK says about losses.
• 23Records and evidence HMRC may ask forTo claim a loss you need to be able to show how you worked it out. What to keep for capital losses, crypto and business losses, how long to keep it, and what HMRC can ask for in a compliance check.
• 24When to get professional helpMany losses are simple enough to claim yourself. Some aren’t — large or complex losses, businesses stopping, partnerships and companies, overseas issues or an HMRC check. Here’s who can help, and how to choose.