How to Amend Your Self Assessment Tax Return: A Complete Guide

Made a mistake on your Self Assessment tax return after submitting it? Don't worry – you can amend your Self Assessment tax return easily with HMRC. Whether you forgot income, claimed the wrong expenses, or spotted a simple typo, correcting your return is straightforward and can prevent penalties or secure a refund. At Mayfair Tax Advisors, we help clients across London and beyond fix errors quickly and compliantly.

Why Might I Need to Amend My Self Assessment?

Common reasons to amend Self Assessment include:

• Forgetting to declare untaxed income (e.g., freelance payments, rental income, dividends)
• Missing allowable expenses or reliefs (e.g., home office costs, pension contributions)
• Typing errors in figures, such as misplaced decimals or wrong amounts
• Changes to personal details (name, address) that need updating
• Realizing you overpaid tax and want to claim a refund

Amending promptly is better than waiting for HMRC to spot the issue – which could trigger an enquiry.

Will Amending Change My Tax Bill?

Updating personal details usually has no impact on your tax. However, adding missed income or removing incorrect expenses might increase what you owe (plus interest), while claiming extra reliefs could reduce your bill or generate a refund. HMRC will recalculate and notify you of any adjustments.

How Long Do I Have to Amend a Self Assessment Tax Return?

You can amend your Self Assessment tax return online (or by paper) within 12 months of the original filing deadline. For the 2024/25 tax year (deadline 31 January 2026), you generally have until 31 January 2027 to make changes. Miss this window? You'll need to write to HMRC with a formal claim – it's more complex, so act fast.

How to Amend a Self Assessment Within 12 Months of Filing

If you filed online (the most common method), follow these steps to correct Self Assessment:

1. Log in to your HMRC online account via gov.uk.
2. Select “Self Assessment” or go to your tax account dashboard.
3. Navigate to “More Self Assessment details” or “At a glance” on the left menu.
4. Choose “Tax return options”.
5. Select the relevant tax year (e.g., 2024 to 2025).
6. Click on your submitted return and select the option to amend or correct it.
7. Make the necessary changes (add/remove income, expenses, etc.).
8. Review everything carefully, then re-submit the updated return.

Note: Wait at least 3 days after original submission before amending online. If you used an accountant or software, inform them first – they may handle the amendment for you.

How to Amend a Self Assessment 12 Months After Filing

If the 12-month window has passed, you can't amend online. Instead:

• Write a signed letter explaining the error, the tax year affected, why you're amending, and the estimated impact on your tax (over/underpayment amount).
• Include your UTR, National Insurance number, and full details.
• For refunds: Add proof of overpayment, payment confirmation, refund method (bank details), and a declaration that you haven't claimed before.
• Send to: Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom.

Use recorded delivery for proof. HMRC may take longer to process – expect 4–8 weeks or more.

What If I’m Claiming a Refund?

If amending results in overpaid tax, HMRC will issue a refund (usually within 4–6 weeks for online changes). Provide bank details for faster payment. For post-12-month claims, include all evidence in your letter to avoid delays.

When Will I Hear Back from HMRC?

Online amendments often update within 3–5 days. Paper or late amendments can take 4–12 weeks. Check your HMRC online account for status updates. Refunds arrive via bank transfer or cheque.

Important Self Assessment Deadlines to Know

• Register for Self Assessment: By 5 October following the tax year
• Paper filing deadline: 31 October
• Online filing & payment: 31 January
• Amendment window: 12 months from filing deadline

Missing deadlines can lead to £100+ penalties – amend early to stay compliant.

How Mayfair Tax Advisors Can Help

Spotting an error on your Self Assessment tax return doesn't have to be stressful. Our expert team handles amendments, reviews your return for missed savings, and ensures everything is HMRC-compliant – often spotting extra reliefs you didn't know about. We've helped clients correct returns, reclaim overpaid tax, and avoid enquiries.

Ready to amend your Self Assessment correctly? Contact Mayfair Tax Advisors today for a free review. Visit www.mayfairtaxadvisors.co.uk or email/call us now. Let's fix it right – and potentially save you money!