A guide to Amending Your UK Tax Return
Mistakes and oversights on tax returns are more common than you might think. Whether you’ve spotted an omitted source of income, a forgotten expense, or a figure that needs updating, HMRC allows you to make changes to your return after it’s been submitted.
Amending your return ensures that your tax position is accurate and up to date. Once a correction is made, HMRC will adjust your tax calculation accordingly, which may result in either a refund or an additional payment.
This guide explains the key deadlines, methods, and steps involved in amending a UK tax return — and what to do if you miss the standard timeframe.
Reasons You May Need to Amend a Tax Return
There are several situations where you might need to update a return you’ve already filed. Some are due to changes in your personal circumstances, while others arise from new information or updated guidance from tax authorities. Here are a few examples:
Change in overseas tax treatment
One of our clients received updated guidance from the Canada Revenue Agency (CRA) regarding how their Canadian pension should be taxed. This change meant their original UK return no longer reflected the correct tax position, so we filed an amendment to align with HMRC rules.
Missed income or benefits
A client realised they had forgotten to include bank interest from a savings account. Adding this income required us to amend the return and recalculate the tax due.
Updated expense claims
A self-employed client initially underestimated their allowable business expenses. Once the correct figures were provided, we submitted an amendment which reduced their overall tax bill.
Late-arriving documents
Sometimes, dividend vouchers or P60/P45 forms arrive after the return is filed. In one case, a client’s investment provider issued an updated dividend certificate, which meant their return needed to be corrected.
Corrections to pension contributions
A client originally reported a lower pension contribution than what was actually paid. Updating this figure increased their available tax relief and reduced their liability.
Step-by-Step: Amending your UK tax return via HMRC Online
Amending a UK tax return through HMRC’s online system is often more straightforward than many people expect, but it’s important to approach the process carefully to make sure the information you provide is accurate. Whether you’ve noticed a mistake, forgotten to include some income, or realised you’re entitled to extra reliefs, HMRC allows you to update your submitted return within specific time limits. Correcting these issues promptly helps you avoid potential penalties and ensures you pay the right amount of tax.
Before you begin, it’s useful to know what you’ll need and how the process works. You’ll need access to your HMRC online account, the details you want to change, and any supporting records to back up your amendments. Once logged in, HMRC guides you through the amendment process in a series of steps, and you can track any changes to your tax calculation once they’ve been processed. In the sections that follow, we’ll walk through the step-by-step process to make amending your return as smooth as possible.
Step 1
Sign in to your HMRC account.
Step 2
From ‘Your tax account’, select ‘Self Assessment account’.
Step 3
Click ‘More Self Assessment details’.
Step 4
From the left-hand menu, choose ‘At a glance’.
Step 5
Select ‘Tax return options’.
Step 6
Pick the tax year you want to amend.
Step 7
Open the return, make your corrections, and file it again.
If You Used Commercial Software
Most tax software has a built-in process for amendments. The exact steps vary by provider, but typically you’ll re-open your submitted return, make the changes, and re-submit directly through the software.
How to Amend your UK self assessment by Post
If you prefer to amend your Self Assessment tax return by post, HMRC still provides a clear process to follow. Instead of updating your details online, you’ll need to complete a paper return using the relevant forms, making sure all changes are clearly marked. This method can be useful if you’re more comfortable working with hard copies or if you don’t have reliable access to HMRC’s online services.
The process involves using the main Self Assessment form (SA100) alongside any necessary supplementary pages, clearly labelling the pages as amendments, and sending them to HMRC at the correct address. Once received, HMRC will review your changes and update your tax bill, issuing a refund if you’ve overpaid or advising you of any extra amount due. While straightforward, paper amendments can take longer to process, so it’s worth allowing extra time before your account is updated.
Step 1 : Get the forms:
Download the SA100 tax return form from HMRC, or Call HMRC to request one by post. Supplementary pages (e.g. SA102 for employment, SA105 for property income) can also be downloaded online.
Step 2: Complete the corrected pages with the updated information.
Make sure to complete the relevant pages with the updated information. It's always important to double check so you don't have to amend your return again.
Step 3: On each page, write:
A self-employed client initially underestimated their allowable business expenses. Once the correct figures were provided, we submitted an amendment which reduced their overall tax bill.
Late-arriving documents
“Amendment” clearly at the top, Your name, Your UTR (found on previous tax returns or HMRC letters)
Step 4: Send your amended pages:
to the address on your Self Assessment paperwork, or use the HMRC Self Assessment address above if unsure.
Step 5: Wait for HMRC to process your amendment.
They will send you an updated tax calculation and bill, and issue any refund directly to your bank (if bank details are provided on the return).
Always keep a copy of the amended pages you send, along with proof of posting (e.g. Post Office receipt). This helps track progress if HMRC queries your amendment.
If You’ve Missed the Deadline to Change Your Return
If you’ve missed the 12-month deadline for amending your Self Assessment return, you can no longer make changes online or by post. Instead, you’ll need to write directly to HMRC to explain what needs correcting and why. This written approach is the only option once the amendment window has closed.
In some cases, you may still be able to reclaim overpaid tax by making a claim for overpayment relief. This can be done up to four years after the end of the relevant tax year, but your letter must include all the required information and evidence. Without the correct details, HMRC is likely to reject your claim, so it’s important to be thorough when setting out your case.
When You Need to Write to HMRC
You must contact HMRC in writing if you need to correct an old return (beyond the 12-month amendment window).
What to Include in Your Letter
For all late amendments, include:
- The tax year you are correcting.
- The reason for the correction (why you paid too much or too little).
- The amount you believe is over or underpaid.
- Restructuring or moving to a different business model.
- Death of a director or shareholder.
- Your signature (no one else can sign for you).
For overpayment relief claims, you must also state:
- That you are making a claim for overpayment relief.
- Whether you have previously appealed the same payment.
- A signed declaration: “The details I have given are correct and complete to the best of my information and belief.”
If you’re unsure whether your situation qualifies for overpayment relief or another correction route, it’s best to seek advice before writing to HMRC — this can avoid delays or rejected claims. Download our example template
Changes in Tax Due on your Amended UK Personal Tax Return
When you amend your tax return, the change may result in either a refund or an additional tax bill. If you have overpaid, HMRC will update your Self Assessment account and issue a repayment—usually directly to your bank account if details are provided, or by cheque if not.
If the amendment means you owe more tax, HMRC will send you an updated bill with a new payment deadline and make any necessary adjustments to your payments on account. Whether you’re due a refund or need to make an extra payment, it can take a few weeks for HMRC to process the changes, especially if the amendment was made by post.
If You’re Owed Tax
If your amendment shows you’ve paid too much tax:
The HMRC will update your Self Assessment statement. Any overpayment will usually be refunded directly to your bank account (provided you’ve given HMRC your bank details). If no bank details are held, HMRC will send a cheque to your registered address.
Refunds can take longer to process after a paper amendment, so it’s a good idea to keep an eye on your Self Assessment account for updates.
Need More Help?
If you need more help or haven't found exactly what you were looking for, feel free to Get in Touch. We have over 20 years of experience helping our clients who have missed filing deadlines.