Filing your self assessment tax return feels great, but what happens when you realise something’s not quite right after you’ve hit submit?
Maybe you missed an expense, forgot to include a source of income, or entered a figure incorrectly. It happens to the best of us. The good news is that you can fix it, as long as you do it in time and in the right way.
Here’s a simple guide to understanding why you might need to amend a return, what the real cost of not correcting it could be and the practical steps to get HMRC updated without stress.
Why you might need to amend a return
Even if you filed on time and thought you got everything right, here are some common reasons you might need to make an amendment:
- You used estimates because you didn’t have all the figures yet – Maybe you didn’t have final statements from investment income or property accounts by the deadline, so you guessed. If new info comes in later, it’s worth amending.
- You missed reporting a source of income – This could be freelance earnings, rental income, dividends or miscellaneous side-gigs. Leaving those off can affect your tax liability (don’t forget HMRC can gets the data from third parties too).
- You missed a deductible expense or allowance – This happens, especially when receipts are spread between bank accounts, wallets and shoeboxes.
- You realise a number was simply wrong – Maybe a pension contribution, charitable gift or business cost was entered incorrectly. The sooner you fix it, the better.
The bottom line is that mistakes aren’t the end of the world but ignoring them could cost you more in tax paid or penalties further down the line.
When you can amend
HMRC gives you a window to make changes:
- You have up to 12 months from the original 31st January filing deadline to amend your online return for that tax year.
- After that 12-month period, those figures are usually fixed unless you have a reasonable excuse and ask HMRC directly (which can often be a complicated process).
The message is don’t delay! Even if you think the change is small, sorting it sooner keeps everything clean and avoids unwanted surprises later.
How to amend your self assessment return
Here’s the simple step-by-step way to fix it:
- Log into your HMRC online account – This is the same place you filed your return originally.
- Go to the tax return you want to change – You’ll see a list of returns filed. Choose the relevant year – Hit “amend return”.
- HMRC will let you change what needs updating, whether it’s income figures, allowances, expenses or another box – Save and submit the changes.
- Once done, HMRC will replace your original submission with the amended one. This generates a new submission receipt – Keep records of what you changed.
- It’s good practice to store notes or proof of why you amended — in case HMRC asks later.
What happens next
- No tax to pay? – If your amendment reduces what you owe, you might get a refund. HMRC will calculate the difference and pay it back.
- Tax owed after amendment – If you now owe more, HMRC will tell you. Interest (and possibly penalties) might apply if payment is late, so settle it as soon as you can.
- If you missed the amendment window – You can still contact HMRC directly, but you’ll need a good reason for missing the 12-month limit and it’s not guaranteed.
The real cost of not amending
Leaving mistakes uncorrected can quietly cost you:
- Overpaid tax that could be refunded
- Incorrect tax bills skewing your financial planning
- Potential HMRC questions or compliance checks later on
When HMRC cross-checks your information with banks, employers and other payers, errors are more likely to flag automatically. Fixing things sooner keeps your tax affairs clear and avoids avoidable stress.
Final thoughts
Amending a self assessment tax return isn’t something to panic about, it’s something to do as soon as you spot the issue. The deadline might have passed, but the opportunity to correct your figures doesn’t disappear straight away.
Be proactive, be accurate and if you’re ever unsure, a quick chat with your accountant can make amending straightforward and headache-free.
Need help getting yours sorted? Why not reach out and make sure you’ve got it right. Give us a call us on 01173 700 079 or e-mail hello@steppingstonesaccountancy.co.uk.
