Realising you’ve made a mistake on your Self Assessment tax return is a horrible moment. Whether it’s a typo in your income figure, a missed allowance, or declaring the wrong amount — the good news is it’s fixable, and you won’t be penalised for correcting an honest error.
Here’s exactly what to do, based on real experiences shared by filers on r/HMRC and HMRC’s own guidance on corrections.
Common Mistakes People Make on Self Assessment
Based on questions from real filers, the most frequent mistakes include:
- Missing the £1,000 trading allowance — especially common for first-time filers who have a side income alongside PAYE employment. One Reddit user earned consultancy income alongside their PAYE job, didn’t know the trading allowance existed, and paid 40% tax on the full amount — overpaying by £400.
- Not claiming allowable expenses because you didn’t know what you could deduct. For example, a sole trader with £8,000 of actual expenses who claimed none is overpaying by £1,600 at 20% tax or £3,200 at 40%.
- Declaring income that doesn’t need to be on a Self Assessment at all — some income types don’t require a return, and declaring them unnecessarily can lead to overpayment.
- Entering figures in the wrong box — for example, not knowing where to put benefit in kind from a P11D (see our guide on declaring benefit in kind).
- Simple arithmetic errors — transposed digits, doubled-up figures, decimal points in the wrong place. If you entered £45,000 instead of £4,500, you could be overpaying by thousands.
- Forgetting to claim capital allowances on equipment purchases over £1,000 — laptops, tools, machinery. A £2,000 laptop unclaimed means £400–£800 in overpaid tax.
One Reddit user, a PAYE employee who did consultancy work for the first time, shared: “I did not know about the trading allowance. I declared the entirety of my side earnings and expected them to be taxed at 40%. I also, naively, expected that the system worked along the lines of ‘I declare it all and then they let me know.’”
This is incredibly common. The Self Assessment system assumes you know what you’re entitled to claim — it won’t prompt you or automatically apply allowances. As another commenter put it: “This isn’t about not having paid taxes, but having paid them on more income than I was supposed to.”
The 12-Month Amendment Window
You have 12 months after the filing deadline to amend your return, as confirmed by HMRC’s corrections guidance. For example:
| Tax year | Filing deadline | Amendment deadline |
|---|---|---|
| 2023-24 | 31 January 2025 | 31 January 2026 |
| 2024-25 | 31 January 2026 | 31 January 2027 |
After this window, you can still correct errors, but you’ll need to use the “overpayment relief” process — it’s slower, more manual, and requires writing to HMRC with evidence. See the section below on overpayment relief for details.
Importantly, you can go back further than 12 months in some cases. One Reddit commenter reported: “I did similar and went back and claimed the trading allowance 2 years later.” This isn’t guaranteed, but HMRC can accept corrections outside the standard window if you can show you overpaid tax. The overpayment relief claim has a 4-year time limit from the end of the tax year in question.
How to Amend Your Self Assessment Return
If you filed online
- Log into your HMRC Online account
- Go to your Self Assessment section
- Select the tax year you want to amend
- Make the corrections in the relevant section
- Submit the amended return
The process is the same as filing — you’re just re-submitting with corrections. HMRC will recalculate your tax automatically. You’ll see the updated tax calculation immediately after submission.
If you filed on paper
You’ll need to download a new SA100 form for the relevant tax year, complete it with the correct figures, and send it to HMRC with a cover letter explaining it’s an amendment. The address is on the form.
How long does HMRC take to process an amendment?
HMRC says amendments are usually processed within 4 weeks, but in practice it can take 6–8 weeks. You won’t get a notification when it’s done — you’ll need to check your HMRC online account to see if the updated figures appear.
If you haven’t heard anything after 8 weeks, call the Self Assessment helpline on 0300 200 3310 to check the status.
What Happens After You Amend
Once you submit the amendment:
- HMRC recalculates your tax — the new figure replaces the old one
- If you overpaid, the excess appears as a credit in your SA account
- If you underpaid, you’ll owe the difference (plus possible interest, but no penalty for voluntary correction)
If you overpaid
The overpaid amount doesn’t automatically land in your bank. It sits in your SA account. You have two options:
- Leave it — it will offset against your next payments on account or balancing payment. For example, if you overpaid by £800 and your next payment on account is £2,500, HMRC will apply the £800 credit and you’ll only pay £1,700.
- Request a refund — you can claim a repayment through your HMRC online account. The money will be transferred to your bank account.
Be aware that refunds can take a long time. HMRC runs security checks on repayments, and some people wait months. One Reddit user reported their refund taking from April until October. See our guide on HMRC repayment delays for what to do if your refund is stuck.
If you underpaid
If your amendment means you owe more tax, you’ll need to pay the difference. HMRC will send you a revised calculation showing the additional amount. You may also owe interest on the underpaid amount from the original due date — but there are no penalties for voluntarily correcting an honest mistake.
For example, if you originally declared £30,000 income but the correct figure was £35,000, and you’re a 20% taxpayer:
- Additional tax due: £5,000 × 20% = £1,000
- Plus interest from 31 January to the date you pay (at the late payment interest rate, currently around 7.5%)
- If you amend in March (2 months late): £1,000 × 7.5% × (2/12) = £12.50 in interest
- No penalty because you corrected it voluntarily
Will You Be Penalised?
No. HMRC distinguishes between:
- Genuine mistakes — corrected voluntarily, no penalty. This is the most common scenario.
- Careless errors — may attract a penalty if HMRC finds them first, but usually 0% if you correct them yourself before HMRC contacts you
- Deliberate underpayment — penalties apply, and they can be severe (up to 100% of the tax owed)
The key is to correct it yourself before HMRC contacts you. As one Reddit commenter put it: “This isn’t about not having paid taxes, but having paid them on more income than I was supposed to.” Overpaying is not a crime — it’s just an error to fix.
Specific Fix: Missed Trading Allowance
If you forgot to claim the £1,000 trading allowance, here’s the specific fix:
- Amend your return
- In the self-employment section, enter £1,000 in the trading income allowance box (box 16.1 on the full SA, or box 10.1 on the short version)
- Make sure you’re not also claiming actual expenses — it’s one or the other
- Submit
At a 40% tax rate, claiming the £1,000 allowance saves you £400 in tax. At 20%, it saves £200. For the Reddit user who prompted this post, the saving was £400 — money that’s now sitting in their SA account waiting to be claimed.
Note: the trading allowance cannot be used if the income came from your employer or your spouse’s employer. It only applies to income from your own business or side hustle with an unrelated party. For a deeper explanation, see our guide on trading allowance vs expenses and HMRC’s trading allowance guidance.
Overpayment Relief: Fixing Mistakes Beyond 12 Months
If you’re past the 12-month amendment window, all is not lost. HMRC has a process called overpayment relief that allows you to claim back overpaid tax for up to 4 years after the end of the tax year in question.
To claim overpayment relief:
- Write to HMRC with details of the overpayment
- Include the tax year, the amount overpaid, and how the error occurred
- Explain why you couldn’t reasonably have known about the error earlier
- Include any supporting evidence
HMRC has 15 days to acknowledge your claim and longer to process it. There are some exclusions — for example, you can’t claim overpayment relief if you already knew about the error and chose not to act, or if the error was caused by HMRC giving you wrong information.
One Reddit user reported going back and claiming the trading allowance 2 years later using this process. It worked, but it took significantly longer than a standard amendment.
Other Common Mistakes and Their Fixes
Entered the wrong income figure
If you entered £45,000 instead of £4,500 (a decimal point error), amend immediately. The difference in tax at 20% is £8,100 — this is a significant overpayment that HMRC will likely flag for security checks when you claim the refund.
Forgot to claim expenses
If you had £6,000 in allowable expenses but claimed none, amending to add them saves:
- At 20% tax: £6,000 × 20% = £1,200
- At 40% tax: £6,000 × 40% = £2,400
See our guide on allowable expenses for sole traders for what you can claim.
Declared benefit in kind in the wrong section
If you put your P11D benefits in the employment income section instead of the “Employment benefits and expenses” section, amend and move them. The tax calculation may be the same, but the form needs to be correct. See our benefit in kind guide for the right boxes.
The Bottom Line
Making a mistake on your Self Assessment is normal, especially for first-time filers. The system is not designed to help you — it assumes you know what you’re doing. But the fix is straightforward: amend within 12 months, correct the figures, and either accept the credit or request a refund.
Don’t let the fear of “getting it wrong” stop you from fixing it. HMRC would rather you correct it than leave it. And if you’re past the 12-month window, overpayment relief gives you up to 4 years to make it right.
Need to correct something specific? See our full guide on how to amend your Self Assessment and our HMRC jargon buster for help with the terminology. If your refund is stuck after amending, see HMRC repayment delays.