Realising you’ve made a mistake on your tax return, or discovering HMRC owes you money, can feel stressful — but both situations are fixable, and the system is designed to let you correct errors and reclaim overpayments without penalty (as long as you come forward voluntarily). The complication is that there are four different routes depending on what went wrong, how you pay tax, and how long ago the error happened. This hub maps all four and links to the detailed guide for each.
1. The four routes — and which one applies to you
Before diving into any one process, work out which route you need:
Route 1 — Amend your Self Assessment return. You filed a Self Assessment return and realised it contains an error (wrong income figure, missed allowance, wrong expense claim). You’re within 12 months of the filing deadline. Fix: log into HMRC Online, correct the figures, resubmit. See our made a mistake on your Self Assessment guide.
Route 2 — Claim overpayment relief. You overpaid tax on a Self Assessment return but you’re past the 12-month amendment window. Fix: write to HMRC with details of the overpayment. You have 4 years from the end of the tax year. See our made a mistake guide for the overpayment relief process.
Route 3 — Claim a P800 PAYE refund. You’re a PAYE employee or pensioner (not in Self Assessment) and you think you’ve overpaid tax. Fix: check your Personal Tax Account or the HMRC app under “Check your Income Tax,” or wait for a P800 letter (sent between June and March). Claim online from there. See our when will I get my P800 guide.
Route 4 — Chase a stuck repayment. You’ve already claimed a refund (through any of the routes above) but the money hasn’t arrived — it’s been weeks or months. Fix: call the Self Assessment helpline, ask about the “no repayment indicator,” and escalate if needed. See our HMRC repayment delays guide.
2. Amending a Self Assessment return (within 12 months)
You have 12 months after the filing deadline to amend your Self Assessment return, as set out in HMRC’s guidance on correcting your Self Assessment. For the 2023-24 tax year (filing deadline 31 January 2025), you have until 31 January 2026. The process is the same as filing — you log into HMRC Online, select the tax year, correct the figures, and resubmit. HMRC recalculates your tax automatically and shows the updated figure immediately.
The most common mistakes people amend for: missing the £1,000 trading allowance (especially common for first-time filers with a side income alongside PAYE), not claiming allowable expenses, entering the wrong income figure (transposed digits, decimal points), and forgetting capital allowances on equipment. For the full list of common mistakes, the step-by-step amendment process, and what happens to the overpaid tax after you amend, see our made a mistake on your Self Assessment guide.
3. Overpayment relief — fixing mistakes older than 12 months
If you’re past the 12-month amendment window, you can still reclaim overpaid tax using overpayment relief. This is a written claim to HMRC, not an online amendment, and you have 4 years from the end of the tax year in which the overpayment occurred.
To claim overpayment relief, write to HMRC with: the tax year, the amount overpaid, how the error occurred, why you couldn’t reasonably have known about it earlier, and any supporting evidence. HMRC has 15 days to acknowledge your claim and longer to process it. There are exclusions — you can’t claim if you already knew about the error and chose not to act, or if the error was caused by HMRC giving you wrong information (that’s a different complaint route). For the full overpayment relief process and a worked example, see our made a mistake guide.
4. PAYE refunds and the P800 — if you’re not in Self Assessment
If you’re a PAYE employee or pensioner and you’ve overpaid tax, you don’t amend a return — there’s no return to amend. Instead, HMRC runs an automatic reconciliation at the end of every tax year, comparing the tax your employer deducted against what you actually owed. If there’s a discrepancy, HMRC sends you a P800 tax calculation letter between June and March after the tax year ends.
You don’t have to wait for the letter. Log into the HMRC app or your Personal Tax Account on GOV.UK, go to “Check your Income Tax,” and look at the current and previous tax year. If you’re owed a refund, it’ll show there with a “Claim a refund” button — and the money arrives within 5 working days if you claim online by bank transfer. If no P800 arrives and you think you overpaid (e.g. you stopped working partway through the year, had multiple jobs, or were on an emergency tax code), you can claim a refund directly using form P50. You have 4 years to claim. For the full P800 timeline, how to check without waiting, and what to do if your P800 says you owe tax, see our when will I get my P800 guide.
5. How PAYE and Self Assessment refunds differ
The refund mechanics are different depending on how you pay tax, and confusing the two leads to wasted effort:
Self Assessment refunds appear as a credit in your SA account after you amend your return or HMRC corrects an error. You can leave the credit to offset future payments on account, or request a repayment to your bank account through HMRC Online. The repayment goes through security checks (see §6) and can take weeks or months.
PAYE refunds (P800) are handled entirely by HMRC’s automatic reconciliation. You don’t request them — HMRC identifies them and sends the P800. You then claim online or by cheque. PAYE refunds are generally faster than SA refunds (5 working days vs weeks) because they don’t go through the same security-check queue.
If you’re in both systems — you have PAYE employment and also file Self Assessment — your SA refund sits in your SA account and your PAYE refund comes through the P800 route. They don’t merge. If HMRC owes you money from both, you’ll need to claim each through its own channel.
6. Chasing a stuck repayment
If you’ve claimed a refund and the money hasn’t arrived, the most likely cause is security checks — HMRC runs automated checks on all repayments, and some get flagged for manual review that takes months. The checks are triggered by first-time repayment requests, large refunds relative to your income, changes to your bank details, refunds that follow an amendment, or random selection.
The fix is almost always calling — and calling again. The pattern reported consistently is: the first call gets “just wait,” the second call gets “just wait,” and the third call gets the repayment issued. When you call the Self Assessment helpline on 0300 200 3310, ask specifically: “Is there a no repayment indicator on my account?” — this is a silent block that agents won’t proactively check for unless you name it. If repeated calls don’t work, file a formal complaint and escalate to the Adjudicator’s Office after 8 weeks. HMRC also pays interest on late repayments (at the repayment interest rate, currently lower than the late payment rate they charge you). For the full chase process, the exact questions to ask, and the escalation route, see our HMRC repayment delays guide.
7. Will you be penalised for correcting a mistake?
No — not for a genuine mistake corrected voluntarily. HMRC distinguishes between three behaviours:
- Genuine mistakes — corrected voluntarily, no penalty. This is the most common scenario.
- Careless errors — may attract a penalty if HMRC finds them first, but 0% if you correct them yourself before HMRC contacts you.
- Deliberate underpayment — penalties apply (up to 100% of the tax owed), and they’re severe.
The key principle: correct it before HMRC contacts you. Overpaying tax is not an offence — it’s an error to fix. The system is designed to let you fix it without penalty, and HMRC would rather you correct it than leave it. For the specific penalty framework and how it interacts with voluntary correction, see our made a mistake guide.
8. The 4-year time limit — and why you shouldn’t wait
All of these routes share one time limit: 4 years from the end of the tax year in which the overpayment or error occurred. For the 2021-22 tax year (ended 5 April 2022), the deadline is 5 April 2026. After that, the claim is time-barred.
There’s no reason to wait. The money earns nothing sitting with HMRC, and the longer you leave it, the more tax years stack up that you might need to claim for. If you think you’ve overpaid for multiple years, claim for all of them at once — the amendment process lets you amend each year separately, and overpayment relief claims can cover multiple years in one letter.
The bottom line
- Work out which route you need — amend (within 12 months), overpayment relief (12 months to 4 years), P800 (PAYE), or chase a stuck repayment.
- Amending is easy — log into HMRC Online, correct the figures, resubmit. No penalty for voluntary correction.
- Overpayment relief goes back 4 years — but it’s a written claim, not an online amendment.
- PAYE refunds come through the P800 — check your HMRC app or Personal Tax Account, don’t wait for the letter.
- If your repayment is stuck, call and ask about the “no repayment indicator” — persistence works, and the third call often gets it issued.
- You won’t be penalised for correcting an honest mistake — the system is designed to let you fix it.
- The 4-year time limit is real — claim now, not later.
For the full step-by-step on amending your Self Assessment return, see our made a mistake on your Self Assessment guide. For how to check and claim a PAYE refund without waiting for a letter, see our when will I get my P800 guide. For chasing a stuck repayment, see our HMRC repayment delays guide. For the complete Self Assessment penalty regime, see our Self Assessment penalties guide.
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