Everyone gets a VAT return wrong eventually. What decides whether that costs nothing or costs a penalty is not the error — it is how you correct it. The rules give you a genuine choice on small errors and take it away on larger ones, and the threshold between the two is not a single number: it is £10,000, or up to £50,000 if the error is under 1% of your box 6 figure.
Method 1: put it right on the next return
You can adjust the error on your next VAT return, with no separate notification, where the net value of the errors does not exceed £10,000 — or where it is between £10,000 and £50,000 and does not exceed 1% of the box 6 (net outputs) figure for the return period in which you find it.
"Net value" means errors in HMRC's favour set against errors in yours, so two mistakes of £8,000 in opposite directions net to nothing and fall comfortably inside Method 1. Adjust box 1 or box 4 as appropriate and keep the working with the VAT records. Two things are not the end of it. A Method 1 adjustment is not a disclosure for penalty purposes: where the error was careless rather than an honest mistake made with reasonable care, you have to tell HMRC separately in writing to get the full reduction. And late payment interest runs on any VAT that was not paid by the due date for the period the error belongs to, whichever method corrects it.
Two limits are easy to miss. Method 1 is not available for deliberate errors at all, regardless of size. And the 1% test uses the box 6 figure of the return you are correcting on, not the one that contained the mistake — so a quiet quarter narrows the route.
Method 2: notify HMRC separately
Above those limits — net errors over £50,000, or between £10,000 and £50,000 and above the 1% threshold — you must notify the VAT Error Correction Team separately, online or in writing. The same applies, whatever the amount, to any error that was deliberate.
A notification should set out the periods affected, the amounts, the reason the error arose and how it was discovered. That last point is not padding: the explanation is what HMRC uses to characterise the behaviour, and the behaviour is what sets the penalty. "Our system mapped a new product to the wrong tax code in March and the reconciliation caught it in August" reads very differently from a bare figure with no narrative.
You can also use Method 2 voluntarily for a small error, and there are situations where that is the better choice — where the same error recurs across several years, where the position is technically arguable, or where you want the disclosure clearly on record.
The four-year wall
Errors can be corrected for four years from the end of the accounting period in which they occurred. That limit cuts both ways: it caps what HMRC can assess in the ordinary case, and it caps what you can reclaim. An overpayment discovered in year five is gone, which is the entire argument for reviewing the VAT treatment of a new product line or a new supply chain early rather than when someone eventually queries it.
The four-year limit does not protect deliberate errors. Where behaviour was deliberate, HMRC can go back twenty years, and the correction obligation has no time restriction at all.
What it costs, and the disclosure discount
Correcting an error does not automatically produce a penalty. A genuine mistake made despite taking reasonable care carries none. Beyond that, penalties are behaviour-based: up to 30% of the tax for carelessness, 20% to 70% for a deliberate understatement, and 30% to 100% where it was deliberate and concealed.
Within each band, the size of the penalty depends on the quality of the disclosure — telling, helping and giving access to records — and, critically, on whether the disclosure was unprompted or prompted. An unprompted disclosure of a careless error can reduce the penalty to nil; the same error disclosed after HMRC has opened a check cannot fall below 15%. That gap is the reason to correct an error the week you find it rather than the quarter after.
Late payment interest runs separately on any tax that was paid late, at base rate plus 4%, and is not affected by the quality of the disclosure.
Finding errors before HMRC does
The recurring sources are predictable enough to test for deliberately:
- New products or services given the wrong VAT code on set-up, then replicated across every subsequent transaction;
- Partial exemption and business/non-business apportionment, particularly where the method was never agreed with HMRC or the business has changed since it was;
- Reverse charge on imported services and in construction — under-declared and over-claimed in equal measure;
- Input tax on entertainment, cars and private use, where the default in most accounting systems is to reclaim;
- Credit notes and bad debt relief claimed in the wrong period or without the six-month condition being met;
- Manual journals posted to VAT control accounts that never reach a return.
An annual reconciliation of the VAT account to the returns and to the statutory accounts finds most of these while Method 1 is still available and the disclosure is still unprompted — which is, in penalty terms, the difference between free and expensive.
Acumon runs these reviews through VAT health checks and handles corrections and disclosures as part of VAT compliance work, including the negotiations where HMRC disputes the behaviour. If you have just found something, the useful question is not how big it is — it is whether anyone at HMRC has asked about it yet.