The relevant rules depend on whether you need to:
Amend your VAT records if you discover they contain errors;
Correct errors on VAT returns you’ve already sent to HMRC;
Claim a refund if you’ve overpaid VAT, or not claimed enough credit on a return.
When you find you have made an error or errors in a VAT return that has already been submitted to HMRC, there are 2 methods that may be used depending on the size of the error.
An adjustment can be made on a current VAT return for a previous VAT return period if the net value of the errors does not exceed the greater of:
1% of the box 6 figure required on the VAT return for the period of discovery (subject to an upper limit of £50,000).
Where the net value of the error is between £10,000 and £50,000 and exceeds 1% of the box 6 figure or where the net value of errors found on previous returns is greater than £50,000 then a notification must me made using the VAT652 form. This form must also be used for any deliberate errors on previous returns.
Subject to the above, if an error is discovered before you’ve submitted a VAT return then you can amend your records, but you must ensure that you keep a clear note to show the reason for the error in case it is questioned in the future.
If you are uncertain how to deal with a past mistake on a return we can advise how best to deal with any necessary amendments or notifications.