HMRC deals with all tax returns filed based on “process now, check later”. Your tax return is your self-assessment of your taxable income and HMRC will not necessarily identify any errors you may have made.
If you discover that you have made a mistake on a tax return, you can amend the return within 12 months of the normal 31st January filing date. If you filed on-line, enter the amended information and the return will be updated. If you filed a paper return, you would need to send a new return, clearly marked “amended return”. If you filed the original return late, you have 12 months from the date it was filed to make any amendment.
If there is additional tax to pay following the amendment, interest will be due on the underpaid tax from the original due date. If the amendment reduces tax due, HMRC will process a repayment on request.
HMRC guidance on correcting a return is at https://www.gov.uk/self-assessment-tax-returns/corrections.
Between 1 and 4 years after the 31st January filing deadline for a tax return it is possible to claim overpayment relief if a mistake in the return has caused you to pay too much tax. There are strict conditions about this relief, and it may not be available in all circumstances. After 4 years the figures become final and late claims are only accepted in very exceptional circumstances.
Tax calculation mistakes and disagreements.
You should check the tax calculation made by HMRC and if you think it is not correct you should contact them by telephone or in writing and ask for an explanation of their calculation. If this shows that there has been a mistake, you should object in writing and ask for a correction. There is no formal process to appeal about a calculation mistake, but if it has not been resolved within a reasonable period, you can make a complaint.
