If an error has been made when completing a self assessment tax return, it is possible to make a correction. If you don’t inform HM Revenue & Customs of an error and it is discovered by HMRC you may face large penalties.
You have 12 months following 31st January which immediately follows the end of the relevant tax year to send amendments to HMRC. For instance, an amendment for a 2010-11 tax return may be made before 31st January 2013. If you originally submitted a paper tax return, you only have to submit the page that contains the amendment. If you file online, it is simpler to make the amendment online.
An error discovered after the 12 months has lapsed should be made in writing to HMRC, giving details of the tax year concerned, the tax paid and documentary evidence. HMRC may want more details from you to ensure the information is correct. If a repayment of tax is due, HMRC will make the repayment either by payable order or directly into your bank account. If correction of the error results in more tax being due, HMRC will ask you to pay the outstanding amount as soon as possible.
If HMRC discover an error and decide that you took reasonable care when completing your tax return but made an error, you will not be charged a penalty. However, if HMRC consider you to have been careless, or made the error deliberately or tried to conceal the error, you will face a penalty.
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