If you submit a self-assessment tax return which results in a repayment of tax, HMRC undertakes a number of routine checks to ensure that the claim is genuine. This means that HMRC may write to you and ask that you confirm your identity. This will include the requirement for you to provide evidence to prove who you are and to answer some questions about the claim that has been submitted in your name. You may also need to provide documentation to support construction industry scheme and employment detail, where appropriate.
If you do not reply to the request from HMRC, then they will remove you from Self Assessment and not make any repayment.
At the moment, these requests are only sent by letter but this may change at at later date.