EM3267 | Discovery: varying an assessment
From HM Revenue & Customs · Enquiry Manual
Sometimes after an assessment has been made, new information may become available that shows the amount assessed is either excessive or too little.
Once an assessment has been made, it can only be varied upon appeal. There are only four situations when it can be amended
If the assessment was not under appeal, it will have become final and determined so a further assessment will need to be considered.
See ARTG2700 for more information about settling appeals by agreement, or where the customer does not respond.