Tax Collection and Management (Wales) Act 2016 section 133

Penalty for failure to notify under-assessment or under-determination

Section 133 imposes a penalty on a person who fails to notify the Welsh Revenue Authority (WRA) within 30 days when a WRA assessment understates their tax liability or the amount they are required to pay.

• If a WRA assessment understates your devolved tax liability or the amount you must pay, you must take reasonable steps to notify WRA within 30 days of the assessment notice being issued.
• WRA will consider whether you knew, or should have known, about the under-assessment when deciding whether reasonable steps were taken.
• The maximum penalty is 30% of the potential lost revenue resulting from the under-assessment.
• These rules apply equally to formal WRA determinations (made when no tax return has been filed), not just to standard assessments.