Taxes Consolidation Act 1997 section 959AU

Date for payment of tax: amended assessments

Section 959AU sets out when additional tax arising from an amended assessment becomes due and payable, including strengthened rules introduced by Finance Act 2025 for cases where the taxpayer's original return was incomplete.

• Additional tax due on an amended assessment is generally treated as due on the same date the original assessment tax was due
• Where the taxpayer filed a full and true return, additional tax on an amended assessment is due one month from the date of the amendment
• Where a return did not contain full and true disclosure and the assessment is amended a second or subsequent time, the additional tax is deemed due on the original assessment date
• The effect of the Finance Act 2025 change is that interest on underpaid tax runs from the original due date, significantly increasing the taxpayer's liability in cases of incomplete returns