Undisclosed income rules impose interest and up to 50% penalty, subject to procedural safeguards and limitation rules. Where a return of undisclosed income required by notice is not filed timely, simple interest at 1.5% per month on the tax determined is payable from the day after the notice deadline until assessment completion, and the Assessing Officer or Commissioner (Appeals) may levy a penalty equal to 50% of that tax, subject to opportunity to be heard, officer approval thresholds for higher penalties, exclusions where tax is paid or seized money adjusted and where the undisclosed income does not exceed the return, and detailed limitation period exclusion and extension rules.