Missing an income tax deadline is expensive because the system charges both interest and penalty, and the two run from different anchors. Interest accrues on the unpaid tax from the date the installment was due, while penalties are fixed sums or multiples that depend on the type of return and the length of the delay. The parking of interest matters most. Because it compounds on the outstanding balance, a business that files three months late is not paying three months of simple interest but a figure built on the whole overdue amount. Separate penalties apply to late VAT, monthly withholding and the annual return, so several overdue filings can multiply quickly. The remedy is to file immediately rather than wait, because every delay adds to the bill. Payment plans exist for genuine difficulty, and the department has shown willingness to accept returns accompanied by interest where the failure was administrative. Advice from an accountant is worth the fee because a correct late filing with a note and full interest costs far less than a reassessment that follows a continued silence.