Monthly, by the 19th of the month following the tax month, which runs from the 6th to the 5th. So the month ending 5 June is due by 19 June. The penalties are unusually harsh for how small the obligation feels: £100 the day you are late, £200 at two months, and then £300 or 5% of the deductions at six and twelve months. Those stack, and they are per return — a contractor who quietly stops filing for a year can accumulate thousands owing nothing in tax. You must file a nil return if you paid no subcontractors in a month, or tell HMRC you are inactive, because HMRC expects a return and penalises its absence rather than its content. That is where most CIS penalties come from.