CIS itself is untouched — monthly returns, verification and deductions all work exactly as they do now, and MTD does not reach them. What changes is your own income reporting. If you are a self-employed subcontractor with qualifying income above the threshold, you are in MTD for Income Tax on the usual dates, so your trade goes into quarterly updates and a final declaration. The interaction worth knowing about is that CIS deductions suffered are credited at the final declaration, not through the quarterly updates, so the estimated tax figure your software shows during the year will look far too high if it ignores the 20% already taken from every payment. That surprises subcontractors, and it is why we set expectations before the first quarter rather than after it.