How do I register for the Construction Industry Scheme as a subcontractor?

You register with HMRC as a CIS subcontractor online through your Government Gateway account, and you'll need your Unique Taxpayer Reference (UTR) — so if you haven't registered for Self Assessment yet, you'll need to do that first.

Once registered, contractors will deduct tax at the standard 20% rate rather than the 30% rate charged to unregistered subcontractors, which makes a real difference to your cash flow through the year even though both rates get reconciled against your actual tax bill at Self Assessment. You can also apply for gross payment status if your turnover and compliance record qualify, meaning contractors pay you in full with no deduction at all and you handle the tax yourself at year end — worth considering once you're established. If you're currently having 30% deducted and haven't registered, that's costing you cash flow for no reason.