20% if they are registered with HMRC, 30% if they are not, and nothing at all if they hold gross payment status — but you do not choose which, you verify. Every new subcontractor must be verified with HMRC before their first payment, and HMRC tells you the rate to apply. Guessing is not an option: deduct 20% from someone HMRC would have told you to deduct 30% from and you owe the difference yourself. Crucially, the deduction applies only to the labour element. Materials, plant hire, and VAT are excluded, so an invoice of £5,000 labour plus £3,000 materials at 20% produces a £1,000 deduction, not £1,600. Subcontractors who do not itemise materials will have deductions taken from the whole amount, which is an expensive way to save typing.