04/09/2026
Your invoice said £8,000 labour, £4,000 materials.
The contractor deducted CIS on all £12,000.
That is an extra £800 sitting with HMRC instead of in your bank. It isn't lost money. It just isn't yours to use until you claim it back, and that takes months.
CIS is a deduction on labour. Before working out the 20%, the contractor is supposed to take off VAT, materials you paid for directly, consumable stores, fuel other than for travel, plant hire, and manufacturing or prefabricating materials.
Two things go wrong, and they go wrong in opposite directions.
First: the materials have to be ones you paid for. If the contractor supplied them, or reimbursed you for them, they don't come off. Plenty of subcontractors deduct materials that were never their cost.
Second: HMRC expects the materials figure to be the direct cost. If you've put a margin on materials, the margin isn't materials. It sits inside the payment and it's within the deduction. And refusing to tell the contractor what you actually paid doesn't protect it. The contractor is entitled to make their own estimate of the split.
So an overstated materials line is a liability sitting with the contractor. An understated one is your cash sitting with HMRC for a year.
What to do about it:
Split labour and materials on every invoice. Not "supply and fix, £12,000."
Keep the purchase invoices. The contractor can ask to see them, and HMRC can ask the contractor.
If you're the one deducting: don't take a materials figure on trust when it looks wrong. The liability for under-deducting is yours, not the subcontractor's.
One job this week: take your last three payment and deduction statements and check the deduction against your invoices. If it's 20% of the whole invoice, ask why.
Which side of this are you on this week: deducting, or being deducted from?