CIS Explained: A No-Nonsense Guide for UK Contractors

If you pay subcontractors in UK construction, you almost certainly need to operate the Construction Industry Scheme (CIS). Get it right and it's a monthly admin task. Get it wrong and HMRC penalties stack up fast — £100 per late monthly return, rising to £3,000 if you keep missing them.

This is a plain-English guide to how CIS actually works in 2026.

Who has to operate CIS

You're a contractor for CIS purposes if:

• You run a construction business and pay subcontractors, or
• You're a "deemed contractor" — a business outside construction that spent more than £3 million on construction in the last 12 months.

You're a subcontractor if you carry out construction work for a contractor. Many businesses are both.

"Construction work" is broader than people expect: site prep, groundworks, actual building, alterations, dismantling, installation of systems (heating, lighting, water, ventilation), painting and decorating, and cleaning inside buildings after construction.

What's out: architecture and surveying, scaffolding hire (without labour), carpet fitting, delivery of materials, and running a canteen or site office.

Registering

Contractors register with HMRC as a new employer for CIS. Subcontractors register separately.

Subcontractors have three possible statuses:

| Status | Deduction rate | How you get it | | --- | --- | --- | | Gross payment | 0% | Pass HMRC turnover, compliance, and business tests | | Registered | 20% | Register with HMRC as a subcontractor | | Unregistered | 30% | Default — don't get caught here |

Verifying every subcontractor before you pay them is your job as the contractor. HMRC tells you which rate to apply.

The monthly cycle

CIS runs on tax months (6th to 5th). Every month you:

1. Verify any new subcontractors before their first payment.
2. Calculate the deduction on the labour portion of each invoice.
3. Deduct materials, VAT, plant hire (with operator only), CITB levy, and fuel from the taxable amount.
4. Pay the subcontractor the net amount.
5. Issue a CIS payment and deduction statement (CIS300) to each subbie within 14 days of the tax-month end.
6. File the CIS monthly return with HMRC by the 19th.
7. Pay HMRC the deductions by the 22nd (electronic) or 19th (post).

If you paid no subcontractors that month, file a nil return — otherwise HMRC assume you forgot.

Materials — the bit everyone gets wrong

You only deduct CIS from labour. Materials the subbie has genuinely bought for the job come off first. But HMRC pushes back if the "materials" figure looks inflated or if the subbie didn't actually pay for them.

Rules of thumb:

• Get an itemised invoice separating labour and materials.
• If the subbie can't evidence they bought the materials, treat the whole invoice as labour.
• Plant hire is materials only if it's without an operator. With an operator, it's a labour cost.

Common penalty triggers

• Late monthly return. £100 immediately, rising with time.
• Not verifying a subcontractor and paying at 20% when they should have been at 30%. You owe the shortfall personally.
• Missing CIS300 statements to subcontractors.
• Treating a worker as a subcontractor when they're really an employee (IR35 / employment status).

VAT reverse charge — don't forget

Since March 2021, most CIS-registered supplies between VAT-registered businesses use the VAT domestic reverse charge. The customer accounts for the VAT, not the supplier. Your invoice should say "reverse charge: customer to pay VAT to HMRC" and show the VAT rate but not add it to the total.

Keeping the audit trail

HMRC can go back six years. You need to keep:

• Verification numbers for every subbie
• Copies of invoices (labour/materials split)
• Copies of monthly returns and CIS300s
• Bank evidence of payment

SiteClick's job costing module captures labour hours, materials receipts (with AI-extracted vendor and totals from a photo), and expense allocations against each project — so when your accountant asks for the labour/materials split, it's already there.

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