CIS Deductions Explained: A Plain-English Guide for UK Contractors

What the Construction Industry Scheme actually is

The Construction Industry Scheme (CIS) is HMRC's way of collecting tax from subcontractors up front. Under CIS, a contractor deducts money from a subcontractor's payment and pays it directly to HMRC. Those deductions count as advance payments towards the subcontractor's tax and National Insurance.

If you pay subcontractors for construction work, you almost certainly need to operate CIS as a contractor. If you're paid by other businesses for construction work, you're a subcontractor — and you can be both at once, which is where a lot of the day-to-day confusion comes from.

This guide walks through how CIS works in practice, so you know what to deduct, when, and what happens if you get it wrong.

Who has to register

• Contractors must register for CIS before taking on their first subcontractor. This includes mainstream construction businesses and "deemed contractors" — organisations that spend heavily on construction even though building isn't their core trade.
• Subcontractors don't have to register, but it's almost always worth it: registered subcontractors are deducted at the standard rate, unregistered ones at the higher rate (more on that below).

Registering as a subcontractor is the single cheapest thing most subbies can do to improve their cash flow.

The three deduction rates

When you pay a subcontractor, the rate you deduct depends on their status with HMRC:

• 20% — the standard rate, for subcontractors who are registered with CIS and verified.
• 30% — the higher rate, for subcontractors you can't verify or who aren't registered.
• 0% (gross payment status) — for subcontractors who meet HMRC's turnover and compliance tests. They're paid in full and settle their own tax later.

The 10-point gap between 20% and 30% is real money out of a subcontractor's pocket every week until they register — which is why verification matters so much.

What you deduct from — and what you don't

This is the part contractors get wrong most often. CIS deductions apply to the labour element of a payment only. You do not deduct from:

• Materials the subcontractor has genuinely paid for
• VAT charged
• Plant hire (where hired from a third party), fuel for that plant, and certain other costs
• CITB levy, where applicable

So if a subcontractor invoices £2,000 for labour plus £800 in materials, a 20% deduction applies to the £2,000 only — £400 — not the full £2,800. Keeping labour and materials clearly separated on invoices protects both sides.

Verifying subcontractors

Before you pay a new subcontractor, you must verify them with HMRC. Verification tells you two things: whether they're registered, and which rate to apply. HMRC gives you a verification number, and if the subcontractor can't be verified you must deduct at 30%.

Skipping verification is one of the most common — and most expensive — CIS mistakes, because if you under-deduct, HMRC can come after you for the shortfall, not the subcontractor.

Monthly returns and statements

Operating CIS means a monthly rhythm:

File a monthly CIS return (CIS300) to HMRC covering every payment to subcontractors in the tax month (the 6th to the 5th). Returns are due by the 19th of the following month.

Pay the deductions you've withheld to HMRC.

Give each subcontractor a payment and deduction statement so they have a record of what was taken.

File a nil return (or tell HMRC) even in months where you paid no subcontractors, or you risk automatic penalties.

Late returns trigger penalties that stack up quickly — an initial fixed penalty followed by further charges the longer it's outstanding.

Where it goes wrong

The recurring CIS failures we see are boringly consistent:

• Not verifying a subcontractor and deducting the wrong rate
• Deducting from materials or VAT by accident
• Missing the 19th-of-the-month deadline
• Forgetting nil returns in quiet months
• Losing the paperwork that proves what was deducted and when

None of these are complicated individually — they're just easy to drop when you're running jobs and the admin piles up.

How ScopeKit helps

ScopeKit handles CIS natively for UK contractors — subcontractor verification, correct labour-only deductions, monthly CIS300 submission to HMRC, and payment-and-deduction statements — so the monthly rhythm runs on rails instead of living in a spreadsheet. Because it's built into the same platform as your quoting and scheduling, the subcontractor details you've already captured feed straight into CIS, rather than being re-keyed.

CIS is one of those parts of running a construction business where a small, repeatable process saves a lot of pain at year end. Get verification and the monthly return right, keep labour and materials separated, and the rest follows.

This guide is general information, not tax advice. For your specific situation, check the current rules on GOV.UK or speak to your accountant.