You’ve landed a run of work for a main contractor, the paperwork arrives, and somewhere in it is a line about CIS. Someone mentions that money will be held back from your invoices. Nobody explains why, or when you get it back.

It’s not a penalty and it’s not optional. The Construction Industry Scheme collects tax on construction work up front, and once you understand what’s actually happening it goes from alarming to merely annoying. Here’s the plain version.

General guidance, not tax advice. Deduction rates, thresholds and deadlines change — take the current figures and rules from gov.uk and talk to an accountant who deals with trades about your own position.

What CIS actually is

Under CIS, a contractor paying you for construction work doesn’t hand over the full invoice amount. They hold back a slice, send it to HMRC against your name, and pay you the rest.

That withheld money is yours, paid early against your tax bill — not a fee, a retention, or the contractor taking a cut. It sits on your HMRC record as tax already paid and is set against what you actually owe at year end. If too much was taken — and for most subbies it is — you get the difference back.

Are you a subcontractor under it?

Two roles matter, and you can be both at once:

Contractor — you pay others to do construction work for you. Main contractors and builders obviously, but also you, the moment you start paying a subbie of your own.
Subcontractor — you’re paid by a contractor to do construction work.

So the one-van sparks who subs for a builder is a subcontractor. The same sparks who then pays a mate to help wire a flat has become a contractor too, with the registration and monthly filing that goes with it. Plenty of trades drift into that second role without realising, and it’s the one with penalties attached.

When it doesn’t apply

A homeowner having work done on their own house isn’t a contractor. Kitchens, bathrooms and rewires for private customers sit outside CIS entirely — which is why many domestic-only trades never touch it.
Some work isn’t a “construction operation” even on site. Architecture and surveying, carpet fitting, delivering materials and running a site canteen are commonly outside it; scaffolding, groundworks, decorating and most trades are firmly inside. If a job sits on the line, check HMRC’s guidance on what’s covered.

How a deduction actually works

The sequence:

You register with HMRC as a subcontractor — once, not per contractor.
Before paying you the first time, the contractor verifies you with HMRC. That check tells them which rate to apply.
You invoice as normal, splitting labour and materials.
They work out the deduction, pay you the balance, and pay the withheld amount to HMRC.
They send you a payment and deduction statement — your proof it was taken.

What the deduction comes off

This is where subbies lose money through sloppy invoicing.

The deduction is worked out on the labour element only. The cost of materials you’ve paid for, plant hire and certain other direct costs are generally excluded — but only if your invoice makes them visible.

Say you invoice £2,000 for a job: £1,400 labour and £600 in materials you bought yourself (illustrative figures — yours will differ). The deduction is calculated on the £1,400. Send that same job through as a single £2,000 line with no breakdown and the contractor may well apply the deduction to the whole lot. You’d get it back eventually, but you’ve handed HMRC an interest-free loan for months for nothing.

So itemise: labour on its own lines, materials clearly separated — the same discipline that makes a properly itemised quote easier to get accepted. Keep the supplier receipts to back the materials figure up.

VAT sits outside the deduction calculation. But if you’re VAT-registered and subbing to another VAT-registered business within CIS, you’re likely in domestic reverse charge territory too — check before you invoice.

Registering, and gross vs net status

The verification check puts you in one of three positions:

Status	What happens to your payments
Registered (net)	The standard deduction comes off your labour. Where most subbies sit.
Not registered	A higher deduction comes off — the scheme’s way of encouraging you to register.
Gross payment status	No deduction at all. You’re paid in full and settle your tax as normal.

Gross payment status is the one to aim for if you’re subbing regularly, because it ends the cash-flow problem below. HMRC applies tests covering your turnover, your record of filing and paying on time, and how you trade; the thresholds change, so get the current position from gov.uk’s CIS guidance. A clean filing history is the price of entry; late returns are how people lose gross status again.

Why it bites your cash flow

Your tax bill is calculated on your profit. CIS deductions are calculated on your labour invoiced — before your van, fuel, tools, insurance, phone, accountant or anything else comes off.

So more tax is collected through the year than is ultimately owed. The refund is real, but it arrives when your return is processed — not when you need it in March.

What that means in practice:

Quote and plan off the net figure. CIS work pays you less than your day rate on the day and tops it up later. Budget for the money that actually lands.
Keep your return prompt. The refund can’t come before the return goes in. Filing in January when you could have filed in May means lending HMRC your money for eight extra months.
Mixed work needs watching. Run CIS subbing alongside private jobs and the private side carries no deductions at all — that tax is still waiting for you. Set it aside as it comes in.

Your monthly statements are money — keep them

Every contractor must give you a payment and deduction statement for each payment period. Treat each as a receipt for tax paid: without it you’re relying on the contractor’s word that the money reached HMRC.

Collect them as you go. Photograph each one the day it arrives and file it by contractor and month. Chasing twelve of them from four contractors in January is miserable, and some will have moved on.
Reconcile them against your own records. Your invoice, the amount that hit the bank and the statement should tell the same story. Mismatches are usually a deduction taken on materials — raise it with the contractor’s accounts while they remember the job.
Chase missing ones in writing. A short email beats a text you can’t find later. The same habit that keeps invoices from going unpaid keeps your tax record straight.

How you get the money back

Sole traders declare the year’s CIS deductions on their Self Assessment return. They’re offset against the tax and National Insurance due, and anything over is refunded.
Limited companies don’t use Self Assessment for it. The company’s deductions are claimed through its payroll scheme and set against what it owes HMRC, with a repayment claim at the tax year end — a different process with its own timing, and a real sole trader versus limited company consideration if most of your work is CIS.

Either way your accountant needs the statements: a complete set makes the claim routine, gaps delay or shrink the refund.

The takeaway

CIS isn’t a tax on you — it’s your own tax collected early by whoever pays you. Register before your first invoice so you’re not on the higher rate, split labour and materials on everything you send so the deduction only hits the labour, keep every monthly statement, and file your return promptly so the over-payment comes back sooner rather than later.

Then do the two things that change the picture: budget from what actually lands in the bank, and ask your accountant whether gross payment status is within reach. Take the current rates, tests and deadlines from gov.uk rather than from a conversation in a site cabin — including this one.

Queezy builds itemised quotes on site with labour and materials on separate lines, so what you send a contractor is already in the shape CIS needs. Download it free on iPhone.

CIS Explained for Subcontractors (Plain English)