If you run a plumbing or heating business, the Construction Industry Scheme probably affects you — even if you’ve never set foot on a building site. CIS catches out more trades businesses than almost any other piece of tax legislation. And the penalties for getting it wrong are not small.
This guide breaks it all down in plain English. No accountancy waffle. Just what you need to know to keep your business on the right side of HMRC.
What Is CIS and Why Should You Care?
The Construction Industry Scheme is a set of HMRC rules that govern how payments are made between contractors and subcontractors in the construction industry. Think of it as a pay-as-you-go tax collection system. Instead of waiting for a subcontractor to file a tax return and pay up months later, HMRC wants some of that tax collected at source — right when the invoice gets paid.
Here’s why it matters to you specifically. HMRC’s definition of “construction work” is far broader than most people realise. It includes plumbing, heating installation, pipework, gas work, bathroom fitting, and boiler installations. If you’re doing any of this work as a subcontractor — or hiring someone else to do it — CIS applies to you.
Ignore it, and you’re looking at fines, penalty deductions at 30% instead of 20%, and a world of stress you don’t need.
Contractor vs Subcontractor: Which One Are You?
This is where most plumbing and heating businesses get confused. Let’s keep it simple.
You’re a Contractor If…
You pay other self-employed people or businesses to carry out construction work for you. It doesn’t matter whether you call them “subcontractors,” “mates helping out,” or “labour-only workers.” If you’re paying someone who isn’t on your payroll to do construction work, you’re acting as a contractor under CIS.
Real-world example: You run a heating company. You win a contract to install a full central heating system in a new-build property. You bring in a self-employed plumber to do the pipework while your own team handles the boiler and controls. In this scenario, you are the contractor and the plumber is your subcontractor. You need to be registered as a contractor with HMRC.
You’re a Subcontractor If…
You carry out construction work for another contractor. This is the more common position for sole traders and smaller heating businesses.
Real-world example: A property maintenance company hires you to install new radiators across a block of flats. They’re paying you to do the work, and they’re responsible for the overall project. You are the subcontractor. You need to be registered as a subcontractor with HMRC.
Can You Be Both?
Absolutely. And many plumbing and heating businesses are. Consider this: you might be a subcontractor to a main building contractor on one job, and then hire a self-employed gas engineer to help you on another. In that second job, you’re the contractor. You’d need to be registered in both roles.
How CIS Deductions Actually Work
Let’s get into the numbers, because this is where it really hits your pocket.
The Standard Deduction: 20%
If you’re a registered subcontractor, the contractor deducts 20% from your labour payments before paying you. Materials are not included in the deduction — only the labour element.
Here’s a worked example. You invoice a contractor £1,000 for a heating installation. The breakdown is £700 labour and £300 materials. The contractor deducts 20% of the £700 labour portion, which is £140. You receive £860. That £140 goes to HMRC as an advance payment against your tax bill.
When you file your Self Assessment at the end of the year, that £140 (and all the other CIS deductions throughout the year) gets credited against your total tax liability. Think of it like a deposit — you’ve already paid some of your tax, so there’s less to settle at the end.
The Higher Deduction: 30%
If you haven’t registered as a subcontractor with HMRC — or if the contractor can’t verify you — the deduction jumps to 30%. Using the same example, that’s £210 deducted from your £700 labour instead of £140. That’s an extra £70 per £1,000 invoice tied up until your tax return is processed.
Over a year, the difference adds up fast. If you’re invoicing £5,000 a month in labour, the difference between 20% and 30% is £500 every single month that you can’t access. That’s £6,000 a year locked away unnecessarily.
Gross Payment Status: 0% Deduction
Some subcontractors qualify for gross payment status, which means no deductions at all. You get paid the full invoice amount and sort your own tax out later. To qualify, you typically need a clean compliance record with HMRC and a minimum annual turnover (currently £30,000 for sole traders, excluding materials and VAT).
It’s worth applying for if you qualify. Having your cash flow uninterrupted makes running your business a lot smoother.
The CIS Process: Step by Step
If you’re acting as a contractor, here’s what you actually need to do. No shortcuts.
Step 1: Register as a contractor with HMRC. You can do this online or by phone. You’ll need your UTR (Unique Taxpayer Reference) and National Insurance number.
Step 2: Verify every subcontractor before you pay them. You do this through HMRC’s verification service. They’ll tell you the correct deduction rate — 0%, 20%, or 30%. You must do this for every new subcontractor, every tax year.
Step 3: Make the correct deductions from payments. Deduct the right percentage from the labour element and pay the subcontractor the net amount.
Step 4: Give the subcontractor a payment and deduction statement. This must show the gross amount, the deduction, and the net payment. The subcontractor needs this for their own tax records.
Step 5: File a monthly CIS return with HMRC. This is due by the 19th of every month following the tax month. Late filing means an automatic £100 penalty — and it goes up from there.
Step 6: Pay the deductions to HMRC. This is due by the 22nd of each month if paying electronically, or the 19th by cheque.
Five Common CIS Mistakes Plumbing & Heating Businesses Make
In my eyes, these are the ones that cause the most damage. Every single one is preventable.
1. Not Registering at All
Some heating engineers simply don’t realise CIS applies to them. They assume it’s only for big building firms. It’s not. If you’re doing any of the work described above and you’re being paid by another contractor, CIS applies. If you’re not registered and someone tries to verify you, they’ll have to deduct at 30%.
2. Failing to Verify Subcontractors
You bring in a mate to help with a big job. He says he’s registered for CIS. You take his word for it and pay him the full amount. Then HMRC comes knocking and wants to know why you didn’t verify him and why no deductions were made. The liability for those missed deductions falls on you, the contractor. Always verify. Every time.
3. Late Monthly Returns
CIS returns are due monthly, not quarterly. Miss the 19th deadline and you’ll get an automatic £100 fine. Miss it by two months and that rises to £200. After six months, HMRC can charge penalties based on the amount of CIS deductions that should have been reported. These stack up quickly.
4. Getting the Employment Status Wrong
This is a big one. There’s a significant difference between someone who is genuinely self-employed and someone who should be on your payroll. If your “subcontractor” works set hours, uses your tools, only works for you, and you tell them exactly how to do the job — HMRC may decide they’re actually an employee.
If that happens, you could owe backdated PAYE, National Insurance, holiday pay, and penalties. For a deeper look at how business structure affects this, take a look at our guide on sole trader vs limited company.
5. Not Separating Labour and Materials on Invoices
CIS deductions only apply to the labour element of a payment. If your invoices don’t clearly split labour from materials, the contractor may have to deduct CIS from the entire invoice — including the cost of that new boiler or those copper fittings. That means more of your money gets tied up with HMRC for no good reason.
Make sure every invoice has a clear breakdown. Labour on one line, materials on another. Simple.
CIS and VAT: How They Overlap
Let’s explore a point that trips people up regularly. CIS deductions are calculated before VAT, not after. So if you’re VAT registered and you invoice £1,200 (that’s £1,000 plus £200 VAT), the CIS deduction is based on the £1,000, not the £1,200.
But here’s where it gets interesting. Since March 2021, the VAT domestic reverse charge applies to most CIS-registered construction services. This means the customer accounts for the VAT, not you. It changes how you raise invoices and what you actually receive.
If you’re dealing with both CIS and VAT, your invoicing needs to be spot-on. Get it wrong and you’ll either charge VAT when you shouldn’t, or fail to charge it when you must. Our VAT guide for plumbing businesses covers this in detail.
How Making Tax Digital Will Change CIS Reporting
Making Tax Digital (MTD) is coming for Income Tax, and it will change how CIS information feeds into your tax affairs. Under MTD for Income Tax Self Assessment, sole traders and landlords earning over £50,000 will need to submit quarterly updates to HMRC using compatible software from April 2026. Those earning over £30,000 follow from April 2027.
For plumbing and heating subcontractors, this means your CIS deductions will need to be tracked and reported digitally throughout the year — not just totted up once at Self Assessment time. Your accounting software will need to handle CIS deduction statements and reconcile them against your quarterly submissions.
If you’re a contractor filing monthly CIS returns, you’re already doing some digital reporting. But MTD will demand more granular, more frequent record-keeping across the board.
The bottom line: if you’re still running your CIS records on spreadsheets or paper, now is the time to move to proper accounting software. We cover the full MTD timeline and what you need to prepare in our Making Tax Digital guide.
What a Specialist Accountant Does Differently
Any accountant can file a CIS return. But there’s a real difference between someone who processes the paperwork and someone who actually understands how plumbing and heating businesses operate day to day.
They Know Your Industry
A specialist accountant understands that you might be a subcontractor on a Monday and a contractor on a Tuesday. They know about the reverse charge, about how materials invoicing works, and about the grey areas around employment status in the trades. They won’t need you to explain what a first fix is or why you sometimes need to bring in extra labour at short notice.
They’re Proactive, Not Reactive
A general accountant might process your CIS deductions after the fact. A specialist will set up your systems so that invoices are raised correctly from the start, subcontractors are verified before they begin work, and monthly returns are filed on time without you having to chase anyone.
They Save You Money You Didn’t Know You Were Losing
Are you claiming all allowable expenses against your CIS income? Are your subcontractor agreements watertight? Could you benefit from gross payment status? Is your business structure costing you more tax than it should? These are the questions a specialist asks — and a generalist often doesn’t.
They Help You Plan Ahead
With MTD on the horizon and employment status rules tightening, having someone in your corner who understands where the legislation is heading — and how it specifically affects heating and plumbing businesses — is worth its weight in copper pipe.
Practical Takeaways: Your CIS Checklist
Here’s what you should be doing right now to make sure your CIS house is in order.
If you’re a subcontractor:
- Make sure you’re registered for CIS with HMRC. If you’re not, do it today.
- Keep every payment and deduction statement you receive from contractors.
- Split labour and materials clearly on every invoice.
- Check whether you qualify for gross payment status — it could transform your cash flow.
- Start preparing for MTD if your income is over £50,000.
If you’re a contractor:
- Register as a CIS contractor with HMRC before you pay your first subcontractor.
- Verify every subcontractor through HMRC’s service before making any payment.
- File your monthly CIS return by the 19th. Set a recurring reminder. No excuses.
- Give subcontractors proper payment and deduction statements.
- Review your subcontractors’ employment status — genuinely self-employed or actually employees?
Everyone:
- Use accounting software that handles CIS properly. Ditch the spreadsheets.
- Review your setup at the start of every tax year.
- Talk to an accountant who understands your trade, not just your tax return.
Not Sure If You’re CIS Compliant?
CIS doesn’t have to be complicated. But it does need to be right. If you’re a plumbing or heating business and you’re not 100% confident your CIS setup is correct, let’s have a chat.
Book a free 15-minute CIS check with our team and we’ll tell you exactly where you stand — no jargon, no obligation, no sales pitch. Just straightforward advice from accountants who work with trades businesses every day.