If you work in the UK construction industry — whether as a contractor, subcontractor, or both — the Construction Industry Scheme (CIS) is something you cannot afford to ignore. Get it wrong and you'll face penalties; get it right and you'll stay compliant, avoid cash flow headaches, and potentially reclaim significant tax overpayments.
Here's a plain-English breakdown of everything you need to know, including a free calculator to take the maths off your plate.
What Is the Construction Industry Scheme?
CIS is an HMRC scheme that requires contractors to deduct money from payments made to subcontractors and pass those deductions directly to HMRC. These deductions count as advance payments towards the subcontractor's Income Tax and National Insurance liabilities.
The scheme applies to most construction work in the UK, including:
- Site preparation and groundwork
- Building, alterations, repairs, and demolition
- Painting and decorating (when part of construction)
- Installing systems such as heating, lighting, and air conditioning
- Civil engineering work
It does not typically cover architecture or surveying, carpet fitting (when not part of construction), scaffolding hire only, or the manufacture of materials.
Who Needs to Register for CIS?
Contractors must register for CIS if they pay subcontractors for construction work, or if their business spends more than £1 million per year on construction in any 3 consecutive years (even if construction isn't their core activity — this catches property developers and large businesses that commission building work).
Subcontractors should register for CIS too, even though it's technically optional. If you're not registered, the contractor must deduct tax at the higher unverified rate of 30% instead of the standard 20%. Registering is free and can save you a significant amount of cash flow every month.
CIS Deduction Rates
There are three possible deduction rates under CIS:
| Subcontractor Status | Deduction Rate | |---|---| | Not registered with CIS | 30% | | Registered — standard | 20% | | Gross payment status | 0% |
Gross Payment Status
If a subcontractor has a strong compliance history and meets HMRC's turnover thresholds, they can apply for gross payment status. This means contractors pay them in full with no deduction, and the subcontractor settles their own tax bill via Self Assessment at year end.
To qualify, subcontractors generally need:
- A net construction turnover of at least £30,000 per year (sole trader), or £30,000 per director/£100,000 for the business (companies)
- An up-to-date tax compliance record with no serious HMRC defaults
Gross payment status is reviewed annually and can be withdrawn if compliance slips.
What CIS Deductions Apply To
CIS deductions are calculated on the labour element only of a payment. Materials costs are excluded.
So if a subcontractor invoices £10,000 — comprising £7,000 labour and £3,000 materials — CIS at 20% applies only to the £7,000 labour element:
- Deduction: £7,000 × 20% = £1,400
- Contractor pays subcontractor: £10,000 − £1,400 = £8,600
- Contractor pays HMRC: £1,400
This distinction is important. Subcontractors who invoice a lump sum without itemising materials may end up having CIS applied to their entire invoice — costing them more than necessary.
Monthly CIS Returns: What Contractors Must Do
Every contractor must file a CIS monthly return with HMRC, even in months where no payments were made (you file a nil return). The deadline is the 19th of each month for the previous tax month (which runs 6th to 5th).
Each return must include:
- The names, UTR numbers, and verification numbers of all subcontractors paid
- Gross amounts paid
- Materials costs (excluded from deductions)
- The CIS deduction made
Failure to file on time results in automatic penalties:
- 1 day late: £100
- 2 months late: £200
- 6 months late: £300 or 5% of deductions (whichever is higher)
- 12 months late: A further £300 or 5%, plus potentially more if HMRC suspects deliberate non-compliance
Late payment of CIS deductions also incurs interest charges, so timing matters.
Verifying Subcontractors
Before making a payment, contractors must verify each new subcontractor with HMRC. This can be done online, via the HMRC CIS online service, or through most accounting software.
Verification confirms the subcontractor's status and the correct deduction rate to apply. You must keep a record of the verification reference number — HMRC can ask for this if they query a return.
If HMRC cannot verify a subcontractor (for example, they're not registered), you must apply the 30% unregistered rate.
How Subcontractors Reclaim Overpaid CIS
CIS deductions are not a final tax bill — they're advance payments. Subcontractors reclaim overpaid amounts through their annual tax return:
- Sole traders: Via Self Assessment. CIS deductions suffered reduce your final tax liability; any overpayment is refunded after filing.
- Limited companies: CIS deductions suffered offset the company's employer PAYE/NIC liabilities via the monthly payroll run. If deductions exceed the PAYE liability, the company can reclaim the balance from HMRC — sometimes monthly.
This is where good record-keeping pays off. Make sure every CIS statement you receive from contractors is filed — you'll need the deduction certificates to substantiate your claim.
CIS for Limited Companies: Both Contractor and Subcontractor
Many construction businesses operate as both a contractor (engaging other subcontractors) and a subcontractor themselves. This dual role means they must:
- Deduct CIS from subcontractors they pay and file monthly returns as a contractor
- Have CIS deducted from their own payments and track deductions suffered as a subcontractor
The offset mechanism allows limited companies to net these off against their PAYE liabilities, making cash flow management critical. If your bookkeeper isn't tracking both sides correctly, you could be overpaying HMRC for months before realising.
Common CIS Mistakes to Avoid
- Deducting from gross (including materials): Always separate labour and materials on subcontractor invoices.
- Forgetting to verify: Applying the wrong rate because you skipped verification can result in underpayment penalties.
- Missing the 19th deadline: Even one day late costs £100. Set a recurring reminder.
- Not filing nil returns: If you had no payments in a month, you still need to file. Silence is not a nil return.
- Ignoring gross payment status reviews: If your status changes, contractors may start deducting again without warning.
Calculate CIS Deductions Instantly
Working out CIS manually is straightforward once you understand the rules, but it's still one more thing to get right every month. Our free CIS calculator handles the maths for you — just enter the gross invoice amount, materials, and the applicable rate, and it gives you the exact deduction and net payment in seconds.
No sign-up required. Works for standard (20%), unregistered (30%), and gross payment status (0%) subcontractors.
CIS is one of those areas where small errors compound quickly — underpayments trigger penalties, overpayments tie up cash flow, and late returns stack up fines fast. If you're managing CIS manually in spreadsheets, it's worth looking at whether the right tools (and a good bookkeeper familiar with construction) could save you both time and money.
Got a CIS question we haven't covered? Drop us a message at support@drakonsystems.com.