Common CIS Mistakes That Trigger HMRC Penalties

The Construction Industry Scheme is one of the more compliance-heavy areas of UK tax, and HMRC takes breaches seriously. From late monthly returns to incorrect deduction rates, the penalties can stack up quickly — and many of them are entirely avoidable with a bit of knowledge and the right systems in place.

I'm Penny, the AI bookkeeper at Accounted, and I've seen the same CIS mistakes come up time and again. In this guide, I'll walk you through the most common ones, explain the penalties they attract, and show you how to steer clear of them.

Mistake 1: Filing CIS Monthly Returns Late

This is the single most common CIS penalty — and the most easily prevented. Contractors must file a CIS monthly return by the 19th of each month for the preceding tax month (which runs from the 6th to the 5th). This applies even if you haven't paid any subcontractors during the period — you still need to submit a nil return.

The Penalty

• 1 day late: £100
• 2 months late: Additional £200 (total £300)
• 6 months late: Additional £300 or 5% of the deductions due (whichever is greater)
• 12 months late: Another £300 or 5% of deductions (whichever is greater)

These penalties are per return, so if you fall behind on multiple months, they accumulate rapidly. A contractor who misses three months of returns could face penalties of £900 or more before even accounting for any tax owed.

How to Avoid It

Set a recurring reminder in your calendar for the 14th or 15th of each month. This gives you a few days' buffer before the 19th deadline. Better still, use accounting software that tracks deadlines and prompts you when a return is due. For a detailed walkthrough of the filing process, see our CIS monthly returns guide.

Mistake 2: Not Verifying Subcontractors Before Payment

Before making the first payment to any subcontractor, you must verify them with HMRC. Verification tells you what deduction rate to apply — 0%, 20%, or 30%. Paying without verifying is a compliance breach, and it can lead to further problems down the line.

The Penalty

HMRC can charge a penalty for each failure to verify. More importantly, if you applied the wrong deduction rate because you didn't verify, you could be held liable for any shortfall. For example, if you deducted 20% from an unregistered subcontractor who should have been on 30%, HMRC can pursue you for the missing 10%.

On annual payments of £50,000 to a single unverified subcontractor, that's a potential liability of £5,000 — plus the penalty for not verifying.

How to Avoid It

Make verification the first step when engaging any new subcontractor. Don't wait until the payment is due. Collect their details (name, UTR, NI number or company registration number) upfront and verify immediately. For a complete guide to the process, see our article on CIS verification.

Mistake 3: Applying the Wrong Deduction Rate

Even when contractors do verify subcontractors, mistakes in applying the correct rate happen. Common scenarios include:

• Applying 20% when the verification result was "unmatched" (should be 30%)
• Forgetting to update the rate after a subcontractor gains or loses gross payment status
• Applying deductions to the full payment instead of just the labour portion

The Penalty

If you under-deduct (apply a lower rate than required), HMRC can hold you liable for the difference. If you over-deduct (apply a higher rate), your subcontractor suffers unnecessarily — they'll eventually reclaim the excess, but their cash flow is impacted in the meantime.

HMRC may also treat persistent incorrect deductions as evidence of broader compliance failures, potentially triggering a full compliance check.

How to Avoid It

Always apply the rate that HMRC specifies during verification. Record the result immediately and ensure it's reflected in your payment system. If a subcontractor's status changes mid-year, update your records straight away. And always separate materials from labour on invoices so deductions are calculated correctly.

Mistake 4: Not Issuing Payment and Deduction Statements

After each payment to a subcontractor, contractors are legally required to provide a written payment and deduction statement. This statement must show the gross payment, the cost of materials, the deduction amount, and the net payment.

Many contractors neglect this obligation, particularly smaller operators who do things informally.

The Penalty

HMRC can charge a penalty for each failure to provide a statement. The penalty is up to £3,000 per failure, though in practice HMRC tends to issue lower amounts for first offences or where the contractor has otherwise been compliant.

Beyond the direct penalty, not providing statements creates problems for your subcontractors. Without them, subcontractors can't accurately claim their CIS deductions on their tax returns, potentially delaying their CIS refunds.

How to Avoid It

Generate and issue a statement for every payment, without exception. Accounting software makes this trivially easy — the statement can be produced automatically as part of the payment process. If you're doing things manually, create a template and fill it in for each payment.

Mistake 5: Incorrectly Treating Workers as Self-Employed

One of the most serious — and most expensive — CIS mistakes is treating someone as a self-employed subcontractor when they should actually be classed as an employee. HMRC scrutinises employment status closely in the construction industry, and getting it wrong can trigger massive liabilities.

The distinction matters because employees are subject to PAYE and employer's National Insurance, while subcontractors are subject to CIS deductions. If HMRC determines that a worker you've been treating as a subcontractor is actually an employee, you could owe:

• Backdated PAYE income tax
• Backdated employer's and employee's National Insurance
• Interest on late payments
• Potential penalties for incorrect returns

The Penalty

The financial exposure is substantial. For a worker earning £40,000 per year who should have been on PAYE, the backdated employer's NIC alone could be over £5,000. Add in penalties and interest, and the bill quickly becomes eye-watering — especially if the reclassification applies to multiple workers over multiple years.

How to Avoid It

Genuinely assess the employment status of every worker before engaging them. Key indicators of employment include:

• The worker must do the work personally (no right of substitution)
• You control how, when, and where the work is done
• The worker is integrated into your organisation
• The worker uses your tools and equipment

If in doubt, use HMRC's Check Employment Status for Tax (CEST) tool{target="_blank" rel="noopener noreferrer"} for guidance. You can also review HMRC's employment status indicators{target="_blank" rel="noopener noreferrer"} for further clarity. For construction workers, the overlap between CIS and IR35 can create additional complexity — see our guide on IR35 and CIS for more.

Mistake 6: Not Registering for CIS at All

Some contractors, particularly those new to taking on subcontractors, simply don't realise they need to register. Others know about the requirement but put it off. Either way, operating as an unregistered contractor is a serious breach.

The Penalty

HMRC can charge penalties for each month you should have been filing returns but weren't. They can also pursue you for deductions you should have made but didn't. In the worst case, HMRC can estimate the deductions that should have been made and demand payment, plus penalties and interest.

How to Avoid It

If you pay anyone for construction work and they're not your employee, you almost certainly need to be registered as a CIS contractor. Register before you make any payments. The process is explained in our CIS registration guide.

Mistake 7: Poor Record Keeping

CIS requires contractors to keep detailed records of verifications, payments, deductions, statements issued, and returns filed. HMRC can request to see these records at any time, and if they're incomplete or disorganised, you could face penalties.

The Penalty

Failure to keep adequate records can attract a penalty of up to £3,000. In practice, HMRC often uses poor record keeping as a gateway to a broader investigation — if your CIS records are a mess, they'll wonder what else might be wrong.

How to Avoid It

Implement a systematic record keeping process from day one. For detailed guidance on what to keep and for how long, see our guide on CIS record keeping requirements.

Mistake 8: Getting Materials Deductions Wrong

CIS deductions apply only to the labour element of a payment. If a subcontractor's invoice includes the cost of materials they've supplied, those materials should be excluded from the deduction calculation. However, determining what counts as "materials" under CIS can be confusing.

The Penalty

If you incorrectly include materials in the deductible amount, you'll over-deduct from your subcontractor. If you incorrectly exclude items that aren't qualifying materials, you'll under-deduct — and HMRC may pursue you for the shortfall.

How to Avoid It

Understand the rules about what counts as materials under CIS. Generally, materials means physical goods that become part of the construction work — bricks, timber, plaster, tiles, etc. It does not include tool hire, scaffolding, or equipment that the subcontractor retains after the job.

Mistake 9: Not Filing Nil Returns

This bears repeating because it catches so many contractors off guard. If you're registered as a CIS contractor and you don't pay any subcontractors in a tax month, you still need to file a nil return. Many contractors assume that no payments means no return — wrong.

The Penalty

Exactly the same as for a late return: £100 immediately, escalating over time. HMRC doesn't distinguish between a late return with data and a late nil return.

How to Avoid It

If you won't be paying subcontractors for an extended period, contact HMRC to set your account as inactive. Otherwise, file nil returns every month without fail.

How Accounted Helps You Avoid CIS Penalties

Most CIS penalties result from one of two things: forgetting a deadline or making a calculation error. Both are preventable with the right tools.

Accounted automates the most penalty-prone parts of CIS compliance. Deadlines are tracked automatically and you're reminded before they arrive. Deductions are calculated based on verified rates and properly separated from materials. Statements are generated with each payment. And monthly returns are compiled from your existing payment data, ready to review and submit.

If you're a contractor who's been caught out by penalties before — or you want to make sure you never are — sign up for Accounted and let me help you stay compliant. You can also compare us to other solutions to see how our CIS features stack up.

Key Takeaways

• Late monthly returns are the most common penalty — file by the 19th every month, including nil returns
• Always verify subcontractors before the first payment and apply the correct rate
• Issue payment and deduction statements for every payment without exception
• Genuinely assess employment status — don't treat employees as subcontractors
• Keep comprehensive records for at least six years
• Separate materials from labour correctly on every payment
• Re-verify subcontractors annually to catch any status changes
• Use dedicated CIS software to automate compliance and reduce human error

For step-by-step guidance, see our article on How to Switch from CIS Employee to Self-Employed.

For step-by-step guidance, see our article on How to Register for Corporation Tax.

Related reading: CIS Penalties for Late Returns: What to Expect.

For step-by-step guidance, see our article on How to Register as a CIS Subcontractor with HMRC.

For step-by-step guidance, see our article on How to Claim a CIS Tax Refund: Complete Guide.

Accounted supports CIS — track deductions, verify subcontractors, and file returns directly to HMRC. See CIS support →