CIS troubleshooting - identify and fix your mistake

CIS mistakes happen to even the most careful contractors. The key is identifying the problem quickly and taking the right corrective action. Acting promptly can significantly reduce penalties and prevent small errors becoming expensive ones.

What type of mistake have you made?

Error on a submitted CIS return - wrong figures, incorrect UTR, missing subcontractor
Missed a CIS deadline - return not filed by 19th, or payment not made by 22nd
Wrong deduction rate applied - deducted 30% instead of 20%, or vice versa
Received a penalty notice - want to appeal or understand your options
Multiple or historical mistakes - problems spanning several months or tax years

Error on a submitted CIS return

If you have discovered an error on a CIS return you have already filed, the correction process depends on when the error occurred and what type of error it is.

How to correct CIS return errors

Step-by-step guide to amending CIS returns - covers current year, previous year, and historical corrections

Missed a CIS deadline

CIS has two key monthly deadlines. Missing either can result in penalties, but acting quickly limits the damage.

What to do if you've missed a CIS deadline

Immediate steps to take when you have missed the 19th filing deadline or 22nd payment deadline

CIS late filing penalties

Penalties escalate the longer you leave it. File as soon as possible to minimise the penalty:

HMRC charges penalties for late CIS monthly returns. Penalties escalate the longer the return is overdue, with additional percentage-based penalties applied at 6 and 12 months.

Late filing (1 day late): £100 fixed penalty
Late filing (2 months late): Additional £200 (total £300)
Late filing (6 months late): Additional £300 or 5% of CIS deducted, whichever is higher
Late filing (12 months late): Additional £300 or 5% of CIS deducted, whichever is higher
Maximum penalty (over 12 months): Up to £3,000 or 100% of CIS deducted, whichever is higher
Late payment: Interest plus PAYE late payment penalty regime applies
Incorrect employment status: Up to £3,000 for declaring someone as subcontractor when they should be employee
Failure to produce records: Up to £3,000 for not showing CIS records when requested by HMRC

Pay a CIS late filing penalty (opens in a new tab)

Wrong deduction rate applied

Applying the wrong CIS deduction rate is the most common contractor mistake. The consequences depend on whether you over-deducted or under-deducted.

Common CIS mistakes and how to avoid them

The top 10 CIS errors contractors make and how to prevent them

Common CIS mistakes

Review these common errors to understand what went wrong and how to prevent it happening again:

HMRC identifies these as the most frequent CIS compliance failures. Each mistake can trigger penalties, additional tax liability, or both. Understanding these common errors helps contractors avoid costly compliance failures.

Mistake 1: Wrong deduction rate applied: Not verifying subcontractor status before first payment - results in incorrect 20% or 30% deduction
Mistake 2: Materials treatment errors: Deducting CIS from materials cost - only labour element is subject to deduction
Mistake 3: Missed re-verification: Not re-verifying returning subcontractors - verification required if not paid in current or previous 2 tax years
Mistake 4: Late return submission: Missing the 19th of the month deadline - triggers £100 penalty escalating to £3,000+
Mistake 5: Incorrect UTR/NI numbers: Using wrong Unique Taxpayer Reference or National Insurance number - causes verification failure
Mistake 6: Not issuing Payment and Deduction Statements: Failing to give subcontractor written statement within 14 days of each payment
Mistake 7: Failing to file nil returns: Not submitting return when no subcontractor payments made - penalties still apply
Mistake 8: Missing electronic payment deadline: CIS deductions due to HMRC by 22nd (electronic) or 19th (post) of month following payment
Mistake 9: Not keeping records for 3 years: Records of all subcontractor payments and deductions must be retained for 3 years after tax year end
Mistake 10: Treating employees as subcontractors: Misclassifying employment status - PAYE and National Insurance shortfall, interest and any applicable penalties

CIS contractor responsibilities (opens in a new tab)

If you deducted too little (e.g., 20% instead of 30%)

You are liable to HMRC for the full amount that should have been deducted
Pay the shortfall to HMRC as soon as possible to minimise interest
Amend your CIS return to show the correct figures
You may attempt to recover the shortfall from the subcontractor, but this is your commercial risk

If you deducted too much (e.g., 30% instead of 20%)

Pay the subcontractor the difference immediately
Amend your CIS return to show the correct figures
HMRC will credit your account for the over-payment
The subcontractor does not need to wait for their tax return - you should pay them directly

Received a penalty notice

If HMRC has issued a CIS penalty, you have 30 days to appeal if you have a reasonable excuse. Even without an appeal, there may be options to reduce the impact.

Appealing a CIS penalty

How to appeal a CIS penalty and what counts as a reasonable excuse HMRC will accept

What HMRC accepts as a reasonable excuse

You may be able to appeal if something unexpected prevented you from meeting your CIS obligations:

A reasonable excuse is something unexpected or outside your control that prevented you from meeting a tax obligation. HMRC considers each case individually, but has published guidance on what is and is not typically accepted.

Accepted: Serious illness or hospitalisation: You or a close family member had a serious or life-threatening illness that prevented compliance
Accepted: Recent bereavement: Death of a close family member or partner shortly before the deadline
Accepted: Fire, flood or natural disaster: Your business premises or records were destroyed by fire, flood, or other disaster
Accepted: HMRC service failure: HMRC's online systems were unavailable when you tried to file, with evidence of attempts
Accepted: Postal delays or strikes: Significant postal disruption prevented paper return delivery (with proof of posting)
NOT accepted: Relied on agent/accountant: Responsibility for filing remains with contractor - agent failure is not a reasonable excuse
NOT accepted: Too busy: Pressure of work is not considered a reasonable excuse for late filing
NOT accepted: Didn't know about deadline: Lack of awareness of CIS obligations is not a reasonable excuse
NOT accepted: Computer problems (routine): Normal IT issues are foreseeable - only exceptional circumstances may qualify
NOT accepted: Insufficient funds: Inability to pay is not a reasonable excuse for late filing (may apply to payment only)

Key requirements for a successful appeal:

The excuse must be genuine and the cause of the failure
You must have acted as soon as reasonably possible once the excuse ended
You should have evidence to support your claim (medical certificates, fire reports, etc.)
The excuse must apply to the specific period of the penalty

How to appeal a CIS penalty

The appeals process has strict deadlines. Act quickly:

You can appeal a CIS penalty if you have a reasonable excuse for the failure, or if you believe the penalty has been incorrectly calculated. Appeals must be made within strict time limits.

Appeal deadline: 30 days from the date on the penalty notice
Late appeal: Possible if you have a reasonable excuse for missing the 30-day deadline
Method 1: Appeal online: Through your Government Gateway account - fastest method with immediate confirmation
Method 2: Appeal by post: Write to the address on your penalty notice with full details and evidence
Stage 1 - HMRC review: HMRC reviews your appeal and responds within 45 days
Stage 2 - Internal review (optional): Request a different HMRC officer reviews the decision
Stage 3 - Tax tribunal: Appeal to First-tier Tribunal if HMRC rejects your appeal
Tribunal deadline: 30 days from HMRC's final decision to appeal to tribunal
Penalty suspended during appeal: You do not need to pay the penalty while a valid appeal is ongoing

What to include in your appeal:

Your name and Unique Taxpayer Reference (UTR)
The penalty reference number from your notice
Why you believe you have a reasonable excuse
What steps you took to comply (or why you could not)
When the circumstances preventing compliance ended
Supporting evidence (medical certificates, fire reports, system screenshots)

Appeal a tax penalty (opens in a new tab)

Reasonable excuses for tax penalty appeals (opens in a new tab)

If you do not have a reasonable excuse

You cannot appeal based on circumstances within your control, but you can:

Pay promptly to stop interest accumulating
Request a payment plan if you cannot pay in full - call HMRC's Business Payment Support Service on 0300 200 3835
Check the penalty calculation - ensure HMRC has applied the correct figures and penalty rates
Learn from the mistake - set up calendar reminders and consider CIS software to prevent recurrence

Multiple or historical mistakes

If you have discovered problems spanning several months or tax years, the situation is more complex. Professional advice is often worthwhile for larger corrections.

When to get professional help

Consider engaging an accountant or CIS specialist if:

Errors span more than one tax year
Total under-deducted amounts exceed £1,000
You are facing an HMRC compliance check
Employment status questions are involved (IR35, deemed employment)
Multiple subcontractors are affected
Your gross payment status (GPS) may be at risk

HMRC CIS contact details

For issues you cannot resolve online or that require HMRC intervention:

Before you call: Have ready your Unique Taxpayer Reference (UTR), the tax periods affected, details of the error, and any penalty reference numbers.

Preventing future CIS mistakes

Most CIS errors are preventable with good processes. Implement these habits to avoid repeating mistakes:

I've made a CIS mistake - how to fix it