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

Current or previous tax year: Amend through CIS online service in your Government Gateway account
Earlier tax years: Contact HMRC CIS helpline on 0300 200 3210
Under-deducted?: Pay the shortfall to HMRC - you are liable even if you cannot recover it from the subcontractor, unless HMRC accepts you took reasonable care and the failure was an innocent error
Over-deducted?: Pay the difference to subcontractor; HMRC credits your account

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

Return deadline: 19th of the month following the tax month (e.g., 19 May for April payments)
Payment deadline (electronic): 22nd of the month following the tax month
Payment deadline (post): 19th of the month following the tax month
Nil return deadline: Same as regular return - even if you made no payments, file by the 19th

CIS late filing penalties

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

HMRC charges a penalty if you file a CIS monthly return late. Under Schedule 55 to the Finance Act 2009 the penalties build up as the return stays overdue. You are charged £100 when the return is late, and a further £200 once it is 2 months late, so £300 in total by then. The £200 is added to the £100, not charged instead of it. Penalties at 6 and 12 months, and any further penalty for returns later than that, are charged on top of those.

Late filing: £100
2 months late: A further £200 (£300 in total)
6 months late: £300 or 5% of CIS deductions, whichever is higher
12 months late: £300 or 5% of CIS deductions, whichever is higher
Returns later than this: Up to £3,000 or 100% of CIS deductions, whichever is higher
Late payment: Interest plus late payment penalties
Failure to produce CIS records: Up to £3,000
Pay or appeal a penalty: Within 30 days of the date on the penalty notice

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

Submit the return immediately

Do not wait until the next deadline. File now to stop penalties accumulating.
Pay any deductions owed

If you have not already paid, pay by electronic transfer to reach HMRC's account as quickly as possible.
Wait for the penalty notice

HMRC will issue a penalty notice after processing. You can appeal once you receive it.
Consider whether you have a reasonable excuse

If something unexpected prevented you from filing, you may be able to appeal.

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:

These are common CIS compliance errors by contractors. Each can lead to penalties, additional tax liability, or both.

Wrong deduction rate: Paying a subcontractor without first verifying them with HMRC, so the 20%, 30% or 0% rate is wrong
Materials errors: Deducting CIS from materials the subcontractor paid for directly; only the labour element is subject to deduction
Missed re-verification: Not verifying again a subcontractor not included on a return in the current or last 2 tax years
Late monthly return: Missing the 19th deadline, which brings a £100 penalty that rises the later the return is
Wrong subcontractor details: Using a UTR or National Insurance number that does not exactly match HMRC records, so verification fails
Missing statements: Not giving the subcontractor a payment and deduction statement within 14 days of the end of the tax month
No return when nothing paid: Not filing a nil return or making an inactivity request in a month with no payments
Late payment to HMRC: Paying deductions after the 22nd (electronic) or 19th (post) of the month following the tax month
Short record keeping: Not keeping CIS records for at least 3 years after the end of the tax year
Employees treated as subcontractors: Wrong employment status on the return can bring a penalty of up to £3,000

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, unless HMRC waives it because you took reasonable care and the failure was an innocent error
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

Appeal a tax penalty online (opens in a new tab)

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

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.

Single mistake, current tax year: Self-correct using CIS online service
Multiple mistakes, current year: Correct systematically; consider accountant review
Mistakes across multiple tax years: Contact HMRC CIS helpline; professional advice recommended
Pattern of errors (same mistake repeated): Fix root cause; may trigger compliance check
Discovered during HMRC compliance check: Cooperate fully; voluntary disclosure reduces penalties

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:

CIS helpline: 0300 200 3210
Opening hours: Monday to Friday, 8am to 6pm
Payment Support Service: 0300 200 3835 (if you cannot pay penalties)
Written queries: PT Operations, HMRC, BX9 1BX

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

Contact HMRC about CIS (opens in a new tab)

Preventing future CIS mistakes

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

Set calendar reminders

Add reminders for the 15th (prepare return), 19th (submit return), and 22nd (payment deadline) each month.
Verify before first payment

Always verify new subcontractors with HMRC before paying them. Never assume their registration status.
Use CIS software

Commercial CIS software can automate verification, calculate deductions, and submit returns correctly.
Keep materials evidence

If deducting materials from the payment, keep invoices or quotes as evidence of actual cost.
File nil returns or request inactivity

If you have no subcontractor payments in a month, file a nil return or request an inactivity period.
Review verification status regularly

Re-verify subcontractors you have not paid in the current or previous 2 tax years.

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

CIS troubleshooting - identify and fix your mistake

Error on a submitted CIS return

How to correct CIS return errors

Missed a CIS deadline

What to do if you've missed a CIS deadline

Submit the return immediately

Pay any deductions owed

Wait for the penalty notice

Consider whether you have a reasonable excuse

Wrong deduction rate applied

Common CIS mistakes and how to avoid them

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

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

Received a penalty notice

Appealing a CIS penalty

If you do not have a reasonable excuse

Multiple or historical mistakes

When to get professional help

HMRC CIS contact details

Preventing future CIS mistakes

Set calendar reminders

Verify before first payment

Use CIS software

Keep materials evidence

File nil returns or request inactivity

Review verification status regularly

Official guidance

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

CIS troubleshooting - identify and fix your mistake

Error on a submitted CIS return

How to correct CIS return errors

Missed a CIS deadline

What to do if you've missed a CIS deadline

Submit the return immediately

Pay any deductions owed

Wait for the penalty notice

Consider whether you have a reasonable excuse

Wrong deduction rate applied

Common CIS mistakes and how to avoid them

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

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

Received a penalty notice

Appealing a CIS penalty

If you do not have a reasonable excuse

Multiple or historical mistakes

When to get professional help

HMRC CIS contact details

Preventing future CIS mistakes

Set calendar reminders

Verify before first payment

Use CIS software

Keep materials evidence

File nil returns or request inactivity

Review verification status regularly

Official guidance

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Fix CIS payment errors

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