Common CIS mistakes and how to avoid them

The 10 most common CIS mistakes

HMRC identifies these as the most frequent CIS compliance failures. Each one can result in penalties, additional tax liability, or both.

Mistake 1: Applying the wrong deduction rate

What goes wrong

You pay a subcontractor without first verifying their CIS status with HMRC. You assume they are registered and deduct 20%, but they are actually unregistered (requiring 30%) or have gross payment status (requiring 0%).

The consequences

• Deducted too little: You may have to pay the shortfall to HMRC yourself
• Deducted too much: Your subcontractor receives less than entitled and may dispute the payment
• Relationship damage: Subcontractors lose trust if their payments are wrong

How to prevent it

• Always verify first: Use the CIS online service to verify every new subcontractor before making any payment
• Record the verification number: HMRC gives you a verification number - keep this with your payment records
• Check the response carefully: HMRC tells you the exact rate to apply - do not assume
• Set up a checklist: Before processing any subcontractor invoice, tick off "verification confirmed"

Mistake 2: Deducting CIS from materials costs

What goes wrong

You apply the CIS deduction to the entire invoice amount, including materials the subcontractor supplied. CIS deductions should only apply to the labour element - materials are excluded.

The consequences

• Over-deduction: Subcontractor receives less than they should
• Disputes: Subcontractor challenges the payment, damaging your working relationship
• Admin burden: You may need to adjust returns and issue corrected statements

How to prevent it

Practical tips for materials

• Require itemised invoices: Ask subcontractors to show materials and labour separately on every invoice
• Request evidence: For significant materials claims, ask for supplier receipts or invoices
• Make reasonable estimates: If no evidence available, estimate materials based on the type of work
• Document your decisions: Note how you calculated the materials deduction for each payment

Mistake 3: Failing to re-verify returning subcontractors

What goes wrong

A subcontractor you have used before comes back after a gap. You assume their status is unchanged and apply the same rate as last time. But their registration status may have changed, or verification is required because too much time has passed.

The rules

You must re-verify a subcontractor if:

• You have not included them on a CIS return in the current tax year, AND
• You have not included them on a CIS return in the previous two tax years

In practical terms: if it has been more than 2 tax years since you last paid them, re-verify.

How to prevent it

• Keep a subcontractor register: Record the last payment date and tax year for each subcontractor
• Check before paying: Before processing payment to a returning subcontractor, check when you last paid them
• When in doubt, verify: If you are unsure, re-verify - there is no penalty for verifying too often
• Set calendar reminders: If you know a subcontractor will return, note when re-verification becomes necessary

Mistake 4: Missing the monthly return deadline

What goes wrong

The CIS return deadline is the 19th of each month for the previous tax month (which runs 6th to 5th). Missing this deadline, even by one day, triggers an automatic penalty.

The consequences

How to prevent it

• Set recurring calendar reminders: Create reminders for the 10th, 15th, and 18th of each month
• Use payroll software: Most payroll software can file CIS returns automatically or remind you
• Build in buffer time: Aim to file by the 15th, not the 19th - this gives you time to fix problems
• Appoint a backup: If you are away, who files the return? Make sure someone else knows how
• Check submission confirmation: Always check you receive HMRC's acknowledgement

Mistake 5: Using incorrect UTR or NI numbers

What goes wrong

You enter the wrong Unique Taxpayer Reference (UTR) or National Insurance (NI) number when verifying a subcontractor. HMRC cannot match the details, so verification fails or returns the wrong status.

The consequences

• Verification failure: HMRC returns "unmatched" status, requiring 30% deduction
• Wrong rate applied: You may deduct at the wrong rate if details partially match a different person
• Subcontractor problems: Deductions may not be credited to the correct person's tax record

How to prevent it

• Get details in writing: Ask subcontractors to provide their UTR and NI number in writing, not verbally
• Check UTR format: UTRs are 10 digits - no letters, no spaces when entering
• Verify NI format: NI numbers are 2 letters, 6 digits, 1 letter (e.g., AB123456C)
• Double-check entry: Before submitting verification, read back the numbers carefully
• Keep copies: Keep a copy of the document showing the subcontractor's details

Mistake 6: Not issuing Payment and Deduction Statements

What goes wrong

You make the deductions and pay HMRC, but forget to give the subcontractor a written statement showing what was deducted. Subcontractors need these statements to reclaim their deductions.

The consequences

• Subcontractor cannot reclaim: Without statements, they cannot prove deductions were made
• Disputes: Subcontractors may challenge payments or refuse future work
• HMRC interest: Failure to issue statements can be flagged in compliance checks

How to prevent it

• Create a template: Set up a standard Payment and Deduction Statement template
• Issue within 14 days: You must issue statements within 14 days of the end of each tax month
• Include all required details: Your employer PAYE reference, tax month, subcontractor name and UTR, gross amount, materials, and deduction amount
• Keep copies: Retain copies of all statements issued
• Automate if possible: Most payroll software can generate these automatically

Mistake 7: Not submitting nil returns

What goes wrong

You have no subcontractor payments to report for a month, so you do not submit a return. But if you are registered as a CIS contractor, you must still submit a nil return - and penalties apply for missing returns even when there is nothing to report.

The current rules

Since 6 April 2026, registered contractors must file a monthly return - including a nil return - for every tax month, unless they notified HMRC in advance (at least 14 days before the start of the tax month) that no subcontractor payments would be made (SI 2026/289). An inactivity request, covering up to 6 months, counts as that advance notification. HMRC has stated penalties will only be issued where contractors have neither submitted an inactivity request nor filed a return.

How to prevent it

• Check your contractor status: If you are registered as a CIS contractor, you must file even with no payments (or notify HMRC in advance)
• Request inactivity in advance: If you will not have subcontractor payments for a period, ask HMRC for inactivity status (up to 6 months) at least 14 days before the tax month starts
• File the nil return otherwise: If you have not notified HMRC in advance, submit a nil return by the 19th
• Set reminders anyway: Even if inactive, check each month whether you need to file

Mistake 8: Missing the electronic payment deadline

What goes wrong

You file the return on time but miss the deadline to actually pay the deductions to HMRC. The payment deadline is different from the filing deadline.

The deadlines

• Filing deadline: 19th of the month following the tax month
• Electronic payment deadline: 22nd of the month (if paying electronically)
• Postal payment deadline: 19th of the month (if paying by cheque - not recommended)

How to prevent it

• Pay when you file: Set up a process to make the payment immediately after filing the return
• Use Direct Debit: HMRC can collect CIS automatically - set this up to avoid missed payments
• Check your bank's processing time: Some banks take 2-3 days to process BACS payments - allow for this
• Set separate reminders: Do not assume filing = paying - remind yourself for both

Mistake 9: Not keeping records for 3 years

What goes wrong

You dispose of CIS records too early, then HMRC asks to see them during a compliance check. Without records, you cannot prove you made correct deductions.

What you must keep

CIS records must be retained for at least 3 years after the end of the tax year they relate to. This includes:

• Subcontractor invoices showing labour and materials breakdown
• Evidence of materials costs (receipts, supplier invoices)
• CIS deduction calculations for each payment
• Verification numbers from HMRC
• Copies of Payment and Deduction Statements issued
• Monthly CIS return submissions
• Payment records to HMRC

How to prevent it

• Set a retention policy: Do not destroy any CIS records until at least 3 years after the tax year ends
• Store digitally: Scan paper records and store securely - easier to keep and search
• Organise by tax year: Keep records organised so you can find them easily if HMRC asks
• Check before destroying: Before any document disposal, check if CIS records are included

Mistake 10: Treating employees as subcontractors

What goes wrong

You engage someone under CIS when their working arrangements actually make them an employee. CIS registration does not determine employment status - the nature of the relationship does.

The consequences

• Penalty up to 3,000 GBP: For incorrectly declaring someone as a subcontractor
• Full PAYE/NI liability: You may owe all the PAYE and National Insurance you should have deducted
• Interest on arrears: From when the tax should have been paid
• Employee claims: The worker may claim employment rights (holiday pay, sick pay, unfair dismissal)

Warning signs someone may be an employee

• You control when, where, and how they work
• They cannot send a substitute to do the work
• They work exclusively for you
• You provide their tools and equipment
• They are paid a regular amount regardless of work done
• They have no financial risk in the arrangement

How to prevent it

• Use HMRC's CEST tool: Check Employment Status for Tax - this gives HMRC's view on status
• Review each engagement: Consider the substance of the relationship, not just the paperwork
• Document the arrangement: Keep records showing why you concluded they are self-employed
• Get professional advice: For borderline cases, consult an employment law specialist

What to do if you have made a mistake

If you discover you have made one of these mistakes, act quickly:

1. Assess the impact: Work out what the error is and how much is involved
2. Correct future payments: Make sure you do not repeat the mistake going forward
3. Consider voluntary disclosure: Telling HMRC before they find out usually results in lower penalties
4. Adjust returns: Some errors can be corrected on a later return
5. Get professional help: For significant errors, consult an accountant who understands CIS

Key point: Voluntary disclosure of errors typically attracts lower penalties than if HMRC discovers the error during a compliance check.

Setting up systems to prevent mistakes

Most CIS mistakes can be prevented with proper systems:

Monthly checklist

• Week 1 (6th-12th): Verify any new subcontractors, collect invoices
• Week 2 (13th-15th): Calculate deductions, prepare return
• Week 3 (16th-19th): Submit return, make payment
• Week 3-4: Issue Payment and Deduction Statements

Document checklist for each subcontractor

• Verification confirmation from HMRC
• Written record of UTR and NI number
• Itemised invoices with materials shown separately
• Materials evidence (where significant amounts claimed)
• Copy of Payment and Deduction Statement issued

Software to consider

• Payroll software with CIS module (automates returns and statements)
• HMRC CIS online service (free for verification and returns)
• Accounting software that tracks subcontractor payments

Next steps

If you have identified mistakes in your current CIS processes:

1. Review recent returns: Check for any of the 10 common mistakes
2. Set up prevention systems: Use the checklists above to improve your processes
3. Consider professional review: An accountant can audit your CIS compliance
4. Train your team: Make sure anyone involved in CIS understands these common pitfalls

Related guides

• Calculate CIS deductions - step-by-step calculation process with worked examples
• Verify a subcontractor's CIS status - how to verify and what the responses mean
• Submit your CIS monthly return - filing process and deadlines
• Appeal a CIS penalty - what counts as a reasonable excuse
• CIS for small builders - simplified guide for contractors with few subcontractors
• Correct CIS return errors - how to fix mistakes on returns you have already submitted