CIS Returns Explained: A Contractor's Guide

What Are CIS Returns?

CIS returns are the monthly reports that contractors in the construction industry must submit to HMRC under the Construction Industry Scheme. These returns detail all payments made to subcontractors during the tax month, the deductions applied, and the verification status of each subcontractor. Filing accurate CIS returns on time is a legal requirement — failure to do so results in automatic penalties and potential HMRC investigations. Whether you are a sole trader contractor, a partnership, or a limited company operating in the construction sector, understanding CIS returns is essential to staying compliant and avoiding costly mistakes.

Who Needs to File CIS Returns?

You need to file CIS returns if you are a "contractor" under the CIS rules. This includes:

• Construction companies that pay subcontractors for construction work
• Main contractors who engage subcontractors on building projects
• Property developers who use subcontractors
• Any business (including non-construction businesses) that spends more than £3 million on construction work in a rolling 12-month period — this includes organisations like local authorities, housing associations, and large retailers undertaking refurbishment or building projects

If you only work as a subcontractor and do not engage other subcontractors yourself, you do not need to file CIS returns. However, you should be registered with HMRC for CIS to benefit from the standard 20% deduction rate rather than the 30% rate applied to unregistered subcontractors.

The CIS Tax Month and Filing Deadlines

The CIS tax month runs from the 6th of one month to the 5th of the next (the same as the PAYE tax month). Your CIS return for each tax month must be submitted to HMRC by the 19th of the following month. For example, the return covering the period 6 June to 5 July 2026 must be filed by 19 July 2026. CIS deductions must be paid to HMRC by the 22nd of the month if paying electronically (19th if paying by cheque).

If you have not made any payments to subcontractors during a particular tax month, you must still submit a nil return. Many contractors are caught out by this requirement — assuming that no activity means no filing obligation. HMRC will issue a penalty for each month a return is late, regardless of whether any payments were made.

What Information Goes on a CIS Return?

Each CIS return must include the following information for every subcontractor you have paid during the tax month:

• The subcontractor's name and Unique Taxpayer Reference (UTR)
• The subcontractor's verification reference number
• The gross amount paid (before deductions)
• The cost of materials included in the payment (these are excluded from deductions)
• The amount of CIS deductions applied
• The deduction rate used (0%, 20%, or 30%)

You must also include a declaration confirming that the employment status of each worker has been considered and that none of the subcontractors listed should actually be treated as employees. This is an important compliance point — HMRC uses CIS returns to identify potential employment status issues.

Verifying Your Subcontractors

Before making a first payment to a new subcontractor (or after a break in payments), you must verify them with HMRC. Verification confirms their CIS registration status and tells you which deduction rate to apply. You can verify subcontractors online through HMRC's CIS system or through CIS-compatible software.

To verify an individual subcontractor, you need their name, UTR, and National Insurance number. For a limited company subcontractor, you need the company name, UTR, and company registration number. HMRC will confirm one of three outcomes:

• Registered — gross payment status: Deduct 0% (pay the full amount)
• Registered — net payment status: Deduct 20% from the labour element
• Not registered: Deduct 30% from the labour element

Keep a record of the verification reference number and the date of verification for each subcontractor. You will need these for your CIS returns.

Calculating CIS Deductions

CIS deductions apply only to the labour element of a payment. If a subcontractor's invoice includes the cost of materials they have supplied, these materials are excluded from the deduction calculation. However, the subcontractor must provide evidence of the material costs (such as supplier invoices). If they cannot substantiate the materials element, you must apply deductions to the full amount.

Here is a worked example:

• A subcontractor invoices you for £5,000, comprising £3,500 for labour and £1,500 for materials
• The subcontractor is registered at the standard rate (20%)
• CIS deduction: 20% of £3,500 = £700
• Amount you pay the subcontractor: £5,000 - £700 = £4,300
• Amount you pay to HMRC: £700

VAT, if applicable, is excluded from the deduction calculation. If the subcontractor is VAT-registered and the £5,000 is plus VAT, you deduct 20% from the £3,500 labour element only, and pay the VAT in full to the subcontractor.

Penalties for Late or Incorrect Returns

HMRC imposes automatic penalties for late CIS returns:

• 1 day late: £100 penalty
• 2 months late: Additional £200 penalty
• 6 months late: Additional penalty of the greater of 5% of the CIS deductions on the return or £300
• 12 months late: Additional penalty of the greater of 5% of the CIS deductions or £300 (can be up to 100% of the deductions in serious cases)

These penalties apply to each return individually, so if you miss several months, the penalties accumulate quickly. In addition to late-filing penalties, HMRC can charge penalties for inaccurate returns, failure to verify subcontractors, and failure to make CIS deductions when required. Interest is also charged on late payments of CIS deductions.

CIS Deductions and Your Own Tax Position

If you are a subcontractor as well as a contractor (which is common in the construction industry), the CIS deductions that other contractors take from your payments can be offset against your own tax liabilities. For sole traders, CIS deductions suffered are credited against your Income Tax and NIC liability on your Self Assessment return. For limited companies, CIS deductions suffered can be offset against your PAYE, NIC, and CIS liabilities through the monthly Employer Payment Summary (EPS). If the deductions exceed your liabilities, you can claim a refund from HMRC.

Keeping accurate records of all CIS deductions — both those you make from subcontractors and those suffered on your own income — is essential for reconciling your position at year-end and ensuring you do not overpay tax.

Common Mistakes to Avoid

• Not submitting nil returns: Even if you made no payments to subcontractors in a month, you must file a nil return. Set up a reminder so you do not forget.
• Failing to verify new subcontractors: Always verify before making the first payment. Applying the wrong deduction rate because you did not verify can lead to penalties.
• Treating employees as subcontractors: If HMRC determines that a worker is actually an employee, you could face backdated PAYE, NIC, penalties, and interest. Consider the employment status of each worker carefully.
• Not deducting from materials incorrectly: Only genuine, evidenced material costs can be excluded from deductions. Inflated or unsupported materials claims will attract HMRC scrutiny.
• Poor record-keeping: Keep all payment records, invoices, verification details, and CIS statements for at least six years.

How We Can Help

CIS compliance is one of our specialist areas at London Accountant. Our CIS returns service covers everything from initial contractor and subcontractor registration, monthly verification of all your subcontractors, preparation and submission of your CIS returns by the deadline each month, calculation and payment of CIS deductions, year-end reconciliation and refund claims, to advice on employment status and Gross Payment Status applications. We work with contractors and subcontractors across London, from one-person operations to large construction firms. Our fixed-fee packages mean you know exactly what you are paying, and our experienced team ensures your CIS obligations are always met on time. Get in touch for a free initial consultation.