CIS monthly returns: a step-by-step guide for contractors

What is a CIS monthly return?

If you are a contractor under the Construction Industry Scheme (CIS), you must submit a monthly return to HMRC for every month in which you make payments to subcontractors. The return tells HMRC how much you paid each subcontractor and how much tax you deducted. This is separate from your own Self Assessment return and must be submitted whether or not you owe any tax.

For a recap of how CIS works overall, see our full CIS guide. This article focuses specifically on the monthly return process.

Who must submit a monthly return?

You must submit a CIS monthly return if you are a contractor, meaning you pay subcontractors for construction work. This applies to main contractors, sub-contractors who themselves use sub-sub-contractors, and businesses that spend more than £3 million on construction in a 12-month period (even if construction is not their primary business, such as property developers or housing associations).

If you have no subcontractors in a particular month, you can submit a nil return or contact HMRC to set up an inactivity period (up to 6 months) to avoid having to file each month. Do not simply ignore the return; HMRC will issue a penalty.

When must returns be submitted?

CIS monthly returns must be submitted by the 19th of each month for the previous tax month (which runs from the 6th to the 5th). So the return for the tax month ending 5 April is due by 19 April. The return for the month ending 5 May is due by 19 May, and so on.

Miss this deadline, and HMRC will issue an automatic penalty. The penalty structure in 2026 is as follows:

• Up to 1 month late: £100
• 2 months late: £200
• 6 months late: £300 or 5% of CIS deductions, whichever is higher
• 12 months late: further £300 or 5%, plus potential additional penalties

These penalties can add up quickly, particularly for small businesses. It's critical to stay on top of these deadlines and ensure all paperwork is in order well before the due date. Consider setting a reminder a week in advance of the deadline to give yourself ample time to address any issues that might arise.

How to submit a CIS monthly return

You can submit online through your HMRC online services account (the CIS section of the business tax account at gov.uk), or through compatible CIS or payroll software. The return requires:

• The contractor's UTR (Unique Taxpayer Reference)
• For each subcontractor: their name, UTR, National Insurance number (for sole traders), gross payment amount, deductions made, and net payment made
• The deduction rate applied (0%, 20%, or 30% depending on the subcontractor's registration status)

Before making any payment, you must verify each subcontractor with HMRC online or by phone. HMRC will confirm whether the subcontractor is registered and what deduction rate to apply. Keep records of all verifications. This could be a simple spreadsheet or a specific accounting software that tracks these details. Accurate record-keeping is not just a requirement but also a good business practice that can save time and prevent errors.

CIS deduction rates explained

There are three rates:

• 0% (gross payment status): The subcontractor has applied for and been granted gross payment status by HMRC. No deductions are made. To qualify, a subcontractor must have a good compliance record and meet turnover thresholds. Many subcontractors aim for this status as it improves cash flow.
• 20% (registered): The subcontractor is registered with HMRC under CIS. HMRC deducts 20% from the labour element of the payment (not materials). This is the most common rate and usually applies to subcontractors who have a good compliance history but do not meet the criteria for gross payment status.
• 30% (unregistered or unverified): Applied when a subcontractor cannot be verified or is not registered with CIS. HMRC deducts 30%, which serves as a strong incentive for subcontractors to register. Unregistered subcontractors should prioritise their registration to avoid higher deductions.

Understanding these rates and applying them correctly is essential for compliance. Incorrect application can lead to penalties and strained relationships with subcontractors. Always ensure you have the latest information on each subcontractor's status before processing payments.

Materials vs labour in CIS

CIS deductions only apply to the labour element of a payment. If a subcontractor's invoice includes materials, you deduct CIS only from the labour portion. To make this work cleanly, ask subcontractors to break down their invoices into labour and materials. If they do not, HMRC may deem the entire payment as labour. Use the CIS deduction calculator to work out the correct amounts before making payments.

For example, if a subcontractor charges £1,000 for a job, with £600 for materials and £400 for labour, CIS deductions apply only to the £400. Suppose the subcontractor is on the 20% rate. In that case, you would deduct £80, paying the subcontractor £320 for their labour, plus the full £600 for materials. Proper documentation and a clear breakdown in invoices can prevent disputes and ensure both parties are clear on the payment terms.

Paying CIS deductions to HMRC

The CIS deductions you collect must be paid to HMRC by the 19th of each month (22nd if paying electronically). These are typically offset against your own PAYE liability. If your CIS deductions exceed your PAYE bill, you can reclaim the excess through your employer payment summary or, for sole traders, through your Self Assessment return. Keep the contractor statement you give to subcontractors — they use it to reclaim their own deductions.

Consider a typical scenario: a contractor has deducted £1,500 in CIS payments from subcontractors in April. The contractor's PAYE liability for April is £1,200. The contractor can offset the CIS amount against their PAYE liability, resulting in a £300 surplus. This surplus can either be reclaimed or carried forward to offset future liabilities. Understanding this mechanism can be a significant financial management tool for businesses, impacting cash flow and budgeting.

Common Mistakes and How to Avoid Them

Even experienced contractors can make errors in their CIS monthly returns. One common issue is failing to verify subcontractors before making payments. Verifying each subcontractor ensures you apply the correct deduction rate, preventing unnecessary errors and potential penalties. Always take a few minutes to verify, especially if working with a new subcontractor or if a subcontractor's circumstances have changed.

Another frequent mistake is missing the submission deadline. A simple calendar reminder can help avoid late penalties. Set up alerts on your phone or email a week before the deadline to give you ample time to prepare and submit the return.

Finally, confusion often arises over how to accurately separate labour and materials costs. Encouraging subcontractors to provide itemised invoices can prevent HMRC from questioning your deductions. If a subcontractor is unsure how to itemise their invoice, offer guidance or suggest a template they could use.

How CIS Affects Cash Flow

CIS can significantly impact a contractor's cash flow. For subcontractors, the deductions represent money that could have been used immediately. For contractors, the responsibility to deduct and remit these payments can create short-term cash flow challenges. Planning is key. Understand your upcoming liabilities, and ensure you have enough funds set aside to meet them.

Consider setting up a dedicated bank account for CIS deductions. This way, the money is separate from your business's working capital, reducing the temptation to use it for other expenses. It also simplifies the process of tracking and paying these deductions to HMRC each month.

For subcontractors, applying for gross payment status can alleviate some cash flow issues. However, this requires a solid compliance history and meeting certain turnover criteria. If you're a subcontractor, regularly review your eligibility for gross payment status. It could be worth the effort if it means better cash flow management.

Final Thoughts

Handling CIS monthly returns properly is a vital part of running a construction business in the UK. While it may seem complex, understanding the process and maintaining good records can help you stay compliant and avoid costly penalties. Regularly review your processes, ensure all documentation is in order, and keep up-to-date with any changes in regulations.

If you're struggling with your returns, consider seeking advice from a professional accountant with experience in the construction industry. They can provide guidance tailored to your specific circumstances, helping you streamline your processes and ensure accuracy. Remember, the goal is to manage your obligations efficiently, keeping both your business and HMRC satisfied.