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

What is a CIS monthly return?

If you're a contractor under the Construction Industry Scheme (CIS), you must send a monthly return to HMRC for each month you pay subcontractors. The return informs HMRC about each subcontractor's payment and tax deductions. This is separate from your Self Assessment return and must be submitted even if no tax is owed.

For a refresher on CIS, check out our full CIS guide. This piece specifically covers the monthly return process.

Who must submit a monthly return?

You need to submit a CIS monthly return if you're a contractor paying subcontractors for construction work. This includes main contractors, subcontractors with their own subcontractors, and businesses spending over £3 million on construction annually, even if construction isn't their main activity, like property developers.

If you have no subcontractors in a month, submit a nil return or contact HMRC to arrange an inactivity period (up to 6 months) to avoid monthly filings. Ignoring the return results in penalties from HMRC.

When must returns be submitted?

CIS monthly returns are due by the 19th of each month for the preceding tax month (6th to 5th). For example, the return for the month ending 5 April is due by 19 April. Missing the deadline results in automatic penalties from HMRC. The 2026 penalty structure is:

• 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

Penalties can quickly accumulate, especially for small businesses. It's important to meet deadlines and have your paperwork ready well in advance. Set a reminder a week before the deadline to handle any issues that may occur.

How to submit a CIS monthly return

Submit online via your HMRC online services account (CIS section at gov.uk) or through compatible CIS or payroll software. You'll need:

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

Verify each subcontractor with HMRC online or by phone before payment. HMRC will confirm registration and the deduction rate. Keep verification records, either in a spreadsheet or specific accounting software. Accurate records are essential for compliance and good business practice.

CIS deduction rates explained

There are three rates:

• 0% (gross payment status): Granted by HMRC, no deductions are made. Subcontractors must have a good compliance record and meet turnover thresholds. Many aim for this status for better cash flow.
• 20% (registered): Registered with HMRC under CIS. HMRC deducts 20% from the labour part of the payment. This is the most common rate, typically for those with a good compliance history.
• 30% (unregistered or unverified): Applied if a subcontractor isn't registered or can't be verified. HMRC deducts 30%, encouraging subcontractors to register to avoid higher deductions.

Applying these rates correctly is essential for compliance. Mistakes can lead to penalties and affect relationships with subcontractors. Always ensure you have current information on each subcontractor's status before making payments.

Materials vs labour in CIS

CIS deductions apply only to the labour part of a payment. If an invoice includes materials, deduct CIS only from the labour portion. Ask subcontractors to itemise invoices into labour and materials. Without this, HMRC might treat the whole payment as labour. Use the CIS deduction calculator to calculate correct amounts before paying.

For instance, if a subcontractor charges £1,000 with £600 for materials and £400 for labour, CIS deductions apply to the £400. At the 20% rate, you'd deduct £80, paying £320 for labour and the full £600 for materials. Clear documentation and itemised invoices prevent disputes and ensure clarity on payment terms.

Paying CIS deductions to HMRC

Pay CIS deductions to HMRC by the 19th of each month (22nd if paying electronically). They're usually offset against your PAYE liability. If CIS deductions exceed your PAYE bill, reclaim the excess through your employer payment summary or Self Assessment return. Keep the contractor statement given to subcontractors; they use it to reclaim their deductions.

Consider this: a contractor deducts £1,500 in CIS payments in April, with a £1,200 PAYE liability. The £300 surplus can be reclaimed or carried forward. Understanding this can aid financial management, impacting cash flow and budgeting.

Common Mistakes and How to Avoid Them

Even seasoned contractors can make mistakes with CIS monthly returns. One issue is failing to verify subcontractors before payments. Verifying ensures the correct deduction rate, preventing errors and penalties. Always verify, especially with new subcontractors or if circumstances change.

Another mistake is missing submission deadlines. Calendar reminders help avoid late penalties. Set alerts a week before the deadline to allow time for preparation and submission.

Confusion can also occur when separating labour and materials costs. Encourage subcontractors to itemise invoices to prevent HMRC from questioning deductions. Offer guidance or a template if needed.

How CIS Affects Cash Flow

CIS can affect cash flow significantly. For subcontractors, deductions are funds that could be used immediately. For contractors, the duty to deduct and remit these payments can create short-term cash flow challenges. Planning is essential. Be aware of upcoming liabilities, and ensure funds are set aside to meet them.

Consider a separate bank account for CIS deductions. This keeps funds separate from working capital, reducing the temptation to use them for other expenses. It also simplifies tracking and paying deductions to HMRC each month.

For subcontractors, applying for gross payment status can ease cash flow issues. This requires a solid compliance history and meeting turnover criteria. Regularly review your eligibility for gross payment status; it could improve cash flow management.

Final Thoughts

Handling CIS monthly returns correctly is key to running a construction business in the UK. Though complex, understanding the process and maintaining good records helps you stay compliant and avoid penalties. Review your processes regularly, ensure documentation is in order, and stay updated on regulation changes.

If struggling with returns, seek advice from an accountant experienced in the construction industry. They can provide guidance specific to your situation, helping you refine processes and ensure accuracy. The aim is efficient obligation management, satisfying both your business and HMRC.