What is the CIS Monthly Return?
If you are registered as a contractor under the Construction Industry Scheme, you must file a monthly CIS return with HMRC showing all payments you have made to subcontractors in the previous tax month.
This is not optional and not based on whether you made any payments — you must file even if you made no payments (a nil return). Missing returns or filing late triggers automatic penalties.
CIS Monthly Return Deadline
Returns for each tax month must be submitted by the 19th of the following month.
Tax months run from the 6th to the 5th:
- Tax month: 6 April – 5 May → Return due: 19 May
- Tax month: 6 May – 6 June → Return due: 19 June
- And so on throughout the year
If you pay HMRC by post, the deadline is the 19th. For electronic payments, it is the 22nd.
What Goes on a CIS Monthly Return?
For each subcontractor you paid during the tax month, you must report:
- Subcontractor's name
- Subcontractor's UTR number
- Total amount paid (gross)
- Amount deducted (the CIS deduction)
- Cost of materials (if any — materials are excluded from the CIS calculation)
- Verification number (from when you first verified the subcontractor with HMRC)
HMRC uses this information to credit each subcontractor's tax account with the deductions made, which they can then claim through their Self Assessment return.
How to Submit the CIS Monthly Return
Online via Government Gateway
The most common method. Log in at www.gov.uk/file-your-cis-subcontractor-tax-return, navigate to the CIS section, and complete the online form.
CIS-compatible Software
Most accounting and construction management software can submit CIS returns directly to HMRC. This automates much of the data entry.
By Post (CIS300 form)
Paper returns are still accepted but must arrive by the 19th. Given postal reliability, submitting online is strongly recommended.
Nil Returns
If you did not pay any subcontractors during a tax month, you must still file a nil return by the 19th.
To avoid filing a nil return every month when you know you won't have payments, you can tell HMRC to treat your returns as nil for up to six months at a time ("notification that no return is due"). Contact the CIS helpline to arrange this.
Paying HMRC: CIS Deductions
After filing the return, you must pay HMRC the total of all CIS deductions made during the month. This is paid together with any PAYE Income Tax and National Insurance owed for employees.
The payment deadline is the same as the return: 19th of the month (22nd for electronic payment).
Penalties for Late CIS Returns
The penalty regime for late CIS returns is automatic and significant:
| Lateness | Penalty |
|---|---|
| 1 day late | £100 |
| 2 months late | £200 |
| 6 months late | Additional £300 or 5% of deductions (whichever is higher) |
| 12 months late | Additional £300 or 5% of deductions |
| Maximum annual penalty per return | Up to £3,000 |
Interest also accrues on unpaid amounts. Penalties apply even for nil returns — missing a nil return still costs £100.
If you have a reasonable excuse (e.g., serious illness, bereavement, IT failure beyond your control), you can appeal the penalty.
Common CIS Return Mistakes
Including the Wrong Gross Amount
Report the gross payment (before the CIS deduction), not the net amount you actually paid. HMRC calculates deductions from the gross figure.
Reporting VAT as Part of the Gross Payment
If the subcontractor charged VAT, do not include the VAT amount in the gross payment reported on the CIS return. The gross amount for CIS purposes is the payment before VAT.
Not Including All Subcontractors
If you have multiple subcontractors across different sites or projects, ensure all of them are included on the return for the relevant tax month.
Incorrect UTR Numbers
If a UTR is wrong, HMRC cannot credit the deduction to the correct subcontractor's record. Always double-check UTRs at the verification stage.
CIS Monthly Return and Your Own Payments to HMRC
Many contractors confuse the monthly return (information) with the monthly payment (money). They are separate:
- The return reports what you deducted
- The payment sends the deducted amounts to HMRC
If you also have employees, your PAYE payment and CIS payment are combined into a single monthly payment to HMRC.
Using FlashBill to Manage CIS
FlashBill tracks all CIS invoices received from subcontractors and records the deductions made. This gives you a clear record of each month's payments, making CIS return preparation straightforward. Export your CIS data for use in your monthly return or share it with your accountant.