What the CIS300 monthly return is

The CIS300 is the monthly return that every CIS contractor must file with HMRC. It reports all payments made to subcontractors in the preceding tax month, the deductions applied, and the verification references used. HMRC uses it to reconcile what you deducted against what subcontractors report they received. Discrepancies trigger compliance checks.

Filing is mandatory regardless of whether you made any CIS payments in the month. From April 2026, nil returns are compulsory. There is no threshold below which you can stop filing — once you are registered as a CIS contractor, monthly returns are required until you formally deregister.

The deadline: the 19th of every month

The CIS300 must be filed by the 19th of each month, covering payments made in the tax month ending on the 5th. So the return for payments made between 6 March and 5 April must be filed by 19 April. This deadline is fixed. HMRC does not grant extensions.

Filing late once costs £100. After two months, the penalty rises to £200. After six months, a further 5% of the tax involved is added. After 12 months, another 5%. The penalties compound. A contractor who stops filing and lets twelve months pass can face penalties exceeding £3,000 plus a percentage of the total deductions that should have been reported.

What to include in each return

For each subcontractor paid in the month, the return must include:

Nil returns contain no subcontractor data but must still be filed by the 19th to confirm you made no payments in the month.

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How to file the CIS300

The CIS300 is filed online through HMRC’s CIS online service, accessible through Government Gateway. You will need your Government Gateway credentials and your employer PAYE reference. Most payroll software can also submit CIS300 returns directly.

If you use a payroll bureau like Bookd, the bureau files on your behalf using its own HMRC agent credentials. You supply the payment data; the bureau processes the return and confirms filing. The confirmation reference is your evidence that the return was submitted.

Correcting errors after submission

CIS300 returns can be amended after submission, but amendments must be made promptly. If you discover an error — wrong deduction amount, incorrect materials figure, missing subcontractor — file an amended return as soon as possible. HMRC treats errors that are corrected promptly very differently from errors that are discovered during a compliance check.

Amendments are made through the same CIS online service. You file a revised return for the same month. If the amendment reduces a deduction amount, you may owe the subcontractor the difference. If it increases it, you owe HMRC the additional deduction plus any interest that has accrued.

What happens if you stop filing

If you stop filing without formally deregistering as a CIS contractor, HMRC continues to expect monthly returns indefinitely. Penalties accumulate. HMRC will eventually issue a penalty notice and, if ignored, pursue the debt. Deregistering from CIS is a separate action from ceasing construction activity — you must actively tell HMRC you no longer need to file.

Yes, from April 2026. Nil returns are mandatory. Missing a nil return carries the same £100 penalty as missing a return with activity.
Yes. Any authorised agent with HMRC agent credentials can file on your behalf. They must be formally authorised through HMRC’s agent services.
The deduction on the CIS300 must match the payment statement issued to the subcontractor. If there is a discrepancy, resolve it before filing or file an amendment promptly after. HMRC will ask both sides for records in any compliance check.
Written by Mark Taylor, Director — Bookd Finance Ltd, Andover, Hampshire. ICB Level 3 Payroll qualified. HMRC Agent Services Account holder.