CIS · Guide · 5 min read

CIS300 monthly return: the complete guide for contractors

A contractor’s monthly CIS checklist: tax months, filing dates, materials evidence, nil returns and reconciliation before submission.

By Trade PA · Published 25 September 2026 · Reviewed 15 September 2026

Most of our CIS guides are written for the subbie side: getting deducted, the statements you should receive, gross payment status. This one is for the other hat, and plenty of sole traders wear both: check whether your subcontracting makes you a CIS contractor.

Who actually has to file

This guide is for contractors operating CIS on relevant subcontractor payments. A private householder paying for work on their own home is not a CIS contractor merely because they hire a builder. Construction businesses and some businesses with substantial construction spending can fall within the scheme. Check registration, employment status and the scope of the work before treating a worker as a subcontractor.

It also covers "deemed contractors": businesses outside construction that spend more than £3 million on construction in a rolling twelve months. That is not most readers of this blog, but if you work for one, it explains why a factory or a retailer is suddenly asking for your UTR.

The month, and the two deadlines

CIS runs on tax months, not calendar months: the 6th of one month to the 5th of the next. Two dates then matter, and people mix them up.

The 19th: the return. Your CIS300 for the tax month ending on the 5th must reach HMRC by the 19th of that same month. So the month ending 5 September has to be filed by 19 September.

The 22nd: the money. The deductions you withheld from subbies are paid over to HMRC by the 22nd if you pay electronically (the 19th if you still pay by post). The return tells HMRC what you owe; the payment settles it. Missing either has consequences, but they are different consequences.

What goes on the return

For each subcontractor paid in the tax month, reconcile the payment before CIS deduction, allowable materials and the deduction withheld. Use payment dates rather than simply listing invoices raised. Keep gross-status payments in your records too.

Materials need evidence. The deductible amount is not automatically whatever a subcontractor labels “materials”. Apply HMRC’s rules to the actual qualifying cost; material mark-up is not the same thing. Remove VAT where the rules require it, and use the verification treatment HMRC supplied. Check HMRC’s deduction guidance.

Unmatched subbies being deducted at 30% also need their verification number shown on the return. And every return carries your declarations: that you have considered each subbie's employment status, that everyone who needed verifying has been verified, and that the information is correct. Those are legal statements with your name on them, not tick-box decoration.

Verification, and where the rates come from

Before you pay a new subcontractor for the first time, you verify them with HMRC. The answer comes back as one of three treatments: gross payment status, so 0% deducted; matched and registered, 20%; or unmatched, 30% until they sort their registration. That rate is HMRC's answer, not your judgement call, and it stays with the subbie until HMRC tells you otherwise. Paying an unverified subbie at 20% because "he seemed legit" is one of the fastest ways to end up owing HMRC the difference yourself.

Nil returns are mandatory again

From 6 April 2026, mainstream contractors must file a nil return or notify HMRC of inactivity when no subcontractors were paid. HMRC distinguishes mainstream from deemed contractors; do not assume the same nil-return requirement for both. Review CIS 340 for your situation.

The penalty ladder

Late filing can attract an initial £100 penalty, another £200 after two months, and further tax-related penalties at six and twelve months. Special rules, caps and deliberate-withholding cases can change the amount. Use HMRC’s notice and current guidance rather than estimating your liability from a short article. Filing and paying are separate obligations.

The statement your subbies are owed

Filing the return is half the monthly job. Within 14 days of the tax month end, every subbie you deducted from is owed a payment and deduction statement showing the same numbers. It is their proof of the tax you took, and they need it for their own return. The format and rules are covered in our CIS statement template guide, and the figures should reconcile exactly with your CIS300, because HMRC can compare both sides.

The mistakes that actually happen

Deducting from materials. Deducting from VAT-inclusive totals. Forgetting the nil months. Paying a new subbie before verifying and guessing 20%. Statements going out late or not at all. And the quiet one: the domestic reverse charge on the VAT side interacting with CIS invoices in ways nobody explained, which is its own guide. None of these are exotic. They are what happens when the monthly routine lives in a spreadsheet and a memory.

Use Trade PA’s CIS records to organise subcontractor payments and statements. Reconcile the figures with bank payments and HMRC verification records before filing through your authorised route. A prepared report is not evidence HMRC received a return: keep the submission receipt and payment confirmation separately.

A five-column month-end check

For a simple illustration, assume £1,000 labour, £200 qualifying materials at cost, no VAT and a verified 20% deduction. The CIS deduction is £200 and the payment is £1,000. Keep the £1,200 amount before deduction distinct from the £1,000 bank transfer. The figures only apply to those stated assumptions.