CIS tax months explained: why the 6th to the 5th matters

The one covering the date you actually paid the subcontractor. CIS is a payment-based scheme, so the invoice date and the work date are both irrelevant to which return the payment lands on.

The trap is the first five days of the month. Run your subcontractor payments on the 1st or the 2nd and every one of them belongs to the tax month that opened five weeks earlier. Firms that treat CIS as a calendar-month exercise routinely file those payments a month late without noticing.

A subcontractor payment made on the 3rd of the month goes on the previous month’s CIS return.

• 5th — the tax month closes.
• 19th — the CIS300 monthly return must reach HMRC.
• 19th — payment and deduction statements must be with every subcontractor you deducted from.
• 19th — the deductions must be paid to HMRC if you pay by post.
• 22nd — the deductions must be cleared in HMRC’s account if you pay electronically.

NoteThe 22nd is a cleared-funds date, not a send date. If the 22nd falls on a weekend or bank holiday, the money must be there by the last working day before it — not after.

Deductions are paid across with your PAYE, on the same reference, in the same monthly payment. Small employers who pay PAYE quarterly can pay CIS quarterly too, but the return is still monthly.

Penalties start the day after the deadline and stack. They apply per return, not per subcontractor.

The penalties are cumulative, so a return six months late has already attracted £100, £200 and then the six-month charge. There is a cap for new contractors: if this is among your first returns, the total penalty for those months is limited to £3,000.

Filing one day late costs £100 whether the return had one subcontractor on it or forty.

You still have to file. A month with no payments is a nil return, and a missed nil return attracts exactly the same £100 penalty as a missed return with £40,000 of deductions on it. This is the single most common CIS penalty and the easiest to avoid.

If you know you will not be paying subcontractors for a while, tell HMRC and ask for the scheme to be made inactive. Inactivity can be requested for up to six months, and while it is in force you do not have to file nil returns. When you start paying subcontractors again, the scheme reopens and monthly returns resume.

NoteDo not simply stop filing and hope. HMRC issues penalties automatically from the scheme record, and cancelling them afterwards takes longer than filing the nil return would have.

Every monthly return carries two statutory declarations, and they are the reason the return is not just a data submission.

1. 01Employment status. That none of the contracts included in the return is a contract of employment.
2. 02Verification. That every subcontractor included has either been verified with HMRC, or has appeared on a return in the current tax year or either of the two preceding tax years.

The first one is the important one. Signing it monthly for someone who works only for you, uses your tools, and takes your instructions is not a formality. If HMRC later reclassifies that person as an employee, the PAYE and National Insurance liability lands on you, together with a stack of declarations you made saying it did not.

The tax period is derived from the payment date, so a payment on the 3rd falls into the previous period automatically rather than depending on whoever is doing the return that month. Periods are labelled in full — "06 Apr – 05 May 2026" — so nobody has to interpret a month number, and an unsubmitted return past its due date is flagged overdue.

Generating a return pulls every unlinked deduction for that period and locks it to the return, so the same payment cannot be reported twice. A period with no deductions is marked as a nil return automatically. Marking a return submitted requires both declarations to be made; without them, Estimark refuses.

What Estimark does not do is transmit the return. Filing a CIS300 requires user-restricted HMRC OAuth and fraud prevention headers that Estimark does not implement, so you file through HMRC’s own service or your accountant’s software and record the reference, the method and the date against the return. That keeps the audit trail complete without pretending to a submission that never happened. More on how CIS works in Estimark.

NoteEstimark does not send CIS deadline reminders. The overdue flag appears when you look at the return, not in your inbox on the 18th.