If you're registered as a contractor under the Construction Industry Scheme, the return covering the tax month 6 September to 5 October is due at HMRC by 19 October 2026, confirmed on GOV.UK. Unlike some HMRC deadlines, there's no grace period here: file it one day late and you get an automatic £100 penalty, whether or not any tax was actually due.
That makes CIS returns one of the least forgiving things on the compliance calendar — monthly, non-negotiable, and penalised the moment the clock ticks over. The good news is that most of the work isn't the return itself. It's making sure the month's subcontractor payments, verifications and deductions are actually straight before you file.
Here's what to check before the 19th.
What the return actually covers
CIS tax months don't follow the calendar. Each one runs from the 6th of one month to the 5th of the next, so the current period is 6 September to 5 October 2026, and the return for it is due by 19 October — roughly two weeks after the month closes.
On the return, you tell HMRC about every payment made to subcontractors during that month, and you have to declare that each subcontractor listed is correctly treated as self-employed rather than an employee. Get that status wrong and HMRC can charge a separate penalty of up to £3,000, on top of anything for late filing — so it's worth being certain, not just hopeful, about each subcontractor's status before the return goes in.
If you made no subcontractor payments in the period at all, you're not off the hook: you still need to either file a nil return or tell HMRC you've temporarily stopped using subcontractors (an "inactivity request"). Simply doing nothing is treated as a missed return.
Step 1: Confirm every subcontractor's verification status
Before you can apply the right deduction rate, each subcontractor needs to be verified with HMRC — either as a first-time check or because their status has changed since you last checked. Verification tells you which rate applies:
- 0% — subcontractor is registered for gross payment status, so no deduction is made.
- 20% — subcontractor is registered with HMRC under CIS but not for gross payment.
- 30% — subcontractor is unregistered, or HMRC can't match the details given.
If anyone new joined your subcontractor list this tax month, or an existing subcontractor's registration status might have changed, verify them through the CIS online service before you finalise the return. Deducting at the wrong rate because verification was skipped is one of the most common reasons a return needs correcting later — and corrections take longer than getting it right the first time. Our free CIS calculator works out the deduction once you know the correct rate, separating labour from materials so you're not accidentally deducting tax on the cost of bricks and timber.
Step 2: Separate labour from materials on every invoice
CIS deductions apply only to the labour element of a subcontractor's invoice, never to materials, VAT, or genuine reimbursed expenses like plant hire. An invoice that lumps labour and materials into one figure forces a guess at the split — and HMRC expects the subcontractor's invoice to show the breakdown clearly, not for you to estimate it after the fact.
If invoices arriving this month are vague about the split, now — before the return is due — is the time to go back to the subcontractor and ask for it in writing, not after you've already filed on an assumption.
Step 3: Reconcile what you've actually paid against what you've deducted
Pull every payment made to subcontractors between 6 September and 5 October and check the deduction withheld on each one matches the rate that subcontractor is verified at. The usual slip is a payment run that goes out before a verification result comes back, so the deduction defaults to 30% when it should have been 20%, or a subcontractor pays through a different legal entity than the one originally verified.
Any mismatch found now can be corrected before filing, which is simpler than amending a submitted return. HMRC's CIS online service lets you correct a return after submission, but it's an extra step worth avoiding where you can.
Step 4: Keep the records, not just the return
Filing the return doesn't close the month's paperwork. HMRC expects contractors to keep full CIS records — what was paid, what was deducted, and the verification evidence — and can charge a penalty if records aren't available on request. If a stack of subcontractor invoices for the month is still sitting in an inbox or a van cab rather than filed against the ledger, that's the gap most likely to cause a problem months from now, not today.
An AI invoice importer reads supplier, date, net amount and total off a photo or PDF of a subcontractor invoice and posts it into Xero, QuickBooks or a Sage-ready CSV, so the record sits against the right payment from the day it's received rather than being reconstructed at year-end. You still confirm the CIS treatment and account code — the point is removing the retyping, not the judgement call.
Step 5: Know what missing the 19th actually costs
HMRC's penalties for late CIS returns are automatic and escalate fast:
- 1 day late: £100 — charged automatically, regardless of whether any deductions were due.
- 2 months late: a further £200.
- 6 months late: a further £300, or 5% of the CIS deductions on the return, whichever is higher.
- 12 months late: another £300 or 5%, rising to up to £3,000 or 100% of the deductions if HMRC finds information was deliberately withheld.
Full detail is on GOV.UK's CIS late filing penalty guidance and the HMRC factsheet CC/FS18b. If you've got a reasonable excuse, you can appeal within 30 days of the penalty notice through HMRC's online service — but the appeal is a backstop, not a plan, since the fixed penalties apply whether or not deductions were actually due.
What "done" looks like by the 19th
- Every subcontractor paid this tax month is verified at the correct rate — 0%, 20% or 30%.
- Every invoice shows labour separated from materials, so the deduction is calculated on the right figure.
- Payments and deductions for 6 September to 5 October reconcile against the ledger.
- A nil return or inactivity request is filed if no subcontractors were paid.
- The return is actually submitted by the 19th, not drafted and left open.
The quick checks, for free
If what you need right now is a fast deduction figure rather than the full reconciliation, our free CIS calculator works out the 20% or 30% deduction on a labour amount in seconds, alongside the rest of the VAT, mileage, CIS and invoicing tools we publish. No sign-up, no email capture.
If the real sticking point this month is a backlog of subcontractor invoices rather than the maths, the free plan on the AI Invoice Importer handles 15 documents a month with no card required — worth running this month's pile through it before the next tax month starts and the backlog grows again. For the detail on claiming back what's overpaid once the year's returns are filed, our CIS tax refund guide covers the process end to end.