If you run subcontractors under the Construction Industry Scheme, the CIS monthly return for 6 August to 5 September is due at HMRC by 19 September — confirmed on GOV.UK. Miss it and the penalty is automatic: £100 the moment you're a day late, rising to £200 at two months, then £300 or 5% of the CIS deductions on the return, whichever is higher, at six and again at twelve months. Get the employment status declaration wrong and there's a separate penalty of up to £3,000.
None of that is new. What catches contractors out every month is not the return itself — it's what has to be true before you can file it: every subcontractor verified, every labour invoice in the books with the right deduction, and no payment made that hasn't gone through the scheme.
Here's a plan to get from "pile of subcontractor invoices" to "return filed" before Friday.
What the 19 September deadline actually covers
CIS tax months run 6th to 5th, not calendar months. The return due on 19 September covers payments you made to subcontractors between 6 August and 5 September. If you've made no payments in that period, you still owe HMRC something: either a nil return, or an inactivity request telling them you've temporarily stopped using subcontractors. Silence is not an option — HMRC doesn't assume "nothing happened," it assumes "nothing filed."
If you've genuinely paid nobody this month, filing the nil return takes two minutes online and closes the door on a £100 penalty for a return you didn't think you needed to send.
Step 1: Verify before you pay, not after
Every subcontractor has to be verified with HMRC before their first payment under your scheme, unless you've paid them under CIS within the current or two previous tax years. Verification tells you their deduction rate: 0% (gross payment status), 20% (registered), or 30% (unregistered, or HMRC can't match the details you gave them).
Skipping verification and guessing the rate is how contractors end up under-deducting and picking up the shortfall themselves. If a new subcontractor's invoice landed this week and you haven't verified them, do it before you process the payment, not after you've filed the return.
Step 2: Separate labour from materials on every invoice
CIS deductions apply to the labour element of a payment, not materials the subcontractor has genuinely bought and can evidence with a receipt or invoice from their own supplier. An invoice that lumps a day rate and a trip to the merchant's into one figure forces you to either deduct on the whole amount (overcharging the subcontractor) or guess a split (a mistake HMRC will query if it looks generous).
Push subcontractors to itemise labour and materials separately on every invoice. It's a habit worth enforcing now, in September, rather than untangling twelve months of lump-sum invoices at year end. Our CIS deductions guide covers the deduction rates and how the maths works if you need the full breakdown, including a free CIS calculator.
Step 3: Get the invoices into your accounting software before you total anything
The return is only as good as the records behind it. If subcontractor invoices for the period are sitting in a van cab, a WhatsApp thread, or an inbox nobody has opened since payment went out, the return gets filed from memory — and memory is where the errors live.
This is where a typing bottleneck usually forms, because subcontractor invoices arrive as photos of handwritten dockets as often as proper PDFs. An AI invoice importer reads the supplier, invoice number, date, labour and materials lines, and VAT from a photo or PDF and posts it into Xero, QuickBooks or a Sage 50-ready CSV, so the record exists in software before you sit down to work out what's owed. You still check the split between labour and materials against what the subcontractor actually did — automation should speed up the entry, not the judgement call.
Step 4: Reconcile deductions against what you've actually paid HMRC
Before you file, cross-check three numbers for the period: the gross amount paid to subcontractors, the CIS deducted, and what's sitting in your CIS liability account waiting to go to HMRC. If a subcontractor was paid but the invoice never made it into the ledger, your deduction total will be understated — and the return you send won't match what actually left your bank account.
A five-minute reconciliation now is cheaper than a correction later. Use the HMRC CIS online service to correct a filed return if you do find a mismatch after the fact — it's built for exactly that.
Step 5: File, then start the next tax month's folder immediately
Once the return is in, don't let 6 September's invoices land in the same pile as the ones you just cleared. Start a fresh folder or intake address for the new tax month the moment you file, so the run-up to 19 October starts from zero rather than from a backlog.
The quick checks, for free
Not every question needs the full workflow. If you just need to check a deduction rate on one invoice or work out gross-to-net on a single payment, our free CIS calculator does the maths in seconds, alongside the VAT, mileage, late-payment and invoicing tools we publish for the same reason — no sign-up, no email capture.
What "done" looks like by 19 September
- Every subcontractor paid in the period is verified, with a deduction rate you can evidence.
- Every invoice is in Xero, QuickBooks or Sage 50, with labour and materials split and the original attached.
- CIS deducted on the ledger matches CIS actually withheld from payments.
- The return is filed — or the nil return / inactivity request is in — before the 19th, not on it.
Subcontractors who've overpaid through the scheme all year can claim it back once their return is done; our CIS tax refund guide walks through that side of it if you're fielding those questions from your own subbies.
If the bottleneck this month is typing up a stack of subcontractor invoices, the free plan on the AI Invoice Importer handles 15 a month with no card. Run this month's pile through it before the 19th and see whether the review screen is faster than the keyboard.