# CIS monthly return: the 19th deadline and what a late one costs > Contractors must send the CIS return by the 19th. The late penalty ladder from £100, with worked examples, and why a nil return is still required. - Page to cite: https://aeolookup.com/trades/cis-monthly-return-19th-late-penalties/ - Publisher: AEO Lookup (https://aeolookup.com/) - Section: Trades and building firms, Rules and paperwork - Published 1 October 2026 - Key fact: We ran the six-month penalty rule (£300 or 5% of deductions, whichever is higher) at three deduction levels; 5% overtakes £300 once deductions pass £6,000. - Sources: GOV.UK, "File your monthly CIS returns", read 2026-10-01; sums saved in tests/gaffer/calc-output.txt. If you pay subcontractors for construction work, you are probably a contractor under the Construction Industry Scheme, and HMRC wants a monthly return from you. It is one of the few deadlines in a small building firm that comes round every single month, which is exactly why it gets missed. GOV.UK says: "Send your monthly returns to HMRC by the 19th of every month following the last tax month." The tax month runs from the 6th to the 5th, so the return for the month ending 5 October is due by 19 October. ## What a late return costs The penalties are fixed, and the first one starts the day after the deadline. From the GOV.UK page, read on 1 October 2026: | How late | Penalty | |---|---| | 1 day late | £100 | | 2 months late | £200 | | 6 months late | £300 or 5% of the CIS deductions, whichever is higher | | 12 months late | £300 or 5% of the CIS deductions, whichever is higher | For returns more than 12 months late, GOV.UK says additional penalties of up to £3,000 or 100% of the CIS deductions may apply. ## The 5% rule: when it overtakes the £300 The two-part rule at six months is easy to read past. We ran it for three firms that deducted different amounts from their subcontractors in the month. - Deductions of £1,500: 5% is £75, so the penalty is the £300 minimum. - Deductions of £6,000: 5% is exactly £300, so the penalty is £300. - Deductions of £12,000: 5% is £600, so the penalty is £600. So for a firm that deducts more than £6,000 in a month, a six-month-late return stops being a flat £300 and starts scaling with the money. These are the figures from our saved sums; they ignore any other penalty HMRC can add and any reasonable-excuse appeal. They are not advice, and an accountant should confirm your own position. ## A nil return still counts A detail that catches firms in a quiet month: if you have made no payments to subcontractors, GOV.UK says you need to "file a return showing your payments were 0 (known as a 'nil return')" or tell HMRC you have temporarily stopped using subcontractors. Doing neither is a missed return, and the £100 clock runs just the same. If your work goes from busy to quiet and back, check that every month has either a return or a stopped-using notice. ## Where it usually goes wrong Three weak points are common in a small firm. This is our reading of how the job is done, not a survey. 1. **The records are in three places.** Payments are in the bank, deductions are in a spreadsheet or the accounts software, and the verification of each subcontractor is in an email from HMRC. The return is only as quick as the slowest of those. 2. **The person who does it is part time.** If the owner's partner does the books two days a week, a 19th that falls on the wrong day is a risk. A reminder on the 12th gives you a week of slack. 3. **Late-paying customers squeeze the cash.** If a customer has not paid you, you may still have tax to account for on what you paid subcontractors. Our guide on [what late payment interest a builder can claim](https://aeolookup.com/trades/late-payment-interest-builders-how-much/) covers recovering some of that cost, and the [unpaid invoice letter template](https://aeolookup.com/trades/chase-unpaid-invoice-letter-template-builders/) covers the chasing. ## A simple routine Set three reminders every month: the 6th (tax month has ended, start gathering), the 12th (return drafted and checked by your accountant or bookkeeper) and the 17th (filed, with two days left). Put a calendar entry in for the nil months too. This is the kind of reminder an AI Paperwork Clerk set up by Overlyft ![](https://aeolookup.com/img/d/ours-badge.png) handles: it reminds you on those days, collects the figures from your accounts software, and drafts the return for you or your accountant to check. Filing stays with a person. ## Short answers from this page **When is the CIS monthly return due, and what is the penalty for missing it?** By the 19th of each month, for the tax month that ended on the 5th. A return one day late is a £100 penalty; two months late adds £200; at six months it is £300 or 5% of the deductions, whichever is higher, and the same again at twelve months. The 5% figure overtakes £300 once the month's deductions pass £6,000. --- ![](https://aeolookup.com/img/d/owner-line.png) General information for UK small firms, not legal, tax or financial advice.