A plain walkthrough of what goes into a CIS300 monthly return, when it's due, and how to avoid the mistakes that trip up first-time contractors.
1 June 2026
If you've taken on subcontractors for the first time, or you're the one now responsible for CIS admin at your firm, the CIS300 return is probably the part that feels most exposed. It's the document HMRC actually sees, so it needs to match what you've paid and deducted to the penny. Here's what goes into it and how to get it right each month.
The CIS300 is the monthly return every registered contractor must submit to HMRC, even in a month where you've made no payments to subcontractors (in which case you file a nil return). It reports, for the tax month just ended, every subcontractor you paid, how much you paid them, how much CIS you deducted, and their verification status.
You're a "contractor" for CIS purposes if you pay subcontractors for construction work, and that includes businesses outside construction that spend heavily on it, not just building firms.
The CIS300 is due by the 19th following the end of the tax month it covers. Since CIS tax months run from the 6th of one calendar month to the 5th of the next, that gives you a two-week window after the tax month closes to pull your figures together and file. Miss it and you're into penalty territory, and repeated lateness escalates the penalties rather than resetting each month.
If you're new to the 6th-to-5th cycle, it's worth reading our guide on CIS tax months explained before your first return, since getting a payment into the wrong tax month is one of the most common reasons a CIS300 doesn't reconcile.
For each subcontractor paid in the tax month, you need:
If any of that is missing, particularly the verification reference, stop and sort it before you file. You should be verifying a subcontractor with HMRC before the first payment, not after. Our guide on verifying a subcontractor with HMRC covers that step if you haven't done it yet.
The return only works if the underlying deduction was calculated correctly in the first place. That means applying the right rate per subcontractor, based on their verified status, and making sure genuine materials costs were excluded from the taxable amount before the rate was applied, not after. Our guide on CIS deduction rates explained walks through the 0/20/30% rates in detail, and if you're unsure what counts as a deductible materials cost, see materials vs labour under CIS.
You can file the CIS300 through HMRC's online service, either directly or via commercial payroll or CIS software, or through your accountant if they handle your monthly filing. The return itself is a summary: it doesn't ask for a line-by-line breakdown per invoice, just the totals per subcontractor for the tax month. That's exactly why keeping clean, subcontractor-by-subcontractor figures throughout the month, rather than trying to reconstruct them from raw invoices at the deadline, saves you the most time.
Filing against the calendar month instead of the tax month. A payment made on 3 June belongs to the tax month ending 5 June, not the one starting 1 June. Get this wrong consistently and your CIS300 and your Payment and Deduction Statements will disagree with each other.
Forgetting the nil return. If you didn't pay any subcontractors in a tax month, you still need to tell HMRC, either by filing a nil return or notifying them you won't be paying subcontractors for a period. Silence isn't the same as compliance here.
Mixing up gross payment and net payment subcontractors. A subcontractor with gross payment status gets 0% deducted, but they still need to appear on your return with the payment recorded, just with no deduction against it.
If you're only dealing with two or three subcontractors, a spreadsheet and a calculator is a perfectly reasonable way to keep on top of your CIS300. Once you're issuing statements to a dozen or more subcontractors a month, working out rates and materials deductions one by one starts eating a disproportionate amount of admin time. If that's where you are, our CIS statement generator takes a spreadsheet of what you paid each subcontractor this tax month and produces every Payment and Deduction Statement plus the totals you need for your CIS300, in one pass.
For more on the paperwork side of CIS, our guides hub covers deduction rates, verification, and statement deadlines in one place.