What is the CIS300 deadline?
The CIS300 is due by the 19th of the month following the tax month. The tax month runs from the 6th to the 5th. For example, payments made between 6 April and 5 May 2026 must be reported by 19 May 2026. The deadline does notmove if the 19th falls on a weekend or bank holiday. From April 2026, nil returns are also mandatory — miss one and it's an immediate £100 penalty.
The one rule
Every CIS300 is due by the 19th of the month following the tax month end. The tax month runs 6th to 5th. So October payments are reported by 19th November, November payments by 19th December, and so on. The deadline doesn't move if the 19th lands on a weekend — file before it.
CIS300 deadline calendar — 2026–27
The full filing calendar for the 2026–27 tax year. Each row is one tax month and the date its CIS300 must be filed by.
| Tax month | Payments made | CIS300 due |
|---|---|---|
| 6 Apr – 5 May 2026 | April 2026 | 19 June 2026 |
| 6 May – 5 Jun 2026 | May 2026 | 19 July 2026 |
| 6 Jun – 5 Jul 2026 | June 2026 | 19 August 2026 |
| 6 Jul – 5 Aug 2026 | July 2026 | 19 September 2026 |
| 6 Aug – 5 Sep 2026 | August 2026 | 19 October 2026 |
| 6 Sep – 5 Oct 2026 | September 2026 | 19 November 2026 |
| 6 Oct – 5 Nov 2026 | October 2026 | 19 December 2026 |
| 6 Nov – 5 Dec 2026 | November 2026 | 19 January 2027 |
| 6 Dec – 5 Jan 2027 | December 2026 | 19 February 2027 |
| 6 Jan – 5 Feb 2027 | January 2027 | 19 March 2027 |
| 6 Feb – 5 Mar 2027 | February 2027 | 19 April 2027 |
| 6 Mar – 5 Apr 2027 | March 2027 | 19 May 2027 |
CIS300 deadline calendar — 2027–28
The next tax year's deadlines, so you can plan ahead.
| Tax month | Payments made | CIS300 due |
|---|---|---|
| 6 Apr – 5 May 2027 | April 2027 | 19 June 2027 |
| 6 May – 5 Jun 2027 | May 2027 | 19 July 2027 |
| 6 Jun – 5 Jul 2027 | June 2027 | 19 August 2027 |
| 6 Jul – 5 Aug 2027 | July 2027 | 19 September 2027 |
| 6 Aug – 5 Sep 2027 | August 2027 | 19 October 2027 |
| 6 Sep – 5 Oct 2027 | September 2027 | 19 November 2027 |
| 6 Oct – 5 Nov 2027 | October 2027 | 19 December 2027 |
| 6 Nov – 5 Dec 2027 | November 2027 | 19 January 2028 |
| 6 Dec – 5 Jan 2028 | December 2027 | 19 February 2028 |
| 6 Jan – 5 Feb 2028 | January 2028 | 19 March 2028 |
| 6 Feb – 5 Mar 2028 | February 2028 | 19 April 2028 |
| 6 Mar – 5 Apr 2028 | March 2028 | 19 May 2028 |
The April 2026 nil-return change
From 6 April 2026, if you made no CIS payments in a tax month you must either (a) submit a nil CIS300 by the 19th, or (b) submit a formal inactivity request to HMRC before the month begins. Missing either triggers an immediate £100 fixed penalty — the same as a late return. Previously, no-payment months could be left unfiled.
This catches bookkeepers with seasonal construction clients hardest. The fix is simple: set a calendar reminder on the 15th of every month, even for inactive clients — confirm nil or file.
The £100 penalty ladder
Miss the deadline and the penalties stack the longer the return is outstanding:
| Late by | Penalty |
|---|---|
| 1 day (immediate) | £100 |
| 2 months | £200 |
| 6 months | £300 or 5% of the deductions, whichever is higher |
| 12 months | A further £300 or 5%, whichever is higher — up to £3,000 in serious cases |
The first £100 lands the moment you miss the 19th — including on a nil return from April 2026. The full penalty rules are covered in how to file a CIS return.
What you need before you file
For each subcontractor you paid in the tax month, have these to hand:
- The subcontractor's UTR
- The HMRC verification number
- The gross amount (labour + materials, ex-VAT)
- The materials amount
- The deduction rate (20%, 30% or 0%)
- The deduction amount (rate × labour only)
Not sure of a deduction figure? Run it through the free CIS calculator — enter labour, materials and the rate and it returns the deduction, net pay and the CIS300 figures. No sign-up.
Have the figures ready before the 19th
The deadline doesn't move — but the slow part is preparing each subcontractor's figures by hand. Forward a subcontractor invoice to CISflow and it extracts the CIS fields — UTR, verification number, labour/materials split and the deduction at the rate you confirm — then lays them out for you to review and approve before a clean CIS bill goes into Xero. The typing is gone; the deadline is one click of preparation, not an evening of re-keying.
It's live and in early access. See it on a real invoice (no signup), or start free — 14 days, no card.
Keep reading
This guide is general information on UK CIS, not tax advice. Deadlines and penalties are as published by HMRC; always check the current rules. CISflow is a data-entry tool, not an accountancy or tax service — you remain responsible for figures submitted to HMRC.
CIS300 deadline questions
- What is the CIS300 deadline?
- The CIS300 is due by the 19th of the month following the tax month. The tax month runs from the 6th to the 5th, so payments made between 6 April and 5 May must be reported by 19 May. The deadline does not change if the 19th falls on a weekend or bank holiday. From April 2026, nil returns are also mandatory — missing one triggers an immediate £100 penalty.
- What if the 19th falls on a weekend or bank holiday?
- The CIS300 filing deadline is fixed on the 19th and does not move if it falls on a weekend or bank holiday. File before the 19th to be safe. (The separate deadline for paying the deductions over to HMRC is the 22nd if you pay electronically — but the return itself is always due by the 19th.)
- Can I file a CIS300 early?
- Yes. You can submit the return any time after the tax month ends on the 5th, up to the 19th deadline. Filing early is fine and often sensible — it removes the deadline risk. You report only the payments actually made in that tax month.
- How do I submit a nil return?
- If you made no payments to subcontractors in a tax month, you must still tell HMRC. Submit a nil CIS300 (declaring no payments) by the 19th, or notify HMRC of a period of inactivity before the month begins. From April 2026 this is mandatory — leaving a no-payment month unfiled triggers an immediate £100 penalty.