The CIS300 is the monthly return a contractor must submit to HMRC showing every payment made to subcontractors in the preceding tax month, together with the CIS deductions withheld from those payments.

Tax months run from the 6th to the 5th of the following calendar month. The CIS300 for each tax month must be filed by the 19th of the following month: payments made in the month ending 5 May are due to HMRC by 19 May. Payment of the deducted CIS must reach HMRC by the 22nd electronically (or the 19th if paying by cheque). Missing the deadline by even one day triggers an automatic £100 penalty, rising to £200 at two months, then to £300 or 5% of the CIS liability (whichever is higher) at six months, and £300 or 100% of the CIS liability at twelve months.

Every contractor registered under the Construction Industry Scheme must file a CIS300 even if they make no payments to subcontractors in a particular month. From 6 April 2026 that nil-return obligation was reinstated (it had been removed in 2015): a contractor with no subcontractor activity must still file, or pre-notify HMRC of inactivity. Payments to local authorities and public sector bodies are excluded from the return under the new Regulation 24ZA exemption, also in force from 6 April 2026.

The CIS300 is also the point at which a contractor confirms the deduction rates it has applied. Before including a subcontractor on the return, the contractor must have verified that subcontractor with HMRC to establish whether they hold Gross Payment Status (0%), are registered (20%) or are unregistered (30%). From April 2026, re-verifying each subcontractor before payment is one of three mandatory due-diligence steps a contractor must document to meet the “knew or should have known” standard under Finance Act 2026.

  • Filing deadline: 19th of the month following the relevant tax month.
  • Payment deadline: 22nd electronically, 19th by cheque.
  • Penalty for one day late: £100 per return.
  • Nil months: a return must still be filed (or inactivity pre-notified) from April 2026.

For a full walkthrough of contractor obligations around the CIS300, including worked examples and the complete penalty ladder, see CIS monthly returns: deadlines, nil returns and penalties. The April 2026 rule changes that affect what must appear on the return are covered in CIS April 2026 rule changes explained.