Why CIS penalties matter more from April 2026
CIS penalties are not new, but the risk has grown sharper this tax year. From 6 April 2026 contractors must file a CIS300 nil return every month they make no payments to subcontractors, an obligation that was removed in 2015 and has been reinstated. That single rule change means a contractor who previously ignored quiet months now faces an automatic £100 penalty for each one left unfiled. For a business with seasonal patterns, three or four quiet months a year could accumulate £300 to £400 in penalties before anyone notices.
Added to that, the Self Assessment penalty ladder that applies to sole-trader subcontractors is steeper than most people realise, and HMRC's interest rate on late payments has been running at a level where even a few months' delay on a mid-sized liability generates a charge worth paying attention to. This guide sets out the full penalty structure, the appeal process, and what HMRC will and will not accept as a reasonable excuse.
For the mechanics of filing the monthly return itself, see our guide to the CIS300 monthly return. For the full picture of what changed from April 2026, see our post on the April 2026 CIS rule changes.
CIS300 late filing penalties
A contractor who misses the 19th of the following tax month filing deadline for a CIS300 return receives an automatic penalty. No warning letter is issued first. The charge is raised the day after the deadline passes, and it escalates the longer the return remains outstanding.
| Lateness | Penalty type | Amount |
|---|---|---|
| 1 day late | Automatic fixed penalty | £100 |
| 2 months late | Second fixed penalty | £200 (cumulative) |
| 6 months late | Tax-geared or fixed (higher applies) | £300 or 5% of the CIS deductions on the return |
| 12 months late | Tax-geared or fixed (higher applies) | £300 or 5% of the CIS deductions on the return |
| 12 months late, information withheld deliberately | Additional penalty on top | Up to £3,000 or 100% of the CIS deductions (whichever is higher) |
The first two tiers are flat amounts and straightforward to calculate. The 6-month and 12-month tiers introduce a tax-geared element: if the return covers a month with significant deductions, the penalty is the higher of £300 or 5% of those deductions, so a month with a large deduction figure can produce a charge above the £300 minimum. The 100% layer is not the standard 12-month penalty. It is an additional charge that only applies where the contractor has deliberately withheld the information, capped at the higher of £3,000 or 100% of the CIS deductions. A return covering £5,000 of CIS deductions left unfiled for 12 months in the ordinary (non-deliberate) case attracts £300 (5% of £5,000 is £250, so the £300 minimum applies), not £5,000.
Nil returns from April 2026
The nil-return obligation reinstated from 6 April 2026 sits on the same penalty ladder. A contractor who makes no payments to subcontractors in a tax month must still file a nil return by the 19th of the following month, or pre-notify HMRC that the business has a period of inactivity. Failing to do either is a late filing, and the £100 automatic penalty follows in the same way as for a return with actual payments. Contractors used to the pre-2026 position, where a quiet month needed no action, must now build the nil filing into their monthly routine.
The practical effect is that a contractor with two active months and two quiet months in a quarter has four CIS300 obligations in that period, not two. Missing the quiet months costs £100 each. Missing all four costs £400 before escalation begins.
Self Assessment penalty ladder for subcontractors
Sole-trader subcontractors file a Self Assessment return each year to account for their CIS income, claim expenses and recover any overpaid deductions. The penalty regime for a late SA return is separate from and more graduated than the CIS300 penalties, and it is the one that catches subcontractors who put off filing.
| Lateness | Penalty type | Amount |
|---|---|---|
| 1 day late (from 31 January) | Immediate fixed penalty | £100 |
| 3 months late (day 91+) | Daily penalties begin | £10 per day, up to 90 days (max £900) |
| 6 months late | Tax-geared or fixed (higher applies) | 5% of outstanding tax or £300 |
| 12 months late | Tax-geared or fixed (higher applies) | 5% of outstanding tax or £300 |
The daily penalties at day 91 are the tier that surprises most people. The online filing deadline for Self Assessment is 31 January. A return filed on, say, 15 May is 104 days late, meaning the £100 immediate penalty plus 90 days at £10 per day equals £1,000 in fixed penalties before the tax owed or any interest is considered. If there is also a significant balancing payment outstanding, the 6-month and 12-month tax-geared charges compound the position further.
These SA penalties are distinct from the CIS300 ladder and must never be merged. A subcontractor can simultaneously face CIS300 penalties for late monthly returns and SA penalties for a late annual return: they run in parallel.
Interest on late payments
Penalties and interest are separate. HMRC charges interest on any tax paid late at the Bank of England base rate plus 4% (the margin rose from 2.5% to 4% on 6 April 2025), applied daily from the payment due date until the amount is cleared.
For Self Assessment, the balancing payment for a tax year is due on 31 January following the year end (so for 2025/26 the due date is 31 January 2027). Interest runs from that date even if the return itself was filed on time. A subcontractor who files on 31 January but does not pay until 31 March has two months of interest on the outstanding liability.
For CIS deductions the contractor has withheld and must pass to HMRC, the payment due date is the 22nd of the following month for electronic payment or the 19th for cheque. Late payment of withheld deductions is therefore both a cash-flow and a compliance risk: HMRC treats the deduction as having been collected and expects it by the 22nd regardless of whether the contractor has passed it on.
Worked example: three missed CIS300 returns
A plastering contractor misses the CIS300 returns for August, September and October 2026, all three months having live payments to subcontractors. By the time they notice in January 2027, the returns are around three to four months late.
- August return: filed 4 months late. Penalties: £100 (day 1) + £100 (day 61 = 2 months) = £200.
- September return: filed 3 months late. Penalty: £100 (day 1) + £100 (day 61) = £200.
- October return: filed 2 months late. Penalty: £100 (day 1) + £100 (day 61) = £200.
Total fixed penalties already charged: £600. If the August return had remained unfiled to 6 months, HMRC would have added the higher of £300 or 5% of the deductions on top, and the same again at 12 months. If the August return covered £4,000 of CIS deductions, the 6-month and 12-month tax-geared charges would each be £300 (5% of £4,000 is £200, so the £300 minimum applies in the ordinary case). The 100% charge of £4,000 would only arise if HMRC found the information had been withheld deliberately. The cost of inaction compounds quickly.
If any of the same months had been nil months (no subcontractor payments) under the post-April-2026 rules, the same penalty ladder applies to the nil returns left unfiled. Three missed nil returns would add another £600 in fixed penalties, for a combined total of £1,200 before escalation.
Reasonable excuse: what qualifies and what does not
HMRC can cancel a penalty where the taxpayer had a reasonable excuse for the late filing or payment. The test is whether a reasonable person in the same circumstances, taking ordinary care of their affairs, could not have met the deadline. HMRC guidance sets out what it will and will not accept.
Examples that typically qualify:
- A serious or unexpected illness that prevented the person from filing, particularly where they had no agent acting for them.
- A bereavement of a close family member shortly before or around the filing deadline.
- A failure by HMRC itself, including a system outage affecting online filing or incorrect advice that led the person to believe no action was needed.
- A documented postal failure outside the person's control, where evidence of posting (such as a certificate of posting or tracked delivery) exists.
Examples that do not qualify:
- Forgetting the deadline.
- Relying on an adviser, agent or employee who failed to act, unless that failure itself was beyond the person's control.
- Pressure of work or busy trading periods.
- Not receiving a reminder from HMRC (HMRC is under no obligation to remind taxpayers of deadlines).
- Not knowing the deadline existed (ignorance of the law is not an excuse, though HMRC may exercise discretion in clear cases of genuine confusion around new rules).
The excuse must also be genuine and continuing: if the obstacle was removed before the deadline but the person still did not file, the excuse does not cover the full delay. HMRC expects filing to take place as soon as the obstacle is resolved.
How to appeal a CIS300 or Self Assessment penalty
The appeal window is 30 days from the date on the penalty notice. Acting within that window is important: a late appeal is harder to progress and requires a separate explanation of why the time limit was missed.
Appealing a Self Assessment penalty
You can appeal online through your HMRC online account, which is the fastest route, or by completing form SA370 (available on GOV.UK) and sending it to the address on the penalty notice. The form asks for the penalty reference number, the grounds for the appeal, and any supporting evidence. State clearly that you are appealing under the Taxes Management Act 1970.
Appealing a CIS300 penalty
CIS300 penalty appeals are made in writing to the HMRC office that issued the notice, again within 30 days. Set out the penalty reference, the return period in question, the reason for the late filing, and any evidence supporting a reasonable excuse. If you were unaware of the nil-return reinstatement from April 2026 and missed a nil return in that first period, note that specifically: HMRC may exercise some discretion around genuinely new obligations in their first months of operation, though this is not guaranteed.
If HMRC rejects the appeal
If HMRC refuses the appeal, you can request an independent review by a different HMRC officer who was not involved in the original decision. If the review also goes against you, the next step is the First-tier Tax Tribunal, which is independent of HMRC. Tribunal applications are made within 30 days of the review conclusion letter. The tribunal process is more formal and involves presenting your case in writing and sometimes in person, so taking advice at that stage is sensible.
Throughout the appeal process, any underlying tax liability continues to accrue interest. Paying the disputed amount "under protest" while the appeal proceeds stops the interest clock and protects your cash position if the appeal ultimately succeeds, because HMRC will refund the payment with interest if you win.
Avoiding penalties: the practical checklist
Most CIS and Self Assessment penalties are avoidable with a consistent routine. The steps below address the common failure points.
- Diarise the 19th. Set a standing reminder for the 19th of every month, not just months where you know you have paid subcontractors. From April 2026, every month is a potential filing month.
- File nil returns promptly. If you had no subcontractor payments in the month, log in and file the nil return the same day you confirm there are no payments to report. It takes two minutes and removes the £100 penalty risk.
- Keep deduction records current. Accurate monthly records mean the CIS300 can be filed quickly. A backlog of paperwork is the usual reason returns are missed or delayed.
- File the SA return before 31 January. Filing early, even in November or December, is a reliable way to avoid the £100 immediate penalty and the daily charges that start from day 91.
- Pay on time even if filing is delayed. If you cannot file by the deadline, make an estimated payment to reduce the interest clock. A payment on account is better than nothing while you resolve the filing position.
If you are a contractor and need support making sure your CIS300 obligations are met every month, including the new nil-return requirement, our team handles the full monthly cycle. See the full range of services on our services page or visit our dedicated gross payment status page if you are also looking to hold payments gross and avoid CIS deductions altogether.