CIS Penalties: What Happens If You Miss a Self Assessment Deadline?
HMRC doesn't wait. The moment you miss the deadline, a £100 penalty is applied automatically — even if you don't owe a penny in tax.
Every year, thousands of CIS subcontractors miss the Self Assessment deadline — and every year, HMRC sends the same automatic penalty regardless of whether you owe any tax. The penalty system doesn't ask questions. It just charges.
What catches most people off guard isn't the first £100. It's what happens next. At 3 months, daily charges start. At 6 months, percentage penalties kick in. At 12 months, another layer is added. By the time some subcontractors deal with it, the bill has grown from £100 to over £1,600 — before interest on any unpaid tax.
This guide lays out exactly what HMRC charges, when, and what you can do to reduce the damage if you've already missed a deadline.
"The penalty applies even if your tax bill is zero — even if HMRC actually owes you money. Filing late costs you either way."
The Key Deadlines You Cannot Miss
| Deadline | What it covers | Miss it and... |
|---|---|---|
| 31 October (paper) | Paper Self Assessment return for the previous tax year | £100 fixed penalty applies immediately |
| 31 January (online) | Online Self Assessment return + any tax owed | £100 fixed penalty + interest on unpaid tax |
| 31 July | Second Payment on Account (if applicable) | 5% surcharge after 30 days |
For most CIS subcontractors filing online, the critical date is 31 January. This covers both the return itself and any tax owed for the previous year. Miss either — even by one day — and the penalty clock starts.
Note on Payments on Account: If your previous year's tax bill exceeded £1,000 and less than 80% was deducted at source (e.g. via CIS), HMRC may require you to make advance payments towards the current year's bill. These are due 31 January and 31 July. CIS deductions often reduce or eliminate POA — but you need to file to confirm this.
Before you deal with penalties — know what HMRC owes you
Many CIS subcontractors facing penalties are actually owed a refund. Use our free calculator to find out your position before you contact HMRC.
CHECK YOUR REFUND NOW →✓ No registration required · 100% Free · HMRC 2026/27 Aligned
The Full Penalty Structure: What HMRC Charges and When
HMRC's penalty system escalates automatically in stages. Here's exactly what happens after each threshold is crossed.
Applied automatically the day after the deadline. No exceptions, no grace period. Applies even if your tax liability is zero or HMRC owes you a refund.
HMRC charges £10 per day for up to 90 days. These accumulate on top of the initial £100 — so by the end of this period you could owe £1,000 in penalties alone, before any tax is considered.
Whichever is greater. This is charged on top of all previous penalties. If your tax bill is £4,000, this adds £200. If it's £8,000, it adds £400. And if you owe nothing, the £300 minimum still applies.
Another charge at the same rate. In cases where HMRC determines you deliberately withheld information, this can rise to 70% or even 100% of the tax due — though for standard late filing this is rare.
What the Penalty Looks Like in Practice
Two subcontractors, same situation — both missed the 31 January deadline. One filed 2 months late, one left it 8 months.
* Interest estimate based on £3,000 unpaid tax at HMRC's current late payment rate. Actual figures vary — use these as illustrations, not exact forecasts.
What About Late Payment — Separate from Filing?
Missing the filing deadline and missing the payment deadline are two separate issues — and HMRC charges for both independently.
If you file on time but don't pay by 31 January, HMRC charges interest from the day after the deadline. After 30 days unpaid, a 5% late payment surcharge is added to the tax outstanding. After 6 months and 12 months, further 5% surcharges apply.
Key point: You can file on time and still face late payment penalties — and you can pay on time but still face late filing penalties. The two systems run independently. File early and pay early to avoid both.
Can HMRC Cancel the Penalty? Reasonable Excuse
HMRC will remove penalties if you have a reasonable excuse — but the bar is higher than most people expect.
What qualifies as reasonable excuse
- Serious or life-threatening illness (yours or an immediate family member's)
- Bereavement of a close family member shortly before the deadline
- A technical failure on HMRC's own systems (with reference number)
- Postal delays entirely outside your control (for paper returns)
- A natural disaster or fire that destroyed your records
What does not qualify
- Forgetting the deadline or not receiving a reminder from HMRC
- Not understanding that you needed to file
- Being too busy with work
- Relying on an accountant who didn't file on your behalf
- Not having the money to pay (note: this is not a reason to delay filing)
How to appeal: Write to HMRC within 30 days of receiving the penalty notice. Explain the reason clearly, include any supporting evidence (medical letters, death certificates, HMRC system error references), and request that the penalty be cancelled. HMRC will respond in writing.
What to Do If You've Already Missed the Deadline
-
1File immediately — even if you can't pay
Every day you delay adds to your penalty exposure. Filing stops the daily £10 charges and percentage penalties from accumulating further. File now, sort the payment separately.
-
2Calculate your actual tax position
Many subcontractors who've missed filing deadlines are actually owed a refund — their CIS deductions exceed their tax liability. Your penalty will be offset against any refund due, reducing the net cost.
-
3Contact HMRC if you can't pay the full amount
Call HMRC's Self Assessment helpline on 0300 200 3310 and request a Time to Pay arrangement. HMRC can spread your bill over monthly instalments — typically up to 12 months. Interest continues to accrue, but surcharges are suspended while the arrangement holds.
-
4Appeal if you have a reasonable excuse
If your situation genuinely qualifies, write to HMRC within 30 days of the penalty notice. Include evidence. HMRC processes appeals in writing — there's no phone appeal process for penalties.
-
5Set up reminders for future deadlines
HMRC does not guarantee reminder letters. Set your own calendar alerts: 1 October (start gathering records), 1 December (file early), 31 January (final online deadline + payment due).
Filing early eliminates the risk entirely. The Self Assessment portal opens on 6 April — the day the new tax year begins. You can file your 2025/26 return from 6 April 2026 onwards. Most CIS subcontractors who file in April or May receive their refunds by June.
The Hidden Cost: Your CIS Refund Gets Delayed
There's a cost to late filing that goes beyond the penalty itself. If HMRC owes you a CIS refund — and for most subcontractors it does — that money sits unclaimed until your return is processed.
On a typical refund of £1,400–£2,500, delaying filing by 6 months means 6 months of that money sitting with HMRC rather than in your account. Add penalties on top, and the real cost of procrastination is considerably higher than the face value of the fines.
CIS Tax Rebate vs Tax Refund: What's the Difference and How Do You Claim? →
What Deadlines Apply to You in 2026/27
| Date | Action required | Who it affects |
|---|---|---|
| 6 April 2026 | Self Assessment portal opens for 2025/26 returns | All CIS subcontractors |
| 31 July 2026 | Second Payment on Account for 2025/26 (if applicable) | Those with POA obligations |
| 31 October 2026 | Paper return deadline for 2025/26 | Anyone filing by post |
| 31 January 2027 | Online return + all tax due for 2025/26 | All CIS subcontractors |
Self Assessment for CIS Subcontractors: Complete Guide →
Pre-Filing Checklist — What You Need Before You Start
- UTR number (10 digits, issued by HMRC when you registered for Self Assessment)
- CIS payment and deduction statements from all contractors you worked for
- Records of all business expenses with receipts or bank statements
- Mileage log if claiming vehicle costs (55p/mile from April 2026)
- Bank account details updated in your HMRC online account (for refunds)
- National Insurance number
- Any PAYE income (P60 or P45) if you had employment alongside CIS work
- Details of any other income: rental, savings interest, dividends
Frequently Asked Questions
What is the penalty for missing the Self Assessment deadline?
The immediate penalty is £100, applied the day after the deadline passes — even if you owe no tax at all. After 3 months, daily penalties of £10 begin (up to £900 over 90 days). After 6 months, a further 5% of the tax due or £300 (whichever is greater) is added. After 12 months, another 5% or £300 is charged. Total penalties on a late return can exceed £1,600 before interest on unpaid tax is added.
What if I owe no tax — do I still get a penalty for filing late?
Yes. The £100 fixed penalty applies regardless of your tax position. Even if HMRC would owe you a refund, you still receive the penalty for filing after the deadline. Filing on time is the only way to avoid it.
Can HMRC cancel a Self Assessment penalty?
Yes, if you have a reasonable excuse. HMRC accepts reasons such as serious illness, bereavement of a close family member, technical failures on HMRC's systems, or circumstances genuinely outside your control. Poor organisation, forgetting the deadline, or not receiving a reminder do not qualify. You must appeal in writing within 30 days of the penalty notice.
What happens if I can't pay my Self Assessment bill?
Contact HMRC before the deadline and request a Time to Pay arrangement. This allows you to spread your tax bill over monthly instalments — typically up to 12 months. Interest applies on the amount outstanding but the late payment penalties are suspended while the arrangement is in place. Do not ignore the bill — HMRC's enforcement escalates quickly once deadlines are missed.
How long does HMRC give you before taking further action?
HMRC sends a penalty notice after the missed deadline and escalates automatically at 3, 6, and 12 months. If unpaid tax remains outstanding beyond 30 days after the due date, a 5% surcharge is applied. After 6 and 12 months, further 5% surcharges are added. HMRC can ultimately pursue debt collection through the courts or instruct bailiffs — so early contact is always the right move.
Does missing a Self Assessment deadline affect my CIS refund?
Yes, indirectly. If you haven't filed, HMRC cannot calculate or pay your CIS refund. Any money owed to you by HMRC sits unclaimed until your return is processed. Penalties also accumulate in the meantime, which are offset against any refund due — reducing the net amount you receive.
Ready to claim your CIS refund? Use our free calculator to find out what HMRC owes you.
CHECK YOUR REFUND NOW →Need professional invoices fast? Create clean, compliant invoices in seconds with our free tool.
TRY FREE INVOICE GENERATOR →