Sourced statutory estimate
Estimate CT600 filing penalties with calendar-month stages, deadline-based flat rates and the correct unpaid-tax snapshot.
Effective scope: standard filing deadlines before/from 1 April 2026. Last reviewed 26 September 2026.
Estimate CT600 filing penalties with calendar-month stages, deadline-based flat rates and the correct unpaid-tax snapshot.
Standard return with a deadline 12 months after accounting-period end. No late notice, extended accounts deadline, multiple returns for long accounts, appeal or disputed tax assessment. The unpaid-tax input is the amount unpaid when the first tax-geared penalty arises, not today's balance.
Rule summary
No filing penalty is charged for filing on or before the deadline. Ordinarily a Company Tax Return is due 12 months after accounting-period end; payment is normally due earlier and is a separate obligation.
For filing deadlines before 1 April 2026 the initial flat amount is £100, with another £100 if filed more than three calendar months late. For deadlines on/after that date the amounts are £200 and another £200. Finance Act 2026 section 265 makes the filing deadline the transition test, not the date the return is submitted.
If the previous two returns were also liable to flat late-filing penalties, the current third-successive-failure amounts are £500 plus £500, or £1,000 plus £1,000 in the new regime.
HMRC calendar periods start on the day after the anchor date and expire at the end of the complete period; the next penalty stage starts the following day. For a 30 September 2025 period end, the first tax-geared stage starts on 1 April 2027, not 31 March 2027. For a 30 April 2026 filing deadline, the three-month flat-penalty period ends on 31 July and the additional flat stage starts on 1 August. For the supported standard deadline, filing after 18 months from period end attracts 10% of tax unpaid when this first percentage penalty arises. Filing after two years attracts 20% in total on that same snapshot. Use tax payable for the period after relevant set-offs, including any applicable Section 455 charge, unpaid at that first incurrence. Do not substitute the remaining balance at the later stage. These are filing penalties, not late-payment interest.
Worked examples
Period ends 31 March 2025, deadline 31 March 2026; filing on 1 October 2026, with £10,000 unpaid when the percentage penalty first arose.
This deadline is before 1 April 2026: £100 + £100 + £1,000 = £1,200, despite filing in the later regime.
Period ends 30 April 2025, deadline 30 April 2026, return filed 1 August 2026, no repeat failures.
More than three calendar months late: £200 + £200 = £400; no tax-geared stage yet.
Period ends 30 September 2025; standard filing deadline is 30 September 2026. At the first tax-geared incurrence, £20,000 is unpaid. No repeated defaults.
Filed 31 March 2027: £400 flat penalties only. Filed 1 April 2027: £400 plus 10% × £20,000 = £2,400. HMRC CTM94070 gives this September-year-end boundary; the same snapshot is used if the total percentage rises to 20% from 1 October 2027.
Check the dates and balances against the relevant HMRC account and records.
Use the paired guide for the rule basis, exceptions and next steps.
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Read the supported rule, assumptions and worked examples behind this tool.
Assumptions and limits
This tool gives a practical estimate and depends on the assumptions shown below.
Reviewed on . Preserved source review from the accepted scoped statutory repair; this contract migration does not re-date that review.
Applies to: Standard filing deadlines before/from 1 April 2026; flat and tax-geared stages have distinct effective rules.
Reviewed scope: Standard return/deadline and correct unpaid-tax snapshot; no late notice, long-account multiple returns, appeal or disputed assessment.
Reviewed model: ct600-late-filing-penalty-calculator: standard filing deadlines before/from 1 April 2026; reviewed 26 September 2026