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CT600 filing deadlines

The ordinary CT600 deadline is the last day of a complete twelve-month filing period after the CT accounting-period end. Learn the leap-year rule and when notices or longer accounts give a different date.

Standard deadline — after checking the whole filing case

For the ordinary Company Tax Return case, the filing deadline is the last day of twelve months from the accounting-period end. HMRC CTM93040 gives 29 February 2028 for a period ending 28 February 2027; the complete period begins on 1 March 2027.

This is not a universal end-date-plus-one-year formula. The legal filing date is the latest applicable period end, including the notice-service and relevant period-of-account rules explained below. The paired calculator only determines the ordinary date when all its scope facts are confirmed.

The return filing deadline is separate from Corporation Tax payment and Companies House accounts filing. Paying tax on time does not fulfil the return obligation, and a Companies House accounts period may contain more than one CT accounting period.

Rule summary

HMRC filing periods and exceptions

Finance Act 1998 Schedule 18 paragraph 14 selects the latest applicable period end: twelve months from the CT accounting-period end, three months from service of the notice, and the relevant period-of-account rule. That accounts rule is twelve months from its end if it lasts at most 18 months, or 30 months from its beginning if longer.

HMRC CTM93040 explains the ordinary twelve-month date: normally the corresponding numbered day/month next year; 28 February immediately before a leap year gives 29 February, and 29 February gives 28 February next year.

A Companies House accounts period is not necessarily one CT accounting period. In HMRC CTM93050's 15-month accounts example, the return for a CT period ending 31 December 2018 is due 31 March 2020 because the whole accounts period ends 31 March 2019. Do not apply this calculator's ordinary rule to that case.

If you cannot confirm the notice and period facts, select Unsure: the result is review required, without an invented filing date. A longer or exceptional period is not automatically overdue twelve months after the first CT period ends.

Worked examples

How the rule behaves in practice

  • Ordinary non-month-end return: One CT accounting period and the whole period of account end on 4 April 2026; the notice-service rule does not give a later date and no exception applies. The complete twelve-month filing period runs from 5 April 2026 through 4 April 2027. The standard CT600 filing deadline is 4 April 2027.
  • February before a leap year: An ordinary supported CT period ends on 28 February 2027, immediately before the leap year 2028. The filing period runs from 1 March 2027 through 29 February 2028. The deadline is 29 February 2028, not 28 February. This follows HMRC CTM93040's explicit February rule.
  • Late notice or missing notice facts: HMRC CTM93050 example 2 has a period ending 31 December 2018 and a notice served on 31 October 2019. Alternatively, the user does not know when the notice was served. The official example's deadline is 31 January 2020 because the notice-service period ends later. Select No / unsure in this calculator: it returns review required, not 31 December 2019. Establish actual notice-service and accounts-period facts before determining the applicable date.

Practical consequences

  • A Corporation Tax accounting period cannot exceed twelve months. Longer accounts normally require separate CT returns; check the whole relevant period of account before deciding either return's filing date. Short CT periods still use the ordinary complete twelve-month filing rule when the supported facts hold.
  • An HMRC notice gives a three-month period after actual service; a late notice can make that date the latest statutory deadline. Service is not necessarily the issue or postage date. No notice, voluntary returns, uncertain service, extensions and specialist filing dates require review, not a guessed deadline.
  • Late filing can lead to fixed and later tax-geared penalties even where the tax payment date was met. Use the separate reviewed CT600 late-filing penalty calculator and guide with a verified applicable filing date; payment interest is a separate question.

If the verified filing date has passed

Use the CT600 late-filing penalty calculator for an estimate and the CT600 late-filing penalties guide for the stages and limitations. Keep tax payment and interest separate from return-filing penalties.

Important limits

  • A CT accounting period cannot exceed twelve months. Longer company accounts can span several CT periods and require more than one return. These cases, late/missing notices, voluntary returns, extensions and special filing dates need review; this form does not collect enough facts to resolve them.
  • Check actual service of HMRC's notice, not just its issue or postage date. Private/public company status does not change this CT filing rule. Companies House first-accounts rules are a different regime.

Primary sources and review date

Reviewed on . Preserved source review from the accepted scoped statutory repair; this contract migration does not re-date that review.

Applies to: Current CTSA ordinary filing-period rules; Finance Act 1998 Schedule 18 paragraph 14.

Reviewed scope: One CT period of at most twelve months coinciding with the whole period of account; served notice does not give a later deadline; no extension/special date.

Review basis and change triggers

Reviewed model: ct600-filing-deadline-calculator: Current CTSA ordinary filing-date rule; Finance Act 1998 Schedule 18 paragraph 14; reviewed 28 September 2026

  • A cited source or statutory regime changes.
  • Reviewed content, runtime adapter or domain implementation changes.
  • A new effective period falls outside the supported scope.

Turn the result into an action plan

  • Use the paired calculator only after confirming the CT period, whole accounts period, notice-service position and absence of a special date.
  • If any fact is missing or an exception applies, gather the notice and accounts records and apply HMRC CTM93030–CTM93050 before assessing lateness.
  • For a missed verified filing date, consult the CT600 late-filing penalty tool and guide; track payment independently in the Corporation Tax payment-deadline tool.

Use the tool

CT600 Filing Deadline Calculator

Calculate a standard Company Tax Return filing date using HMRC's complete twelve-month rule, including February/leap-year boundaries. Exceptional or uncertain cases require review.

FAQ

Is 28 February 2027 followed by a 28 February deadline?

No. In the ordinary filing case, HMRC CTM93040 gives 29 February 2028 because the following year is leap.

Can this calculate a long first accounts period?

Not from an end date alone. A long accounts period can contain multiple CT periods and the relevant accounts-period rule may give a later deadline. Select No / unsure and review the full facts.

Does a late HMRC notice affect the date?

Yes. The three-month period after actual service may end later than the ordinary twelve-month period. Missing or late notice facts are outside the calculator's supported case.