The two deadlines every CT600 workflow should separate

A Company Tax Return has a filing deadline and a payment deadline. They are related, but they are not the same control. HMRC's main Company Tax Returns guide says the return deadline is 12 months after the end of the accounting period it covers. The Corporation Tax payment deadline is usually 9 months and 1 day after the end of the accounting period.

That gap creates a common practice problem: the company often needs to know its tax liability before the CT600 itself is due. A good CT600 workflow should therefore treat tax calculation, client approval, payment instruction, and final filing as connected stages, not as one last-minute upload.

What counts as the CT600 package?

A Company Tax Return is more than the main CT600 boxes. The filing package normally includes the CT600, any required supplementary pages, accounts, computations, and supporting detail. For most companies, accounts and computations need to be included in iXBRL format when filing online.

That matters for deadlines because a return can be operationally late long before it is legally late. Missing accounts, an unapproved computation, an unresolved CT600A loan question, or a client who has not approved final figures can block the filing even when the core tax calculation is nearly finished.

Late filing penalties

HMRC charges penalties when a Company Tax Return is filed after the deadline. The standard GOV.UK penalty ladder starts with a fixed penalty when the return is one day late, then adds another fixed penalty after three months. If the return is more than six months late, HMRC can estimate the Corporation Tax due through a tax determination and add a tax-related penalty. A further tax-related penalty can arise after 12 months.

HMRC also says the initial fixed penalties can increase for companies that are late three times in a row. For accountants, repeated lateness is not just a cost issue. It is evidence that the internal workflow needs stronger client-chasing, earlier accounts intake, and clearer approval deadlines.

Tax determinations are a red flag

If a return is more than six months late, HMRC may issue a tax determination: HMRC's estimate of the Corporation Tax bill. GOV.UK says a company cannot appeal against the determination itself. The company still needs to file the return, pay the Corporation Tax due, and HMRC will recalculate interest and penalties from the filed figures.

In workflow terms, a determination should trigger an immediate recovery process: confirm the accounting period, complete accounts and computations, resolve supplementary-page points, file the return, and reconcile any amount already paid or demanded.

Deadline controls for accountancy practices

Practices need a deadline system that does more than list annual return dates. Useful controls include:

Accounting period end and CT600 filing deadline for every company.
Separate Corporation Tax payment deadline and payment-reference status.
Accounts received, trial balance imported, and iXBRL attachment status.
Supplementary-page review flags, including CT600A, CT600C, CT600L, CT600N, and CT600P where relevant.
Manager review status and query list before client approval.
Client approval date, approver identity, and final filing authority.
HMRC submission acknowledgement, rejection, amendment, or determination status.

The highest-risk cases are usually not the large clean jobs. They are small owner-managed companies with director loans, uncertain dividends, late bookkeeping, missing accounts, or a belief that no tax due means no filing risk.

What directors often get wrong

Directors often focus on the tax payment and forget the separate filing obligation. A nil-tax return can still be due if HMRC has issued a notice to deliver a Company Tax Return. Dormant or near-dormant companies also need careful handling because Companies House accounts filing and HMRC Corporation Tax filing are separate obligations.

A director who previously used HMRC's joint online accounts and Company Tax Return service also needs a new route. That service closed on 31 March 2026, so directors who still need to file a CT600 generally need commercial software or professional help.

How Robocount supports deadline-safe CT600 work

Robocount is built for the full CT600 workflow: preparation, computation, supplementary-page review, iXBRL attachments, approval, filing, and post-filing evidence. Deadline safety depends on making blockers visible before the final filing date.

Connects CT600 figures to supporting computations and attachments.
Keeps supplementary-page review visible in the filing workflow.
Helps practices separate payment advice from final return submission.
Gives directors and accountants a clearer review trail before approval.
Supports evidence retention for HMRC acknowledgements, rejections, and amendments.

FAQ

When is a CT600 due?

HMRC's general rule is that the Company Tax Return is due 12 months after the end of the accounting period it covers. Always check the company's actual accounting period and any HMRC notice to deliver a return.

When is Corporation Tax due?

For many companies, Corporation Tax is due 9 months and 1 day after the end of the accounting period. Some companies have different payment rules, so practices should review payment timing separately from the filing date.

What happens if a CT600 is late?

HMRC can charge fixed penalties, tax-related penalties, and interest. If the return is more than six months late, HMRC may issue a tax determination estimating the tax due.

Does no Corporation Tax due mean no CT600 is needed?

Not automatically. If HMRC has asked for a Company Tax Return, the company usually needs to file one even if no tax is due.

Useful HMRC references

This guide is general product and filing workflow information, not tax advice. Check current HMRC guidance and the company's facts before filing.

CT600 filing deadlines, payment dates, and late return penalties.