Why I Cannot File for Accounting Periods Ending After 31 March 2026

id: ct600-april-2026-taxonomy-limit title: Why I Cannot File for Accounting Periods Ending After 31 March 2026 category: ct600-filing tags: [taxonomy, CT2025, period end, April 2026, not supported, filing blocked, CT2024, iXBRL, corporation tax] applies_to: [trading, dormant, all] difficulty: beginner last_updated: 2026-04-07 related: [extended-accounting-period, editing-period-dates]

If your accounting period ends after 31 March 2026, TinyTax will show you this message when you try to file:

> Period end date (X April 2026) is after 31 March 2026. The updated taxonomies required for these periods are not yet supported. Please check back later and try again once they are available.

This is a known temporary limitation. Here is what it means and what to do.

Why Does This Happen?

Corporation tax returns filed with HMRC must use an approved iXBRL taxonomy — a technical standard that defines how financial data is tagged and validated by HMRC's systems. Each taxonomy version covers a specific range of period end dates:

Taxonomy	Covers Periods Ending
CT2023	1 April 2022 – 31 March 2024
CT2024	1 April 2024 – 31 March 2026
CT2025	1 April 2026 onwards — not yet published by HMRC

The current taxonomy (CT2024) only covers periods ending up to 31 March 2026. For periods ending from 1 April 2026 onwards, HMRC requires CT2025 — a new version that HMRC has not yet released. Until HMRC publishes CT2025, TinyTax cannot generate a valid CT600 or iXBRL accounts for these periods.

When Will Support Be Available?

We are monitoring HMRC's official taxonomy publication page and will add support as soon as CT2025 is released. HMRC does not announce a fixed release date in advance.

ℹ We will update this guide and add support for periods ending after 31 March 2026 as soon as CT2025 is published. Check back in a few weeks.

Do I Need to File Urgently?

For most companies, no. Corporation tax returns are due 12 months after the end of the accounting period:

Period End	CT600 Deadline
5 April 2026	5 April 2027
30 April 2026	30 April 2027
30 June 2026	30 June 2027
31 March 2027	31 March 2028

There is no need to rush — TinyTax will support your period well before your deadline.

What Should I Do Now?

Simply check back in a few weeks. Once TinyTax adds CT2025 support, the error message will disappear and you can complete your filing as normal.

If your filing deadline is approaching sooner than expected, please contact us and we will advise on your options.

Common Questions

I read that HMRC published CT2025 in early 2026 — why can't I still file?

There is an important distinction between two things HMRC does at different times:

Updating their guidance page — HMRC sometimes lists an upcoming taxonomy on their Taxonomies Accepted by HMRC page before the technical files are ready. This announces that CT2025 will be required, but does not make it usable yet.

Publishing the technical schema files — Filing software like TinyTax needs to download the actual XSD schema files (the machine-readable taxonomy package) and integrate them before it can generate valid CT600 returns. These files are separate from the guidance page and are released later.

As of the date this article was last updated, the CT2025 technical schema files are not yet available. We check regularly and will implement support as soon as they are.

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