Filing Periods Ending After 31 March 2026

Why Can't I File?

HMRC requires all CT600 returns to use a specific iXBRL taxonomy — a standardised data format that changes each tax year. For periods ending after 31 March 2026, the required taxonomy is CT2025.

HMRC has not yet published CT2025. Until they do, no CT600 software — including TinyTax — can submit returns for these periods.

This is not a limitation of TinyTax. It is an HMRC dependency that affects all filing software providers equally.

Which Periods Are Affected?

Any CT600 with an accounting period end date of 1 April 2026 or later requires the CT2025 taxonomy.

Common scenarios:

What Should I Do?

If you're not in a rush: Simply wait. We are actively monitoring HMRC's taxonomy publications and will implement CT2025 support as soon as it becomes available. Your saved form data will not be lost.

If you have an urgent deadline: HMRC is aware that CT2025 has not been published. Filing deadlines for periods ending after 31 March 2026 will not result in penalties while the taxonomy remains unavailable. If you are concerned, you can contact HMRC's Corporation Tax helpline on 0300 200 3410 to confirm.

Will I Be Notified?

Yes. Once CT2025 support is live, we will notify affected customers by email so you can complete your filing straight away.

Companies House Accounts

The same limitation applies to Companies House accounts filings for periods ending after 31 March 2026. The FRC publishes a separate taxonomy for statutory accounts, and TinyTax will update both the CT600 and accounts taxonomies together.

Current Taxonomy Coverage

Last updated: April 2026