What changes to UK Corporation Tax filing rules apply to companies in the 2024/25 financial year?

Overview: what “changes” means for 2024/25

When people ask what has “changed” about UK Corporation Tax filing rules in the 2024/25 financial year, they often mean one of three things:

First, whether the legal filing obligations themselves have moved (for example, deadlines, what must be included in a Company Tax Return, and whether filing must be online). Second, whether HMRC has introduced new forms, data fields, validation checks, or mandatory attachments that change how you prepare and submit a return. Third, whether connected company-law reforms (especially at Companies House) affect the practical route you use to file accounts and tax information, even if the core Corporation Tax law has not been rewritten.

In practice, 2024/25 (the UK financial year running from 1 April 2024 to 31 March 2025) is a period where the headline Corporation Tax filing framework remains familiar for most companies, but the “how” and “what extra information” around certain claims has become more demanding. If your company claims specialist reliefs (particularly R&D or creative industry reliefs/credits), you are far more likely to experience genuine filing-rule change in this period than a straightforward trading company with no special claims.

Quick refresher: what counts as the “2024/25 financial year” for Corporation Tax?

UK Corporation Tax is assessed by reference to a company’s accounting period (usually its financial year, but it can be shorter or longer in some circumstances). The term “2024/25 financial year” is often used in government announcements and administrative planning, and it runs from 1 April 2024 to 31 March 2025.

That is not the same thing as your company’s own accounting period unless your year-end happens to align. Many companies have year-ends such as 31 March, 30 June, 30 September, or 31 December. So, a company with a 31 December 2024 year-end straddles the 2024/25 financial year only partially (April–December 2024). Another company with a 31 March 2025 year-end fits perfectly within the 2024/25 financial year.

Why does this matter? Because many “changes” are triggered by a specific start date tied to accounting periods (for example, “accounting periods beginning on or after 1 April 2024”). Two companies can be preparing returns during the same calendar months but be caught by different rule sets because their accounting period start dates differ.

What has NOT changed: the core obligation to file a Company Tax Return

For most companies, the basic filing obligations in 2024/25 remain stable:

You must submit a Company Tax Return (typically the CT600 and supporting iXBRL attachments) to HMRC for each accounting period for which you are within the charge to Corporation Tax. Your return must include your tax computation and statutory accounts in the required electronic format, unless you are legitimately exempt from online filing.

The familiar timing framework still applies in the usual way for many companies: the tax return is due within 12 months after the end of the accounting period, while the Corporation Tax payment deadline is often earlier (commonly 9 months and 1 day after the period end for “standard” companies, with different payment regimes for larger companies). The key point for 2024/25 is that the deadlines themselves are not the main story; the story is increasingly about data quality, digital submission routes, and additional information requirements for claims that HMRC views as higher-risk.

Change 1: mandatory “additional information” for creative industry claims (from 1 April 2024)

One of the clearest filing-rule changes that applies squarely in the 2024/25 financial year is the introduction of a mandatory additional information form for creative industry tax reliefs and creative industry expenditure credits.

From 1 April 2024, companies making claims in the creative industries space are required to submit an additional information form alongside the Company Tax Return. This is not merely “nice to have” supporting detail. It changes the practical filing package: your CT600 submission is no longer complete (for these claims) unless the required additional information is provided in the prescribed way and at the right time.

The requirement also alters process. The additional information must typically be filed before or on the same day as the Company Tax Return. That means teams can no longer treat “supporting evidence” as something to be prepared later in case HMRC asks. The submission itself becomes the moment at which your support must be organised, structured, and ready to pass validation.

If your company has historically claimed film, high-end television, children’s television, animation, video games relief, or newer expenditure credit variants, 2024/25 is the year you should expect more structured data demands. Practically, that often means:

• allocating ownership of the creative claim pack earlier in the timetable;
• ensuring your accounting and production records reconcile cleanly to the figures you claim;
• checking that your software or agent workflow can create and submit the additional information form correctly; and
• building in time for validation errors and resubmission if your first attempt is rejected.

Even if the underlying relief economics are familiar to you, the administrative bar is higher. Companies that treat this as a “last-minute formality” are more likely to miss the procedural requirement and end up with a claim that is delayed, queried, or rejected.

Change 2: the merged R&D scheme affects what you report and how you support it (periods beginning on/after 1 April 2024)

R&D is another area where 2024/25 is significant, not because all companies suddenly claim R&D, but because the companies that do claim face a changed landscape.

For accounting periods beginning on or after 1 April 2024, R&D relief rules were reshaped through the introduction of a merged scheme structure (with different treatment depending on intensity and eligibility in certain circumstances). This type of policy change is not just about how much relief you get. It also impacts how you complete the return because the scheme mechanics determine which boxes you use, which supplementary pages apply, and what explanatory material HMRC expects to see in your computations and attachments.

On top of that, the separate “additional information form” requirement for R&D claims (introduced earlier) continues to bite. In 2024/25 it has become part of the everyday filing reality for R&D claimants. So, when you ask “what filing rules changed,” one answer is: the filing package and the evidence expectations for R&D claims are now more structured, more standardised, and less tolerant of vague narratives.

For companies in 2024/25, the practical implications include:

• confirming whether your accounting period start date triggers the post-1 April 2024 regime;
• ensuring you understand which R&D mechanism you are using and that your CT600 entries match it;
• preparing a computation narrative that clearly explains the claim methodology (because HMRC’s risk filters are sensitive to inconsistencies); and
• treating the “additional information” step as a core deliverable with its own deadlines, not an optional appendix.

If you file without aligning the numbers, the narrative, and the prescribed forms, you increase the likelihood of enquiry or processing delay—both of which can create cashflow issues if you are expecting a payable credit.

Change 3: increased emphasis on digital validation, formats, and iXBRL discipline

Many companies experience “rule change” as a sudden rejection message: the submission is not accepted because a field is missing, a tag is wrong, or a format does not match current validation rules. In 2024/25, this reality matters more because HMRC and Companies House systems have continued to evolve their digital requirements.

For Corporation Tax, online filing typically involves:

• CT600 (the structured tax return);
• iXBRL-tagged accounts (or appropriately formatted accounts submitted in iXBRL); and
• an iXBRL computation (or a computation in an accepted electronic form), plus any supplementary pages that apply.

Even where the law says “file a return,” the practical gatekeeper is the software and the validator. If you are using older accounts production tools, older tax software, or a workflow that relies on manual conversions, 2024/25 is a good moment to reassess. The risk is not that you are “non-compliant” in theory, but that you cannot get a return accepted on time because your submission package fails validation.

In particular, companies that prepare bespoke accounts formats, use unusual tagging approaches, or operate group structures (where multiple entities may share templates) should be careful. A small template issue can replicate across entities and become a multi-return filing problem right at the deadline.

Practical steps that help in 2024/25 include:

• running “test validations” earlier than you used to, especially if your accounts layout changed;
• ensuring your software supports current taxonomies and that your accounts team and tax team agree on who owns tagging quality;
• reviewing any rejection messages carefully and documenting fixes so they can be repeated consistently; and
• keeping records of exactly what was submitted (not just the final PDF accounts) so you can evidence what HMRC received.

Change 4: Companies House reform affects the joint-filing environment

Although Corporation Tax is administered by HMRC, many companies experience compliance as a joined-up process: statutory accounts go to Companies House, and the accounts and computation support the tax return to HMRC. During the 2024/25 financial year, the wider reform programme at Companies House (driven by economic crime legislation and an increased focus on data integrity) has practical implications.

From early March 2024, Companies House gained stronger powers to query information and introduced new requirements around key company details such as registered office addresses, confirmation statement content, and lawful purpose statements. These are not “Corporation Tax rules” as such, but they affect company administration and can influence how smoothly your annual cycle runs—especially if you rely on a single compliance timetable that bundles accounts filing, confirmation statements, and tax preparation together.

The important takeaway for 2024/25 is that Companies House is moving towards a more active gatekeeper role, and that increases the value of consistency across your filings. If the registered office address is challenged, if names or details are queried, or if corporate information looks inconsistent across periods, you may spend more time on administrative resolution—time you might otherwise spend finalising accounts and tax.

For groups and owner-managed businesses, this has a real practical consequence: the “company secretarial” details are no longer a background task that can be tidied up later. They should be part of the same governance rhythm as year-end accounts and tax.

Change 5: planning for the closure of the legacy HMRC/Companies House joint filing service (effective 31 March 2026, but relevant to 2024/25 returns)

A major practical change is on the horizon: the closure of the older online service used by some small companies to file company accounts and the Company Tax Return. While the closure date is 31 March 2026 (outside the 2024/25 financial year), it matters to companies with 2024/25 accounting periods because those returns are often filed during 2025 and 2026.

In other words, you can have an accounting period that falls within 2024/25, but your filing date may be close to (or after) the switchover if you file late in the window. Companies that used the older service because their affairs were “simple” should treat 2024/25 as the transition-planning year. That means choosing commercial software (or appointing an agent), learning the workflow, and ensuring you can retrieve and save past submissions in an accessible format before access ends.

Even if you are not worried about the 2026 deadline today, the smart move is to avoid being forced into a tool change at the last minute. If you trial software during the 2024/25 cycle, you can fix process issues while you still have time, rather than discovering them when a return is due.

Change 6: more structured disclosure culture for “higher scrutiny” items

Beyond specific new forms, a broader shift is visible in 2024/25: HMRC is increasingly designing Corporation Tax administration around structured disclosure. This is most obvious in areas like R&D and creative claims, but it also shows up in how companies handle attachments, narrative explanations, and consistency between computations and accounts.

Historically, many companies assumed that if they “got the numbers right,” the return would be accepted and any questions could be dealt with later. The direction of travel in 2024/25 is different. HMRC’s systems and compliance approach encourage:

• earlier, clearer disclosure of claim methodology;
• standardised additional information submissions where required;
• internal consistency (accounts, computation, CT600 entries all agreeing); and
• digital record-keeping that can support the return if HMRC asks follow-up questions.

This doesn’t necessarily mean every company faces more scrutiny, but it does mean the cost of poor organisation is higher. A company that cannot quickly produce a coherent audit trail may find that enquiries take longer and are more disruptive.

How these changes play out for common company types

It can help to translate the 2024/25 changes into what different kinds of companies actually need to do.

Owner-managed trading company with no special claims

If you are an owner-managed company with straightforward trading income and no R&D or creative claims, you may experience little “new” in the CT600 itself during 2024/25. Your main exposure is operational:

• keeping digital filing capability robust (software, tagging, authentication, access rights);
• ensuring Companies House data is up to date and not vulnerable to query; and
• planning ahead for the eventual retirement of older joint filing routes if you currently use them.

For many such businesses, the biggest risk is complacency: leaving software migration and compliance housekeeping too late, then discovering in a busy period that the tools or data are not ready.

Company claiming R&D

If you claim R&D, 2024/25 can be a turning point in administrative effort. Even if you’ve claimed successfully for years, you should assume:

• the claim needs more structured explanation and support than before;
• an additional information form is a procedural requirement, not optional; and
• scheme changes tied to accounting periods beginning on or after 1 April 2024 can affect the way figures flow through the return.

This is also an area where “filing rule changes” can appear as a software issue: if your software does not support the right boxes or supplementary pages, you may need to update, switch tools, or coordinate with an agent to file correctly.

Company claiming creative industry reliefs/credits

Creative claimants are among the most directly affected in 2024/25 because of the mandatory additional information form that applies from 1 April 2024. If you are in film, TV, animation, games, theatre, or related creative sectors, you should treat the claim as a project with a deliverables list, not simply a line in the computation.

Good practice includes:

• locking down the cost ledger mapping to qualifying/non-qualifying categories early;
• ensuring production finance teams and tax teams reconcile on the same numbers;
• agreeing who signs off the narrative and evidence pack; and
• submitting the additional information in the correct sequence relative to the CT600.

Groups and companies with multiple entities

Groups can be affected even when individual entities seem simple. If a group has one entity claiming R&D, another claiming a creative credit, and others trading normally, you can end up with multiple filing “standards” across the group.

In 2024/25, group controllers should consider creating a compliance matrix that records, entity by entity:

• which supplementary pages apply;
• whether an additional information form is required (and which one);
• which software is used for each entity (and whether versions are consistent);
• who owns tagging and submission; and
• the submission timetable, including validation time and contingency.

This sounds bureaucratic, but it often saves time. A group that standardises early can reduce the chance that one entity’s filing problem disrupts everyone else’s timetable.

Practical checklist for 2024/25 Corporation Tax filing

To make the 2024/25 changes actionable, here is a practical checklist you can apply to your next filing cycle. Think of it as a way to surface whether you are in a “no major change” position or whether you have additional procedural requirements.

Step 1: Confirm your accounting period start date

Many changes are triggered by “accounting periods beginning on or after 1 April 2024.” Confirm the start date of the period you are filing for, not just the year-end. If you changed your year-end, split periods, or had a short period, double-check the trigger dates.

Step 2: Identify whether you have claims that trigger extra filing requirements

Ask: are we claiming R&D relief/credit? Are we claiming creative industry reliefs/credits? Do we have any other specialist claims or supplementary pages? If yes, assume there are additional procedural requirements and build them into your timetable.

Step 3: Confirm your software/workflow can produce the required submission package

This is especially important if:

• you rely on older or low-cost filing tools;
• you previously used a legacy online service designed for simple businesses; or
• you have had filing rejections in the past due to formatting or tagging.

Run a dry check early. If you only discover incompatibility at the deadline, your choices become limited and stressful.

Step 4: Validate iXBRL tagging and attachment quality before the final week

Treat tagging and file structure as a first-class compliance requirement. Keep copies of what you submit, in a form you can reproduce later if needed. Make sure internal teams know where the “final submitted files” live.

Step 5: Align your Companies House administration with your tax timetable

Because Companies House is more active in querying or policing information quality, make sure your registered office address, confirmation statement information, and key company details are accurate and up to date. Don’t let basic administration derail the accounts-and-tax cycle.

Step 6: Build contingency for new “additional information” steps

If you are a claimant affected by mandatory additional information forms, treat them like a mini-project. Decide who drafts, who reviews, and who signs off. Keep a clear record of what was submitted and when.

Common pitfalls in 2024/25 (and how to avoid them)

Even well-run finance teams can fall into predictable traps when administrative rules evolve. In 2024/25, the most common pitfalls include:

Leaving additional information forms to the last minute. These forms often require input from outside finance (for example, technical leads for R&D, production teams for creative claims). If you only start gathering inputs in the final days, you risk missing the procedural submission window.

Assuming “the numbers are right” is enough. Structured disclosure means that numbers without a coherent explanation and evidence trail are more likely to attract questions or delay.

Relying on outdated software or templates. Validation failures can turn into deadline failures. Software changes are easiest when you have time, not when a return is due.

Separating Companies House admin from tax compliance. In a world of increased data scrutiny, inconsistencies and outdated details can generate friction and distraction at exactly the wrong time.

Avoiding these issues usually comes down to one thing: moving key checks earlier in the timetable and treating “digital acceptance” as part of compliance, not an afterthought.

So, what should companies do differently in 2024/25?

If you want a simple summary of what companies should do differently in the 2024/25 financial year, it’s this:

For straightforward companies, keep your filing process modern and robust: confirm your digital submission tools work, keep iXBRL and attachments clean, and align corporate admin with tax deadlines. For companies making R&D or creative claims, elevate compliance project management: identify the required additional information steps early, assign clear ownership, and build a defensible evidence trail that matches the figures in the CT600 and computation.

Finally, regardless of company type, treat 2024/25 as a planning window for the next stage of digital compliance. The direction of travel is clear: more structured information, more reliance on software, and less tolerance for informal or last-minute filing practices. Companies that adapt their workflow now will find future filing cycles less disruptive and more predictable.