Quick Answer
From 1 April 2026, all UK limited companies must file annual accounts with Companies House in iXBRL format using commercial software. The free WebFiling service and HMRC's CATO service both closed on 31 March 2026. You need commercial software before your next filing deadline.
What Changed on 1 April 2026: iXBRL Accounts Filing Is Now Mandatory
From 1 April 2026, the way UK limited companies file annual accounts with Companies House changed permanently. The free WebFiling service for accounts closed on 31 March 2026, and iXBRL (Inline eXtensible Business Reporting Language) format is now mandatory for all submissions. Accounts filed via paper or through the old WebFiling portal are no longer accepted.
The change is driven by the Economic Crime and Corporate Transparency Act 2023 (ECCTA), which gives Companies House new powers to scrutinise, validate, and query submitted accounts data. iXBRL tagging allows Companies House systems to read individual line items in your accounts automatically. The official guidance is on GOV.UK: Use a software provider to prepare package accounts for Companies House.
At the same time, HMRC closed its free Company Accounts and Tax Online (CATO) service on 31 March 2026. This changes the filing workflow for every UK limited company director, not just those who handled their own accounts.
Who Is Affected: Every UK Limited Company Without Exception
The iXBRL mandate applies to all UK limited companies regardless of size, revenue, or trading status. There is no small company exemption.
- Micro-entities (turnover up to £1 million, balance sheet up to £500,000, up to 10 employees — thresholds increased from 6 April 2025) must file using the FRS 105 iXBRL taxonomy
- Small companies filing full or abridged accounts must use the small companies iXBRL taxonomy
- Dormant companies that still file dormant accounts with Companies House are included
- Companies filing under FRS 102 (the standard used by most medium and larger limited companies) must also file in iXBRL
For UK digital and creative agency directors, this typically means your agency itself (the limited company) must file iXBRL accounts. If you also have a holding company or a separate property company, each entity has its own filing obligation.
The Hidden Trap: Joint Filing Has Ended
Most agency directors who "handled their own CT600" were using HMRC's CATO service, often without realising that CATO was also acting as their iXBRL conversion layer. CATO took plain financial data and prepared a simple Corporation Tax return and accompanying accounts in one workflow. That route is now gone.
The practical consequence is that the joint filing route — one submission that simultaneously satisfied both Companies House and HMRC — has ended for most companies. The two filings are now separate:
- Annual accounts to Companies House — in iXBRL format via commercial software, within 9 months of your accounting year-end
- CT600 Corporation Tax return to HMRC — also in iXBRL format, within 12 months of your accounting year-end (payment due within 9 months and 1 day)
If you are on FreeAgent, CT600 filing is included in the subscription for micro-entities, so FreeAgent handles both filings within a single platform. If you are on QuickBooks, note that QuickBooks does not currently file CT600 directly to HMRC. You would need a separate CT600 tool such as TinyTax or Taxpipe to complete the HMRC filing.
This is the gap many directors are discovering: software that handles bookkeeping and VAT returns does not automatically handle CT600 filing. Check your software before your next deadline.
If you are unsure how your current setup fits together, our guide to Making Tax Digital for agencies covers the broader compliance picture, and our MTD for Income Tax guide for limited company directors addresses a related but separate change that went live on 6 April 2026.
Which Software Do You Need for iXBRL Accounts Filing?
GOV.UK lists approved Corporation Tax software suppliers at gov.uk/government/publications/corporation-tax-commercial-software-suppliers. The right choice depends on whether you file yourself or use an accountant, and whether you need CT600 as well as Companies House accounts.
Software options for small UK limited companies
- FreeAgent — Subscription from around £14.50/month. Includes CT600 filing for micro-entities within the same platform. Good fit if you want bookkeeping, VAT, and year-end accounts in one tool.
- TinyTax — From around £20/year for micro-entities. Standalone tool for iXBRL accounts and CT600 filing. Straightforward option for directors who handle their own filing without a full bookkeeping system.
- Taxpipe — Approximately £59 per filing. Pay-per-use model for CT600 and accounts. No annual subscription needed, which suits companies that only file once a year.
- Xero — Subscription from £16/month. Handles Companies House accounts filing. CT600 requires a third-party add-on such as Taxfiler. Common in agencies already using Xero for bookkeeping.
- Sage — Various plans including Sage One and Sage Accounting. Handles iXBRL accounts filing. CT600 availability depends on the plan.
If you already use Xero or QuickBooks for bookkeeping, check whether your plan includes Companies House and HMRC filing or whether you need an add-on.
For agencies comparing accounting software more broadly, our Xero vs QuickBooks guide for marketing agencies covers the differences in more depth. The CT600 filing gap is one of the practical distinctions covered there.
Step by Step: How to File iXBRL Accounts with Companies House
Once you have chosen your software, the filing process follows five steps. Your software handles the iXBRL tagging automatically — you do not need to understand XBRL schemas to file correctly.
- Check your next Companies House deadline. Private limited companies must file accounts within 9 months of their accounting year-end. Confirm your date on GOV.UK or in Companies House correspondence.
- Choose and set up your iXBRL software. Select a product from the GOV.UK-approved list that covers your company size and filing needs. Set it up before your deadline, not on the day.
- Enter your financial data and generate accounts. Input your income, expenses, assets, and liabilities for the accounting period. The software generates iXBRL-tagged accounts using the correct Companies House taxonomy for your company type.
- File to Companies House. Submit the iXBRL package via the software's direct integration or through the Companies House software filing API. You will need your Companies House authentication code, which is a six-character code sent by post when you registered.
- File your CT600 to HMRC separately. Submit your Corporation Tax return using the same software (if it supports CT600) or a standalone tool. The CT600 deadline is 12 months after your accounting year-end. Payment is due within 9 months and 1 day.
If you have lost your Companies House authentication code, you can request a new one through the Companies House service. Allow several days for postal delivery.
Late Filing Penalties to Avoid
Failing to file accounts on time with Companies House carries automatic financial penalties. For private limited companies, the penalties are:
- Up to 1 month late: £150
- 1 to 3 months late: £375
- 3 to 6 months late: £750
- More than 6 months late: £1,500
These penalties double if accounts are filed late for two consecutive years. In persistent cases, Companies House can apply to strike off the company, which creates significant complications for directors and shareholders.
The transition to iXBRL filing does not extend your deadline. If your accounts are due on 31 August 2026, they must be filed by that date in iXBRL format regardless of whether you have set up new software.
Do not assume your accountant has this covered
If you use an accountant but have not spoken to them since March 2026, confirm they are filing via iXBRL-compliant software. Some smaller practices were slow to migrate. Your filing obligation sits with you as a director — a missed deadline from a software failure at your accountant does not remove your personal liability for the penalty.
What Agency Directors Specifically Need to Check
For digital and creative agency directors, the iXBRL change surfaces a few practical questions that general guidance does not always address.
Are you on a calendar year or a split year-end? Many agencies incorporated mid-year have year-ends that do not align with the April tax year. Your Companies House accounts deadline is 9 months after your accounting year-end, not 9 months after April. If your year-end is 31 March, your accounts are due by 31 December 2026.
Do you have a holding company or group structure? Each entity in a group files separately. If you separated your trading company from an IP holding company, each needs its own iXBRL accounts and CT600. The same applies to any dormant shell you have not yet dissolved.
Has your accountant switched software? Some practices migrated their client base to a new system during Q1 2026. If your accountant sends you documents from a different system than before, or mentions that accounts look slightly different this year, this is likely the reason. Ask them which software they are using and whether your filing workflow has changed.
If your agency is going through a growth phase and you are spending more time on financial admin than on clients, it may be worth considering whether a specialist accountant can absorb this complexity. Our guide to switching accountants in the UK covers the process, including professional clearance and mid-year handovers.
More broadly, staying on top of compliance deadlines is easier when you have monthly management accounts telling you where the business actually stands. Our salary calculator is a useful starting point if you are also reviewing how to structure your own director remuneration for the 2026/27 tax year.
How Alto Can Help
Alto Accounting is an ACCA registered practice specialising in UK agencies. We handle iXBRL accounts preparation and filing, CT600 submissions, and the full year-end compliance cycle for agency founders and directors. If your current setup leaves you unsure whether your accounts are being filed correctly and on time, we can review the position and take it over if needed.
Book a free consultation to speak with a specialist.
Frequently Asked Questions
What is iXBRL and why is it now mandatory for UK companies?
iXBRL stands for Inline eXtensible Business Reporting Language. It is a digital tagging format that allows regulators and software to read, compare, and validate financial data automatically. From 1 April 2026, all UK limited companies must submit annual accounts to Companies House in iXBRL format using commercial software. This was driven by the Economic Crime and Corporate Transparency Act 2023 (ECCTA), which gives Companies House new powers to verify and analyse accounts data digitally.
Does the iXBRL mandate apply to dormant companies and micro-entities?
Yes. The requirement applies to all UK limited companies regardless of size or trading status. Micro-entities filing abbreviated accounts under FRS 105, small companies filing full or abridged accounts, and dormant companies filing dormant accounts all need commercial iXBRL software from 1 April 2026. There is no size exemption. The only distinction is the iXBRL taxonomy your software uses: FRS 105 accounts use the Companies House small companies taxonomy rather than the full UK GAAP taxonomy.
Can I still file my own company accounts without an accountant?
Yes, but you now need commercial software to do it. HMRC's free Company Accounts and Tax Online (CATO) service, which allowed directors to file CT600 returns and prepare simple accounts without software, closed on 31 March 2026. You can file yourself using affordable software such as TinyTax (from around £20 per year for micro-entities), FreeAgent (included in its monthly subscription), or Taxpipe (approximately £59 per filing). Alternatively, you can appoint an authorised agent to file on your behalf.
Do I now need to file separately with Companies House and HMRC?
In most cases, yes. The joint filing route that allowed you to submit accounts and your CT600 to both Companies House and HMRC in a single transaction through the CATO service has ended. You typically need to file your annual accounts with Companies House and your Corporation Tax return (CT600) with HMRC separately. Some commercial software products include both in a single workflow, but they are submitted to two different systems. Check whether your chosen software handles one or both filings.
What are the late filing penalties if I miss the Companies House deadline?
Companies House late filing penalties depend on how late the accounts are. For private companies, the penalties are: up to 1 month late = £150; 1 to 3 months late = £375; 3 to 6 months late = £750; more than 6 months late = £1,500. These penalties double if accounts are filed late for two consecutive years. In serious cases, Companies House can also strike off the company from the register.
Which accounting software supports iXBRL filing for Companies House?
GOV.UK publishes a list of approved commercial software suppliers for Corporation Tax at gov.uk/government/publications/corporation-tax-commercial-software-suppliers. Common options used by UK agencies and small limited companies include FreeAgent, Xero (with a third-party CT600 add-on), Sage, QuickBooks (with a separate filing tool), TinyTax, and Taxpipe. FreeAgent is notable because CT600 filing is included in its monthly subscription and covers micro-entities directly.
What happened to HMRC's Company Accounts and Tax Online (CATO) service?
HMRC's CATO service, also known as the free online Company Tax Return service, closed permanently on 31 March 2026. It had been available since 2007 and allowed smaller companies to prepare and file both their CT600 Corporation Tax return and Companies House accounts without paying for software. HMRC closed it because it could not support iXBRL tagging requirements or the enhanced validation rules introduced by the ECCTA 2023. Any data previously stored in CATO is no longer accessible.
Is there a free alternative to HMRC's CATO service?
No direct free replacement exists. HMRC has not announced a new free service. Some affordable options are available: TinyTax offers plans from around £20 per year for the simplest micro-entity accounts. FreeAgent has a free plan for sole traders, but limited company CT600 filing requires a paid subscription. Some accountants include software costs within their fees. The GOV.UK software finder tool lists approved products and their pricing so you can compare options before committing.