What changed for directors

Companies House announced that the joint online filing service used by many small companies would close on 31 March 2026. GOV.UK now points companies toward commercial software for Company Tax Return filing. That matters for directors who previously treated the CT600 as a short online add-on to accounts filing.

A Company Tax Return is not just the main CT600 form. HMRC guidance describes the return package as the CT600, company accounts, tax computations, any required supplementary pages, and supporting attachments. For most online filings, accounts and computations also need to be in the required electronic format, commonly iXBRL.

Can a director still self-file a CT600?

Yes, in the sense that a director can still prepare and file the company's Corporation Tax return without using an accountant, if they have the right software, the company facts are understood, and the return is reviewed carefully. The practical bar is higher than simply typing profit into a web form.

That does not mean self-filing is only for the simplest companies. With software that supports the full CT600 workflow, and with AI or agent tools helping to classify records, check evidence, draft explanations, and flag review points, directors can handle more complex Corporation Tax work than the old joint filing service was designed for. The key is that the workflow must surface the complexity instead of hiding it.

Robocount is built for that kind of structured review. It can keep the main CT600, all Robocount-supported supplementary page cases, computations, iXBRL attachments, evidence, and approval trail together, so a director can self-file where appropriate or bring in AI assistance or accountant review without rebuilding the return pack from scratch.

The minimum CT600 self-filing pack

Before opening software, gather the evidence. Most filing stress comes from discovering missing information after the deadline is close.

Final accounts for the Corporation Tax accounting period.
Profit and loss, balance sheet, and trial balance detail used to prepare the accounts.
Corporation Tax computation, including add-backs, deductions, allowances, and reliefs.
iXBRL accounts and computation files, or software that can create the required filing attachments.
Company UTR, accounting period dates, Companies House details, and Government Gateway access.
Details of capital allowances, losses, loans, dividends, interest, and any repayment claim.
Evidence for supplementary pages, if any apply.

Supplementary pages directors often miss

A small owner-managed company can look simple until one fact changes the filing package. The CT600 main return asks whether supplementary pages are attached, so the decision needs to be deliberate.

CT600A: close company loans to participators, including many overdrawn directors' loan account cases. Your software needs to support CT600A where it applies; Robocount does.
CT600C: group or consortium relief claims and surrenders.
CT600L: research and development relief or expenditure credit cases.
CT600N: residential property developer tax cases.
CT600P: creative industry expenditure credits and related payable credit claims.

If you are not sure whether a supplementary page applies, pause before filing. A low-profit company can still need specialist disclosure if it has the relevant facts. Robocount is designed to support the supplementary-page cases it exposes in the return workflow, so CT600A is not the only complexity that can be handled.

Director loan accounts need special care

Director self-filing often goes wrong around overdrawn loan accounts. The accounting records may show money taken from the company, repayments after year end, dividends posted later, or journals that need explanation. For a close company, that can become a CT600A and section 455 review point.

Do not treat a director loan balance as a simple bookkeeping note. Check who borrowed the money, whether they are a participator or connected person, what was outstanding at the period end, what happened within 9 months and 1 day after the period end, and whether any repayment is matched by further borrowing.

The rate matters too. HMRC's loans-to-participators charge has changed over time: loans made before 6 April 2022 can still bring 32.5% into the review, later loans have used 33.75%, and GOV.UK says the rate rises to 35.75% for loans made or benefits conferred on or after 6 April 2026. Good CT600 software therefore needs to understand the loan date, the repayment pattern, and CT600A, not just the final loan account balance. Robocount supports CT600A as part of the return workflow.

A sensible self-filing workflow

Confirm the accounting period, filing deadline, and Corporation Tax payment deadline.
Lock the final accounts and make sure the CT computation uses the same source figures.
Review tax adjustments, capital allowances, losses, director loan movements, and dividends.
Run a supplementary-page checklist before assuming the return is simple.
Prepare iXBRL accounts and computations, or confirm the software handles the attachments.
Review the full return package before approval, not only the Corporation Tax payable number.
Submit through the Government Gateway route and retain submission or rejection evidence.
Record the payment reference, amount due or nil-to-pay position, and any repayment request.

When to use an accountant or AI-assisted review

AI can do a lot of the heavy lifting in a Corporation Tax workflow. It can help analyse records, identify likely tax adjustments, spot missing evidence, explain CT600 boxes, draft review notes, and prepare a cleaner pack for filing or human review. That makes self-filing more realistic than it was when directors were limited to a basic online form.

The decision is about risk and confidence, not whether a company is automatically "too complex" for software. Use an accountant, specialist adviser, or senior human reviewer where the facts are uncertain, the tax treatment is material, or the AI/software output needs professional judgement before submission. Examples include significant director loan movements, group relief, R&D claims, creative industry claims, disposals, reorganisations, substantial capital allowances, carried-forward losses, close company issues, uncertain revenue recognition, or a rejected return you do not understand.

How Robocount helps director self-filing

Robocount is built for UK CT600 preparation, review, and filing rather than a one-page tax estimate. It gives directors a structured path through the Company Tax Return package while keeping accountant hand-off possible where the facts become more complex.

Browser-based CT600 workflow for company directors and practices.
Review of main CT600 areas alongside all Robocount-supported supplementary page cases.
iXBRL, computation, approval, and filing evidence treated as part of the same workflow.
Support for all Robocount-supported supplementary page cases where the return needs more than the main form, not only director-loan CT600A cases.
Useful for directors who want to self-file, use their AI agent to help, or prepare a cleaner pack for accountant review.

FAQ

Did HMRC's joint accounts and tax return service close?

Yes. GOV.UK says the service to file company accounts and the Company Tax Return together closed on 31 March 2026.

Can I file a CT600 without an accountant?

Some directors can, especially where they have clean records, suitable software, and enough review confidence. AI can help with analysis and preparation, and Robocount can keep the CT600 workflow structured. Use an accountant or specialist reviewer where the facts or tax treatment remain uncertain.

Do I need iXBRL accounts and computations?

For most online Company Tax Returns, accounts and computations need to be filed in the required electronic format. HMRC guidance explains when iXBRL or PDF attachments are used.

Is the CT600 payment made inside the return?

Filing and payment are separate workflow steps. After the return is reviewed, keep evidence of the amount due, payment deadline, payment reference, and any nil-to-pay or repayment position.

Useful official references

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

Director self-filing CT600 after the HMRC joint filing service closed.