CT600 review controls

CT600 pre-filing review checklist for small accountancy practices.

A practical manager-review checklist for UK Corporation Tax returns before submission: CT600, computations, iXBRL, supplementary pages, client approval, payment communication, and evidence.

Why a pre-filing checklist matters

A CT600 can look complete while still being risky. The return may have the right headline tax number but the wrong accounting period, missing iXBRL attachment, unreviewed director loan, unchecked loss claim, or no client approval evidence. Small practices need a repeatable review flow that catches these issues before the Government Gateway submission.

HMRC's CT600 form notes that a return includes the Company Tax Return form, any supplementary pages, accounts, computations, and relevant information. A pre-filing review should therefore check the whole return package, not only the main form.

1. Company and period checks

  • Confirm the company name, registration number, Unique Taxpayer Reference, and Corporation Tax accounting period.
  • Check whether the accounting period is short, long, final, or split across two CT periods.
  • Confirm that HMRC has issued a notice to deliver a return where relevant.
  • Check the filing deadline and payment date separately.

GOV.UK says the Company Tax Return deadline is normally 12 months after the end of the accounting period. Corporation Tax payment is usually due 9 months and 1 day after the period end for companies outside quarterly instalment payment rules.

2. Accounts, computations, and iXBRL attachments

Reviewers should check that accounts and computations match the CT600 figures and are attached in the expected format. HMRC's XBRL guidance explains that online Company Tax Returns generally require accounts and computations in Inline XBRL, with limited exceptions.

  • Accounts agree to the final trial balance and statutory accounts.
  • Tax computations reconcile accounting profit to taxable profit.
  • Disallowable expenses, capital allowances, losses, and tax adjustments are explained.
  • iXBRL files are the final approved versions, not draft exports.
  • Attachment names and periods make sense to a future reviewer.

3. Supplementary-page triage

Supplementary pages are a common missed-control point. The reviewer should not ask only whether a page was completed. They should ask whether the client facts make a page necessary.

  • CT600A: close company loans, overdrawn directors' loan accounts, section 455 tax, repayment claims, releases, or write-offs.
  • CT600C: group relief claims or surrenders, including consent evidence.
  • CT600L or CT600P: R&D and creative industry credits, including additional information and payable credit checks.
  • Other specialist pages: CFCs, charities, insurance, tonnage tax, cross-border royalties, ring fence trades, restitution tax, Freeports, Investment Zones, or RPDT.

If a supplementary page is not needed, document the review reason where the file contains facts that could otherwise suggest one.

4. Tax calculation and payment communication

The review should separate filing from payment. A return can be ready to submit while the client still needs a clear payment instruction, repayment expectation, or nil-to-pay confirmation.

  • Check taxable total profits, rates, marginal relief, associated-company thresholds, and credits.
  • Compare the tax due to any payments on account, repayments, losses, or credits.
  • Check whether quarterly instalment payments may apply for larger companies.
  • Use the accounting-period-specific Corporation Tax payment reference when sending payment instructions.
  • Keep client communication on file with the return evidence.

5. Client approval and filing evidence

Client approval should happen before submission and be tied to the actual final package. The file should show what was approved, who approved it, when it was approved, and what was filed.

  • Final CT600 and computations sent to the client.
  • Final accounts or accounts extract included where appropriate.
  • Client approval recorded before submission.
  • HMRC response and submission timestamp retained after filing.
  • Post-filing payment, repayment, or rejection follow-up tracked.

How Robocount supports the review

Robocount is built to make CT600 review visible before filing. The goal is to give small practices a structured package: main return, computations, iXBRL attachments, supplementary-page prompts, client approval, and submission evidence.

  • One workflow for CT600 preparation, review, and filing readiness.
  • Supplementary-page awareness for specialist Corporation Tax scenarios.
  • Evidence-led review for owner-managed companies and practice teams.
  • Browser-based access for preparers, reviewers, and approval workflows.
  • API and MCP-ready structure for firms automating intake or pre-review checks.

FAQ

What should be checked immediately before a CT600 is filed?

Check the company and period, final accounts, computations, iXBRL attachments, supplementary pages, tax calculation, client approval, payment communication, and HMRC submission route.

Is client approval enough if the CT600 later changes?

Approval is only useful if it relates to the final package. If figures or attachments change after approval, the practice should refresh the approval or document the change and authority clearly.

Should payment instructions be reviewed with the CT600?

Yes. Filing and payment have different deadlines. The practice should check the amount, due date, and Corporation Tax payment reference before sending payment instructions to the client.

Official references

This guide is general product and workflow information, not tax advice. Check current HMRC guidance and the facts of the company before filing.