File a CT600 without an accountant

A company director is entitled to file the company’s own Corporation Tax return. Nothing in the rules requires an accountant. What the rules do require, since 31 March 2026, is recognised software: HMRC’s free online filing service for accounts and Corporation Tax has closed, and HMRC accepts CT600 returns only through commercial software that has passed its recognition process.

What a CT600 filing actually consists of

• The CT600 form itself: the company’s details, the accounting period, and the boxed figures from the computation.
• A Corporation Tax computation in Inline XBRL, tagged against HMRC’s computational taxonomy for the relevant year.
• The statutory accounts for the same period, also in Inline XBRL, tagged against the FRC taxonomy.

The three are submitted together in one envelope. The computation is where most of the work is, and where an error propagates into a wrong tax bill.

The computation, step by step

1. Start with the profit or loss shown in the accounts.
2. Add back expenditure that is not deductible for tax: depreciation, client entertaining, fines and penalties, the disallowed share of anything else.
3. Deduct income that is not taxable trading income, such as dividends received, and deduct capital allowances in place of depreciation.
4. Set trading losses brought forward against the trading profit.
5. Deduct qualifying charitable donations to reach taxable total profits.
6. Apply the rate. For financial year 2023 the small profits rate is 19% up to £50,000 and the main rate 25% above £250,000, with marginal relief between. The limits are divided by one plus the number of associated companies and prorated for short periods.
7. If the accounting period straddles 1 April, apportion profits between the two financial years by days and compute each separately.

FilingKit does exactly this and shows you each step with the rule that produced it and the CT600 box it fills. You can run the computation now, free, with no account.

Where directors get caught

• Treating depreciation as deductible. It is not; capital allowances replace it.
• Missing that the annual investment allowance limit is prorated for a period shorter than a year.
• Forgetting that dividends received, although not taxed, still count towards the profits that decide the rate.
• Filing the accounts with Companies House and the return with HMRC from two different sets of figures.

Deadlines and scope

FilingKit’s first release handles a single UK trade with accounts prepared under FRS 105, accounting periods of up to twelve months ending on or before , main-pool capital allowances, losses brought forward, donations and marginal relief. Anything outside that is refused with an explanation rather than guessed at. The full list is on how it works.

Price

£69 files the accounts and the CT600 together. The computation is free; pay when you file.