Company Tax Return (CT600) Explained for Directors

A Company Tax Return is the return a limited company sends to HMRC to report its profit or loss and work out its Corporation Tax. It is made up of the CT600 form, the company's accounts and its tax computations, and it must be filed online within 12 months of the end of the accounting period.

New directors often assume that filing accounts at Companies House is the whole job. It is not. The Company Tax Return is a separate filing, to a separate body, with its own deadline and its own penalties. This guide explains what the CT600 is, what goes into it, when it is due and how to avoid the common slips.

Key takeaways

• The Company Tax Return is the CT600 form plus accounts and tax computations.
• File it within 12 months of the end of the accounting period.
• Corporation Tax is usually due sooner: 9 months and 1 day after the period ends.
• A return is required even if the company made a loss, once HMRC has asked for one.
• Late filing starts at a £100 penalty and escalates.

What the CT600 is

The CT600 is HMRC's form for a Company Tax Return. On it the company reports its turnover, its taxable profit or loss, any reliefs and allowances claimed, and the Corporation Tax due. The form is not sent on its own. A complete return has three parts:

• The CT600 form, with any supplementary pages that apply.
• The company's accounts for the period.
• Tax computations, showing how the accounting profit was adjusted to reach the taxable profit.

The accounts and computations must be submitted in a tagged digital format known as iXBRL, which accounting and tax software produces automatically. HMRC's overview is on GOV.UK's Company Tax Returns page. HMRC has closed its free online service for filing accounts and a Company Tax Return together, so companies now need commercial software or an agent to file; check GOV.UK for the current filing options.

From accounting profit to taxable profit

The profit in your accounts is rarely the profit you pay tax on. The tax computation starts with the accounting profit and adjusts it. Typical adjustments include:

• Adding back depreciation, which is not allowed for tax, and claiming capital allowances on equipment instead.
• Adding back disallowable costs, such as client entertaining and some fines.
• Deducting reliefs the company is entitled to.
• Using losses brought forward from earlier periods, where the rules allow.

The result is the taxable profit, to which the Corporation Tax rate is applied. The rate depends on the level of profit, and companies between the lower and upper limits can claim marginal relief; see our guide to marginal relief and check the current rates on GOV.UK.

The three deadlines directors mix up

Notice the order. The tax is due before the return. You cannot know how much to pay without doing the computation, so in practice the return needs to be prepared within nine months, not twelve. Different rules apply to a company's first accounts and to larger companies that pay in instalments. Our guides to the Corporation Tax payment deadline and company accounts deadlines cover each in detail.

Accounting periods and first-year returns

An accounting period for Corporation Tax cannot be longer than 12 months. That matters for new companies. A company's first accounts often cover slightly more than a year, because they run from the date of incorporation to the end of the month a year later. Where the accounts cover more than 12 months, the company must file two Company Tax Returns: one for the first 12 months and one for the remainder, each with its own payment deadline. HMRC explains this on its page about accounting periods for Corporation Tax.

Late filing penalties

Penalties for a late Company Tax Return are set out on GOV.UK:

• 1 day late: £100.
• 3 months late: another £100.
• 6 months late: HMRC estimates the Corporation Tax bill and adds a penalty of 10% of the unpaid tax.
• 12 months late: another 10% of any unpaid tax.

If the return is late three times in a row, the £100 penalties increase to £500 each. Interest is charged separately on tax paid late. These penalties are on top of any Companies House penalty for late accounts, which is a separate regime.

Step by step: preparing a Company Tax Return

1. Confirm the accounting period shown on HMRC's notice to deliver a return.
2. Finalise the bookkeeping: reconcile the bank, review debtors and creditors, and post year-end adjustments.
3. Prepare the annual accounts for the same period.
4. Prepare the tax computation: adjust for depreciation, capital allowances, disallowable costs and losses.
5. Complete the CT600 and any supplementary pages, such as those for loans to directors.
6. Pay the Corporation Tax by 9 months and 1 day after the period end, using the correct payment reference.
7. File online using commercial software, and keep the submission receipt.
8. Keep the records that support the return for at least 6 years.

Worked example (illustrative example)

"Harrow Lane Design Ltd", an invented illustrative example, has a year end of 31 March 2026. Its accounts show a profit of £38,000 after £3,000 of depreciation and £500 of client entertaining. It bought a £4,000 computer system in the year and claims capital allowances on the full cost.

• Accounting profit: £38,000
• Add back depreciation: £3,000
• Add back entertaining: £500
• Deduct capital allowances: £4,000
• Taxable profit: £37,500

The company's key dates are: accounts to Companies House by 31 December 2026, Corporation Tax paid by 1 January 2027, and the Company Tax Return filed by 31 March 2027. The director prepares all three together in the autumn so the tax figure is known well before it is due. The company and figures are invented and simplified to illustrate the process.

Common mistakes

• Thinking Companies House accounts are enough and never filing the return with HMRC.
• Treating 12 months as the real deadline when the tax is due at 9 months and 1 day.
• Not filing because there was a loss or no trading, despite a notice from HMRC.
• Filing one return for a first period longer than 12 months instead of two.
• Forgetting the supplementary pages, for example where a director owes the company money.
• Leaving depreciation in and not claiming capital allowances.

Have your Company Tax Return handled

Our Corporation Tax service starts from £250 per year and covers the computation, the CT600 and online filing, with your company accounts from £350 per year. We prepare both together so the tax figure is ready well before the payment date.

Return due soon, or already late? Talk to us or see our pricing.