The CT600 is the official HM Revenue & Customs form that limited companies must file to report their profits and calculate the corporation tax they owe for each accounting period.
For agency founders who operate through a limited company, the CT600 is your annual tax return to HMRC. It must be submitted online within 12 months of the end of your company's accounting period, though any tax due must be paid within 9 months and 1 day of that period end. The form captures your company's turnover, allowable expenses, capital allowances, and any reliefs or claims, then applies the correct corporation tax rate to work out your final liability.
In 2025/26, the rate depends on your company's profits. If your agency's taxable profits are £50,000 or less, you pay the small profits rate of 19%. Between £50,000 and £250,000, marginal relief gradually increases the effective rate. Above £250,000, the main rate of 25% applies. For many agency founders, especially those drawing a modest salary and taking dividends, profits often fall within the 19% band, making corporation tax relatively straightforward.
The CT600 includes supplementary pages if your agency claims R&D tax credits, has complex group structures, or makes certain elections. Most agency founders will file the standard CT600 with no extra schedules. You must also submit your company's statutory accounts and a corporation tax computation alongside the return. HMRC uses the CT600 to check your figures against your filed accounts, so consistency is essential.
Common mistakes include missing the filing deadline, which triggers an automatic £100 penalty (or more for repeated delays), and incorrectly calculating marginal relief if profits hover near the £50,000 or £250,000 thresholds. If your agency's profits are close to these boundaries, it pays to plan ahead, for example, by making pension contributions or capital purchases to reduce taxable profits.
From 6 April 2026, the main rate of corporation tax remains at 25%, but the small profits rate rises to 19% (it stays at 19% for now). The CT600 itself does not change format, but the rates applied to your profits will shift.
When this matters for agency founders: You must file a CT600 every year your limited company trades, even if you made no profit or were dormant. Missing the deadline costs you money and HMRC attention. Understanding what is CT600 and how it interacts with your dividend strategy, salary, and allowable expenses helps you minimise tax legally and avoid penalties.