Corporation Tax Advice & CT600 Filing

What is corporation tax?

Corporation tax is the tax that UK limited companies and certain other organisations pay on their taxable profits — including trading profits, investment income and chargeable gains. Unlike sole traders, who report business profits through Self Assessment, a company is a separate legal entity that calculates its own tax, files a Company Tax Return (the CT600) and pays HMRC directly. Getting the figures, reliefs and deadlines right is essential, because penalties and interest apply automatically to mistakes and delays.

Corporation tax rates and marginal relief

Since 1 April 2023 the UK has operated a tiered system rather than a single flat rate. The rate your company pays depends on its level of taxable profit:

Taxable profit	Rate
Up to £50,000	19% (small profits rate)
£50,000 – £250,000	Marginal relief (tapered effective rate)
Over £250,000	25% (main rate)

Marginal relief smooths the transition between the two rates, so a company just above £50,000 is not suddenly taxed at the full 25%. The £50,000 and £250,000 thresholds are also divided between associated companies, which can push a group into higher rates. We apply the official HMRC calculation for you. Rates and thresholds can change at fiscal events, so always confirm the current figures on GOV.UK.

Corporation tax deadlines

Corporation tax has an unusual quirk: you pay the tax before the return is formally due. For most companies the key dates, measured from the end of your accounting period, are:

• Pay corporation tax: 9 months and 1 day after your accounting period ends.
• File your CT600: 12 months after your accounting period ends.

For example, a company with a year ending 31 March must pay by 1 January and file by 31 March the following year. Large companies with profits over £1.5 million pay in quarterly instalments instead. Missing the filing deadline brings an immediate £100 penalty, rising with continued delay, while late payment accrues interest.

The CT600 and the filing process

Your Company Tax Return is made up of the CT600 form, your statutory accounts and detailed tax computations that bridge your accounting profit to your taxable profit. Returns must be filed online with HMRC in the required iXBRL format, and accounts must also be filed at Companies House. We prepare all three documents in tandem, reconcile your bookkeeping, adjust for disallowable expenses and capital items, and submit everything correctly so HMRC and Companies House records stay consistent.

Reliefs that reduce your bill

Claiming the right reliefs is where corporation tax planning adds real value:

• Capital allowances and full expensing — deductions for qualifying plant, machinery and equipment, including 100% full expensing on many new assets.
• R&D tax relief — enhanced deductions or credits for companies carrying out qualifying research and development.
• Annual Investment Allowance — full relief on qualifying capital spend up to the annual limit.
• Loss relief — carrying trading losses back or forward to offset profits.
• Salaries, pension contributions and other allowable costs.

Eligibility rules are detailed and HMRC scrutinises claims such as R&D closely, so it pays to have these prepared properly.

How Accuprime helps

We handle the full corporation tax cycle for owner-managed and growing companies: preparing year-end accounts, building the tax computation, calculating marginal relief, identifying and claiming reliefs, filing the CT600 and accounts, and telling you exactly what to pay and when. We also plan ahead — advising on profit extraction, director salary and dividend mix, and timing of capital investment — to keep your overall tax efficient and legitimate.

If you are setting up a company we can guide you through limited company formation, and we pair corporation tax with payroll, VAT and Making Tax Digital compliance. Explore our full tax services, see how we work as your local accountant in Reigate, or read HMRC’s official guidance on corporation tax.