Corporation tax · Director guide · Updated May 2026
Corporation tax explained: how it works for UK limited companies.
Written by Iftikhar Rashid FCCA — Managing Partner, RR Accountants. 16 years in practice, specialist in limited company directors.
What is corporation tax?
Corporation tax is the tax on UK limited company profits. It is charged at rates between 19% and 25% depending on profit level — with marginal relief for profits between the small and main rate thresholds. Every company must file a CT600 return and pay any tax due within 9 months and 1 day of the accounting year end. Unlike income tax, corporation tax is on profit — after allowable expenses, capital allowances, pension contributions, and reliefs have been deducted. [VERIFY: gov.uk/corporation-tax-rates]
Who this applies to: All active UK limited companies — including SPV property companies, contractor PSCs, and trading companies.
Corporation tax rates (2026/27)
[VERIFY all figures at gov.uk/corporation-tax-rates]
| Profit level | Rate [VERIFY] | Notes |
|---|---|---|
| Up to small profits threshold | 19% (small profits rate) [VERIFY] | Most small companies fall here |
| Between thresholds | Marginal relief applies [VERIFY] | Effective rate tapers between small and main rate |
| Above main rate threshold | 25% (main rate) [VERIFY] | Companies with higher profits |
Associated companies rule: If a director owns or controls multiple companies, the profit thresholds for marginal relief are divided between them. This can push companies into the marginal or main rate band sooner than expected. [VERIFY: current associated companies rules at gov.uk]
How corporation tax is calculated
Company turnover (total revenue)
− Allowable business expenses
− Employer pension contributions
− Capital allowances / Annual Investment Allowance
− R&D enhanced deduction (if applicable)
− Other allowable deductions
= Taxable profit
× Corporation tax rate (19%–25%) [VERIFY]
− Marginal relief (if applicable)
= Corporation tax due
Allowable deductions (reduce CT)
- ✓Salaries + employer NI
- ✓Employer pension contributions
- ✓Capital allowances on equipment
- ✓Professional fees (accountancy, legal)
- ✓Advertising and marketing costs
- ✓Interest on business loans
- ✓R&D qualifying expenditure
- ✓Business travel and subsistence
NOT deductible (do not reduce CT)
- ✕Dividends paid to shareholders
- ✕Personal expenses of directors
- ✕Fines and penalties
- ✕Most entertainment costs
- ✕Client hospitality above HMRC limits
- ✕Personal use portion of mixed-use assets
Corporation tax deadlines
CT600 return filing
12 months after accounting year end
Filing deadline is one year after year-end. This is different from the payment deadline.
CT payment due
9 months + 1 day after accounting year end
For a 31 March year-end: CT due 1 January. For a 31 December year-end: due 1 October the following year.
Large company quarterly payments
Months 7, 10, 13, 16 of accounting period
Companies above the large company threshold pay quarterly. [VERIFY current threshold at gov.uk]
Legitimate ways to reduce corporation tax
Employer pension contributions
Fully deductible. No employer NI. No employee NI. One of the most tax-efficient forms of director remuneration. Subject to annual allowance limits. [VERIFY]
Annual Investment Allowance (AIA)
100% first-year deduction on qualifying plant and machinery. Reduces taxable profit in the year of purchase. [VERIFY: current AIA limit at gov.uk]
Research and development relief
Enhanced deduction on qualifying R&D expenditure. Only where activity genuinely qualifies — HMRC scrutiny is high since 2023. [VERIFY current R&D scheme at gov.uk]
Salary optimisation
Directors can adjust their salary level to optimise CT efficiency versus NI cost. Works in conjunction with the salary vs dividend strategy.
Loss relief
Trading losses in one period can be carried forward against future profits, or carried back against prior year profits. SPV property losses have different rules. [VERIFY]
Timing of capital expenditure
Purchasing qualifying assets before year-end brings forward the tax relief. Timing of large purchases can reduce the CT bill in a specific year.
Corporation tax — frequently asked questions
What is corporation tax?
Corporation tax is the tax levied on the profits of UK limited companies. Unlike income tax (which individuals pay), corporation tax is charged on company profits — after allowable expenses, capital allowances, and reliefs have been deducted. Every UK limited company must file a CT600 return and pay any tax due. [VERIFY: current rates at gov.uk/corporation-tax-rates]
What is the UK corporation tax rate in 2026?
The UK corporation tax rate depends on the level of company profits. Profits up to a lower threshold are taxed at the small profits rate. Profits above the upper threshold are taxed at the main rate. Profits between the two thresholds attract marginal relief — a tapered rate between the two. [VERIFY: current rates and thresholds at gov.uk/corporation-tax-rates]
What is marginal relief for corporation tax?
Marginal relief applies to companies with profits between the small profits threshold and the main rate threshold. It tapers the effective tax rate between the two rates — so profits in this band are not simply taxed at the higher rate from the first pound. The relief is calculated using a formula based on the profit level and the relevant thresholds. [VERIFY: current marginal relief thresholds and calculation at gov.uk]
When is corporation tax due?
For most companies, the CT600 return and corporation tax payment are due 9 months and 1 day after the end of the company's accounting year. For a company with a 31 March year-end, both the return and payment are due by 1 January the following year. Large companies with profits above a certain threshold must pay in quarterly instalments. [VERIFY: current deadlines and large company rules at gov.uk]
What expenses can a limited company deduct from corporation tax?
Allowable deductions include: salaries and employer NI, rent and business rates, equipment costs (via capital allowances or AIA), professional fees (accountancy, legal), advertising and marketing, travel on business, software and subscriptions, pension contributions, and interest on business borrowing. Personal expenses of directors are generally not deductible. The rules on expenses are detailed — specific categories require care.
What is the CT600 and who needs to file one?
The CT600 is HMRC's corporation tax self-assessment return. Every active UK limited company must file one annually — even if the company made a loss and owes no tax. A dormant company may not need to file a CT600 but may need to notify HMRC. The return covers the company's accounting period and reconciles profit with taxable profits after adjustments.
Can a company reduce its corporation tax through pension contributions?
Yes. Employer pension contributions are fully deductible as a business expense, reducing taxable profit. For a company paying the main rate of corporation tax, £10,000 of employer pension contributions reduces the tax bill by the applicable rate. Pension contributions made by the company on behalf of a director do not attract employer NI — making them one of the most tax-efficient ways to remunerate a director. [VERIFY: current pension allowances at gov.uk]
What is R&D tax relief for limited companies?
R&D (Research and Development) tax relief allows qualifying companies to deduct a higher percentage of qualifying R&D expenditure from their taxable profits, or claim a cash credit. The schemes and rates have changed significantly since 2023. Only activities that advance science or technology and involve genuine technical uncertainty qualify. HMRC has increased enquiries into R&D claims — contemporaneous evidence is essential. [VERIFY: current R&D relief rates and schemes at gov.uk]
Related guides and services
- Corporation tax service — CT600 filing
- Accountants for limited company directors
- Salary vs dividend for directors
- Company formation
- Annual accounts and CT600
- R&D tax credits
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