UK Corporation Tax Guide 2026: Rates, Filing Deadlines & Payment Rules

Running a limited company in the UK means dealing with UK corporation tax rules that change faster than most business owners expect. Get it wrong, and you face penalties from HM Revenue and Customs (HMRC) that can eat into your profits. This guide breaks down exactly how much you owe, when to pay, and how to file without the stress.

Key Takeaways

• The standard rate for 2026/27 is 25% on profits over £250,000.
• Companies with profits under £50,000 pay a reduced 19% rate.
• You must file your CT600 return within 12 months of your accounting period ending.
• Tax payments are due 9 months and 1 day after your accounting period ends.
• Large companies (turnover >£25m or balance sheet >£12.5m) must make quarterly instalments.

Understanding the 2026/27 Tax Bands

Corporation tax isn't a flat fee; it depends on your total taxable profits. For the financial year starting April 6, 2026, HMRC uses three bands. The "small profits" rate applies if your profits are £50,000 or less. The "main rate" kicks in for profits between £50,001 and £250,000, but there's a tapering rule here. If you fall in this middle band, your effective tax rate slides up from 19% to 25% as profits grow. Once you cross £250,000, the full 25% main rate applies to all profits, not just the excess.

This structure encourages smaller businesses to stay lean while ensuring larger firms contribute more. However, be careful with the tapering zone. A single large contract could push you over £250,000, triggering the higher rate on your entire income. Planning your timing of revenue recognition can sometimes keep you in the lower bracket, though you shouldn't distort normal business operations just for tax savings.

Calculating Your Taxable Profit

Your turnover isn't what you pay tax on. You pay on taxable profit, which is your income minus allowable expenses. Allowable expenses include staff salaries, rent, utilities, marketing costs, and depreciation on assets. But watch out for non-allowable items like entertainment for clients, fines, or part of private car use. These need to be added back to your accounts before calculating the final figure.

If you have associated companies, their profits count toward your threshold too. For example, if you own 51% or more of another company, its profits merge with yours for determining which tax band you fall into. This often surprises new directors who think each entity stands alone. Always check your group structure before estimating your liability.

Filing Deadlines and the CT600 Form

Every limited company must file a Company Tax Return, known as the CT600 form, even if you made a loss. The deadline is 12 months after your accounting period ends. If your year ends March 31, 2026, you must file by March 31, 2027. Missing this date incurs automatic late filing penalties, starting at £100 for being one day late and rising quickly.

Filing online via HMRC’s Business Tax Account is now mandatory for most companies. Paper forms are largely phased out unless you have specific exemptions. Online filing allows real-time updates and reduces errors. Keep digital records of all transactions for at least six years, as HMRC can investigate past returns during this window.

Payment Rules and Instalment Options

Unlike personal income tax, corporation tax is usually paid in one lump sum. The payment deadline is nine months and one day after your accounting period ends. So, if your year ends March 31, 2026, you must pay by January 1, 2027. Late payments attract interest charges calculated daily, which can add up fast if you’re short on cash.

However, if you’re a larger company, you might qualify for Quarterly Instalment Payments (QIP). You’re required to use QIP if your previous year’s tax bill was over £10,000, or over £15,000 if you were part of a group. QIP spreads your liability across four payments throughout the year, improving cash flow. Smaller companies can opt into QIP voluntarily if they want smoother budgeting, provided their tax bill is under £10,000.

Common Pitfalls to Avoid

One major trap is confusing dividends with salary. Directors often take money out as dividends, thinking it’s tax-free. While dividends aren’t subject to corporation tax, they are taxed personally when received. Over-relying on dividends without considering personal tax implications can lead to unexpected bills. Another pitfall is forgetting about Capital Gains Tax. When you sell assets, the gain is taxed separately from trading profits. Don’t forget to declare these gains in your return.

Also, beware of dormant companies. Even if you haven’t traded, you may still need to file a confirmation statement and potentially a tax return if you had any activity in the past. Dormant status doesn’t automatically exempt you from all compliance duties. Check with an accountant to ensure you’re meeting minimal requirements.

How to Prepare for Filing

Start preparing your accounts three months before your deadline. Gather all bank statements, invoices, and expense receipts. Reconcile your books to ensure every transaction is recorded. Use accounting software like Xero or QuickBooks to automate categorization, but always review the output manually. Errors in categorization can skew your taxable profit and lead to incorrect tax calculations.

If you’re unsure about complex items like R&D credits or foreign income, seek professional advice. Getting it right the first time saves money on penalties and interest. Consider setting aside 25% of your monthly net profit into a separate tax account to avoid cash flow shocks when the bill arrives.