What is Corporation Tax? UK Guide for Business Owners

At Towerstone Accountants we provide specialist limited company accountancy services for directors and owner managed businesses across the UK. We created this webpage for company owners who want clear guidance on Corporation Tax, including how it is calculated, when it is due, and how it should be paid. Our aim is to help you understand your obligations, avoid penalties, and manage your company tax position with confidence.

Corporation Tax is one of the core taxes that UK companies must understand, yet it is often misunderstood, oversimplified, or confused with personal tax. I regularly speak to directors who know they have to pay Corporation Tax but are unclear on what it is actually charged on, when it becomes payable, or how much control they really have over the final bill.

In simple terms, Corporation Tax is a tax on the profits made by companies. It applies to limited companies and some other organisations, but not to sole traders or ordinary partnerships. While the concept sounds straightforward, the detail matters a great deal, because small misunderstandings can lead to unexpected bills, cash flow problems, or compliance issues.

In this guide, I will explain what Corporation Tax is, who pays it, how it is calculated, when it is due, and how it fits into the wider picture of running a company in the UK. I will also cover common mistakes I see in practice and how good planning makes Corporation Tax far more manageable.

What Corporation Tax actually is

Corporation Tax is a tax charged on the taxable profits of a company during its accounting period.

Taxable profits are not the same as money in the bank. They are calculated using accounting rules and tax adjustments, which means the figure used for tax can differ from what a director feels the business has earned in cash terms.

Corporation Tax is administered by HMRC and applies across the UK to companies that are within the charge to tax.

Who pays Corporation Tax

Corporation Tax is mainly paid by:

• Limited companies registered in the UK

• UK branches of overseas companies

• Some clubs, societies, and associations

If you run a business as a sole trader or a normal partnership, you do not pay Corporation Tax. Instead, you pay Income Tax through Self Assessment.

This distinction is important, because Corporation Tax applies to the company itself, not to the individual directors or shareholders.

What profits are subject to Corporation Tax

Corporation Tax is charged on a company’s taxable profits. These usually fall into three main categories.

These are:

• Trading profits from normal business activity

• Investment profits such as rental income or interest

• Chargeable gains from selling assets

All of these are combined to arrive at the total taxable profit for the accounting period.

Trading profits explained

Trading profits are the profits generated from the company’s main business activities.

This is broadly calculated as:

• Turnover

• Less allowable business expenses

• Adjusted for tax rules

Allowable expenses are costs incurred wholly and exclusively for the purposes of the business.

Some costs that appear in the accounts may be disallowed for tax, which increases taxable profit.

Investment income and Corporation Tax

Companies can also earn investment income.

Common examples include:

• Rental income from property

• Interest on bank accounts

• Interest on loans made by the company

• Dividends from other companies

Most UK dividends received by companies are exempt from Corporation Tax, although there are exceptions. Other forms of investment income are usually taxable.

Chargeable gains

When a company sells an asset for more than it paid for it, the profit is known as a chargeable gain.

Assets that can give rise to gains include:

• Property

• Shares

• Business equipment

• Intellectual property

Chargeable gains are included in taxable profits and taxed as part of Corporation Tax.

Unlike individuals, companies do not have an annual capital gains allowance.

Corporation Tax rates

Corporation Tax is charged at rates set by the government.

The rate depends on:

• The level of company profits

• Whether marginal relief applies

• The accounting period in question

The existence of different rates and thresholds means that Corporation Tax is no longer a single flat rate for all companies. Understanding where your company sits within those thresholds is important for planning.

Accounting periods and Corporation Tax

Corporation Tax is calculated for each accounting period.

An accounting period is usually:

• 12 months long

• Aligned with the company’s financial year

In some cases, the first accounting period may be shorter or longer, depending on when the company started trading.

A company can have more than one accounting period within a set of statutory accounts, which can be confusing for new directors.

When Corporation Tax is due

One of the most common issues I see is confusion around payment deadlines.

For most small companies, Corporation Tax is due:

• Nine months and one day after the end of the accounting period

This means the tax bill often falls due before accounts are finalised or long before dividends are taken.

Large companies may have to pay Corporation Tax by instalments, but this does not usually apply to small owner managed businesses.

Filing the Corporation Tax return

In addition to paying Corporation Tax, companies must file a Corporation Tax return.

This return includes:

• A detailed profit and loss account

• Tax adjustments and computations

• Supporting schedules

The filing deadline is usually:

• 12 months after the end of the accounting period

Paying late or filing late can result in penalties and interest.

The difference between accounting profit and taxable profit

This is one of the most important concepts to understand.

Accounting profit is calculated using accounting standards. Taxable profit is calculated using tax rules.

Common differences arise because:

• Some expenses are disallowed for tax

• Capital items are treated differently

• Timing differences apply

• Special reliefs or allowances are used

This is why the Corporation Tax bill can feel higher or lower than expected.

Allowable and disallowable expenses

Allowable expenses reduce taxable profit. Disallowable expenses do not.

Common allowable expenses include:

• Staff costs

• Rent and utilities

• Professional fees

• Business insurance

• Marketing costs

Common disallowable expenses include:

• Client entertaining

• Fines and penalties

• Personal expenses

Understanding this distinction is key to managing Corporation Tax properly.

Capital allowances and Corporation Tax

Capital allowances are a major part of Corporation Tax planning.

Instead of deducting the cost of certain assets as expenses, companies claim capital allowances.

Assets that commonly qualify include:

• Equipment and machinery

• Computers and IT

• Vehicles in some cases

• Fixtures in commercial property

Capital allowances can significantly reduce taxable profits in the year of purchase.

Losses and how they affect Corporation Tax

Companies do not pay Corporation Tax if they make a loss.

Losses can usually be:

• Carried forward to offset future profits

• Carried back to offset previous profits in some cases

Loss relief can be valuable, especially in the early years of trading.

Dividends and Corporation Tax

Dividends are paid out of profits after Corporation Tax.

This means:

• Corporation Tax is calculated first

• Tax is paid by the company

• Dividends are then declared from remaining profits

Dividends do not reduce Corporation Tax. They are a distribution of profits, not an expense.

This is a common misunderstanding among new directors.

Director salaries and Corporation Tax

Unlike dividends, salaries paid to directors are an allowable expense.

This means:

• Salaries reduce taxable profit

• Employer National Insurance is also deductible

• PAYE and National Insurance apply

Choosing the right balance between salary and dividends is an important part of tax planning.

Corporation Tax and cash flow

One of the biggest challenges with Corporation Tax is cash flow.

The tax is often due:

• Months after profits were earned

• Before cash has been extracted

• Regardless of whether money is still in the bank

Successful companies plan for Corporation Tax by setting money aside regularly rather than treating it as an afterthought.

Common Corporation Tax mistakes I see

In practice, the most common issues include:

• Forgetting the payment deadline

• Confusing profit with cash

• Claiming disallowed expenses

• Not planning for tax early enough

• Paying dividends without checking profits

These mistakes are avoidable with proper systems and advice.

How Corporation Tax fits into overall tax planning

Corporation Tax should not be viewed in isolation.

It interacts with:

• Personal tax on dividends

• PAYE and National Insurance

• Pension planning

• Investment decisions

Good planning considers the total tax position rather than just the company’s bill.

The role of an accountant in Corporation Tax

An accountant helps with far more than just calculating the bill.

This usually includes:

• Preparing accurate tax computations

• Claiming available reliefs

• Advising on timing of income and expenses

• Helping manage cash flow

• Ensuring compliance with deadlines

In my experience, proactive advice saves more tax than reactive corrections ever could.

Why Corporation Tax often feels confusing

Corporation Tax feels complex because:

• The rules differ from personal tax

• The timing does not match cash movements

• The figures are adjusted for tax purposes

Once these differences are understood, the system becomes far more predictable.

Corporation Tax for new companies

New companies often underestimate Corporation Tax.

Common early mistakes include:

• Not setting money aside

• Paying dividends too early

• Assuming tax will be small

• Missing the first payment deadline

Understanding Corporation Tax from day one avoids unpleasant surprises later.

What happens if Corporation Tax is not paid

If Corporation Tax is not paid on time:

• Interest accrues

• Penalties may apply

• HMRC may take enforcement action

Ignoring Corporation Tax issues rarely makes them go away. Early communication is always better.

Final thoughts

Corporation Tax is a fundamental part of running a limited company in the UK. It is not just a bill that arrives once a year. It is a predictable outcome of how a business earns money, spends it, and plans ahead.

In my experience, the companies that struggle least with Corporation Tax are not those that earn the least, but those that understand how it works and plan for it early. With good records, realistic cash flow planning, and proper advice, Corporation Tax becomes a manageable and expected part of business life rather than a source of stress.

Once you understand that Corporation Tax is a tax on profits, not on cash, and that timing matters as much as totals, the whole system starts to make far more sense.

You may also find our guidance on how much is corporation tax and how to calculate corporation tax useful when dealing with related Corporation Tax questions. For a broader overview of Corporation Tax rules and support, you can visit our corporation tax help hub.