Corporation Tax Explained: What Companies Need to Understand in 2025/26

Corporation Tax is one of the most misunderstood taxes for company owners. Many directors know the headline rate, assume it applies uniformly, and only really think about Corporation Tax when the bill arrives.

In reality, Corporation Tax in 2025/26 is more complex than it first appears. Rates depend on profit levels, marginal relief applies in certain cases, and the way profits are calculated is just as important as the rate applied to them. Small timing errors, missed adjustments, or misunderstandings around expenses can significantly increase the tax a company pays — often unnecessarily.

This post explains how Corporation Tax actually works, how rates apply in 2025/26, where companies commonly go wrong, and what directors can do to stay compliant while avoiding avoidable overpayments.

What Is Corporation Tax?

Corporation Tax is charged on the taxable profits of UK limited companies and certain other organisations.

Taxable profits include:

• Trading profits
• Investment income
• Capital gains (with different rules from personal CGT)

Corporation Tax is paid by the company, not the director, and is completely separate from personal tax on salaries or dividends.

One of the most important things to understand is this:

Corporation Tax is not based on cash in the bank.
It is based on taxable profit, calculated using tax rules — not just the accounting profit shown in the accounts.

Corporation Tax Rates in 2025/26

Since April 2023, the UK has operated a tiered Corporation Tax system, and this continues in 2025/26.

The key rates are:

• 19% — Small profits rate
• 25% — Main rate
• Marginal relief applies between the two

How the thresholds work:

Companies with profits between £50,000 and £250,000 do not pay a flat rate. Instead, marginal relief gradually increases the effective rate.

Marginal Relief: Where Confusion Sets In

Marginal relief reduces the Corporation Tax bill for companies with profits between the lower and upper thresholds.

This means:

• You do not suddenly jump from 19% to 25%
• The tax rate increases gradually as profits rise

However, marginal relief is often misunderstood because:

• It is calculated using a formula
• It is not a separate “discount” you apply manually
• It interacts with associated company rules

Many directors only find out how marginal relief works when their accountant explains why their tax bill is higher than expected.

Associated Companies: A Common Trap

The profit thresholds (£50,000 and £250,000) are shared between associated companies.

Companies are associated if:

• One company controls another, or
• Both are under common control (usually by the same director/shareholder)

For example:

• Two associated companies split the thresholds in half
• The small profits threshold becomes £25,000 each
• The main rate threshold becomes £125,000 each

This means companies can reach the 25% rate far sooner than expected.

Many directors are unaware their companies are associated, particularly where:

• Multiple companies exist under the same ownership
• A property company sits alongside a trading company
• A dormant company still technically exists

How Taxable Profits Are Calculated

Corporation Tax is calculated starting with accounting profit, then adjusted for tax purposes.

Common adjustments include:

• Disallowing non-business expenses
• Capital allowances replacing depreciation
• Adjusting for entertaining costs
• Timing differences (accruals vs payments)

This is why two companies with identical accounting profits can have very different Corporation Tax bills.

Expenses That Commonly Cause Problems

One of the biggest causes of Corporation Tax errors is incorrectly claimed expenses.

HMRC frequently challenges:

• Personal expenses put through the company
• Entertaining costs
• Mixed-use expenses without clear apportionment
• Motor expenses with weak records
• Use of home claims with no evidence

If an expense is disallowed:

• Taxable profit increases
• Corporation Tax increases
• The issue may trigger wider HMRC scrutiny

This is where delayed or weak bookkeeping often leads directly to higher tax.

Capital Allowances: Often Missed or Misunderstood

Capital allowances allow companies to deduct the cost of certain assets from taxable profits.

Common qualifying assets include:

• Equipment and machinery
• Computers and office equipment
• Fixtures and fittings
• Certain vehicles (with restrictions)

Full Expensing and Annual Investment Allowance (AIA)

Most companies can claim:

• 100% relief on qualifying expenditure under the AIA (up to £1 million)

However, the rules depend on:

• The type of asset
• Whether it is new or second-hand
• Whether it is a car
• The emissions level

Failing to claim allowances correctly can result in paying tax earlier than necessary.

Timing Issues That Increase Corporation Tax

Corporation Tax is heavily influenced by timing.

Common timing issues include:

• Declaring dividends before profits exist
• Paying expenses late
• Delaying bookkeeping until year end
• Missing opportunities to accelerate allowances
• Not considering the accounting period end date

Small timing changes can move profits into a higher tax band or affect marginal relief calculations.

Corporation Tax vs Cash Flow

One of the most frustrating experiences for directors is paying Corporation Tax when the company feels “short on cash”.

This usually happens because:

• Tax is based on profits, not cash
• VAT collected is not profit
• Loan repayments reduce cash but not profit
• Capital expenditure doesn’t always give immediate relief

Understanding this difference early helps avoid nasty surprises.

Corporation Tax Deadlines

Companies must:

• File Corporation Tax returns within 12 months of the accounting period end

• Pay Corporation Tax within 9 months and 1 day of the accounting period end

Missing deadlines can result in:

• Interest
• Penalties
• Increased HMRC attention

Unlike personal tax, Corporation Tax has no January deadline — but the shorter payment window often catches directors out.

Why HMRC Focuses on Corporation Tax

HMRC pays close attention to Corporation Tax because it often reveals:

• Poor expense discipline
• Weak record keeping
• Informal director withdrawals
• Incorrect dividend treatment

Once HMRC questions a company’s Corporation Tax position, they often review:

• Director’s loan accounts
• PAYE and benefits
• VAT
• Personal tax returns

Corporation Tax enquiries rarely stay narrow.

How to Reduce Corporation Tax — Properly

Reducing Corporation Tax is not about aggressive planning. It’s about:

• Claiming legitimate expenses correctly
• Using capital allowances properly
• Timing income and expenditure sensibly
• Understanding marginal relief
• Keeping accurate, up-to-date records

Most overpayments happen not because tax rates are high, but because the numbers going into the calculation are wrong or incomplete.

Final Thoughts

Corporation Tax in 2025/26 is no longer a simple flat-rate calculation. Profit levels, associated companies, marginal relief, and correct profit adjustments all matter.

For many directors, the biggest risk is not underpaying tax — it’s overpaying it unnecessarily, or triggering HMRC attention because the underlying records are weak.

Understanding how Corporation Tax really works allows companies to plan properly, manage cash flow, and avoid unpleasant surprises.

How Merranti Accounting Can Help

If you want clarity on your Corporation Tax position, help reviewing your profits, or confidence that you’re not paying more tax than necessary, Merranti Accounting can review your company’s position and explain where you stand.

A short review now can prevent unexpected bills and give you confidence in your numbers before deadlines arrive.