Understanding Associated Companies: How They Affect Your Corporation Tax

If your limited company has links to other companies — whether through shared ownership, control, or common participation — those connections can have a real and significant impact on the corporation tax you pay. Since 1 April 2023, when the UK introduced a two-rate corporation tax structure, the concept of associated companies has become more important than ever.

Many company directors and small business owners overlook this detail, assuming the standard £50,000 small profits rate threshold and £250,000 upper threshold apply uniformly. In reality, these thresholds are divided by the number of associated companies plus one, which can push a company into a higher tax bracket or reduce the marginal relief it receives.

In this comprehensive guide, we explain exactly what associated companies are, how they affect your corporation tax calculation, and how WeFile handles this automatically within the CT600 filing process — ensuring your tax computation is accurate every time.

Two companies are considered associated for corporation tax purposes if one company controls the other, or if both companies are under the common control of the same person or group of persons. Control, in this context, generally means holding the majority of voting rights, the right to the majority of distributable profits, or the right to the majority of assets on a winding up.

The rules are set out in sections 25E and 25F of the Corporation Tax Act 2010 and apply for accounting periods beginning on or after 1 April 2023.

Here are some common scenarios where companies are considered associated:

• A director owns 100% of Company A and 100% of Company B — both companies are associated with each other because they are under common control of the same individual.
• A parent company owns a subsidiary — the parent controls the subsidiary, making them associated.
• A husband owns Company X and a wife owns Company Y — if they are connected persons (which spouses are under tax law), HMRC may treat these as associated. However, there is a substantial commercial interdependence test that may apply.
• A person holds more than 50% of shares in multiple companies — all such companies are associated with each other through common control.

It is important to note that dormant companies and certain companies that have not carried on a trade or business at any point during the accounting period are generally excluded from the associated companies count. However, the precise rules can be nuanced, and directors should consider their specific circumstances carefully.

Before April 2023, the UK had a flat corporation tax rate of 19% for all companies regardless of profit levels. The concept of associated companies existed in previous legislation but became dormant when the small profits rate was abolished in 2015.

From 1 April 2023, HMRC reintroduced a two-tier rate structure:

• Small profits rate: 19% on taxable profits up to £50,000
• Main rate: 25% on taxable profits above £250,000
• Marginal relief: A gradual increase for profits between £50,000 and £250,000, using a marginal relief fraction of 3/200

The critical point is that the £50,000 lower limit and £250,000 upper limit are not fixed for every company. They must be divided by the total number of associated companies plus one (i.e., the company itself plus its associates).

This means that if your company has associated companies, the profit thresholds at which higher rates kick in are significantly reduced.

The formula is straightforward:

Adjusted Lower Limit = £50,000 ÷ (1 + number of associated companies)

Adjusted Upper Limit = £250,000 ÷ (1 + number of associated companies)

Let's look at some practical examples:

Example 1: No associated companies

• Lower limit: £50,000 ÷ 1 = £50,000
• Upper limit: £250,000 ÷ 1 = £250,000
• A company with £40,000 profit pays 19% = £7,600

Example 2: One associated company

• Lower limit: £50,000 ÷ 2 = £25,000
• Upper limit: £250,000 ÷ 2 = £125,000
• A company with £40,000 profit now falls in the marginal relief band (between £25,000 and £125,000), resulting in a higher effective tax rate than 19%

Example 3: Four associated companies

• Lower limit: £50,000 ÷ 5 = £10,000
• Upper limit: £250,000 ÷ 5 = £50,000
• A company with £40,000 profit is now within the marginal relief band and approaching the upper limit, paying significantly more tax than it would without any associated companies

As you can see, even modest profits can attract higher tax rates when a director controls multiple companies. A company that would comfortably pay 19% with no associates could find itself paying close to 25% when several associated companies exist.

When an accounting period is shorter than 12 months, the adjusted lower and upper limits are further pro-rated by the number of days in the accounting period relative to the number of days in a full year (or the total accounting period length, whichever is greater).

For example, if a company with one associated company has a 6-month accounting period:

• Adjusted lower limit: £50,000 ÷ 2 × (183/365) = approximately £12,534
• Adjusted upper limit: £250,000 ÷ 2 × (183/365) = approximately £62,671

This pro-rating ensures that short periods are treated fairly and prevents companies from gaining a tax advantage by using shortened accounting periods.

WeFile automatically handles this pro-rating when your filing period is shorter than 12 months, taking into account both the number of associated companies and the exact length of the accounting period.

Corporation tax rates are set per financial year (which runs from 1 April to 31 March). If your accounting period spans two financial years — for example, 1 January 2024 to 31 December 2024 — your profits are apportioned between the two financial years on a time basis.

The associated companies adjustment is applied to the limits for each financial year separately, with the limits also pro-rated based on the number of days falling in each financial year.

WeFile handles this split automatically. When you enter your associated companies count, the system calculates the correct thresholds for each financial year portion, applies the marginal relief formula independently for each period, and combines the results into the total corporation tax liability shown on your CT600.

WeFile has built full associated companies support into every stage of the CT600 filing process. Here is how it works:

Step 1: Filing Details

In the first step of the filing wizard, alongside your company details and accounting period, you will see a field labelled "Number of Associated Companies". This is where you enter the count of companies associated with yours during the accounting period. If you have no associated companies, simply leave it at zero.

The field includes a helpful tooltip explaining that associated companies are those under common control or ownership, and that this number affects the corporation tax rate thresholds.

Step 2: Automatic Tax Calculation

Once you have entered your profit and loss figures, WeFile automatically uses the associated companies count to calculate the correct corporation tax. The system:

• Divides the £50,000 lower limit and £250,000 upper limit by (associated companies + 1)
• Pro-rates the limits for the exact length of your accounting period
• Handles financial year boundary splits (e.g., periods crossing 1 April)
• Applies the 3/200 marginal relief fraction correctly against the adjusted thresholds

You can see the resulting corporation tax figure update in real-time as you enter your financial data.

Step 3: Review and Submission

On the review page, the Account Type tab displays an "Associated Companies" section showing the number you entered and a clear explanation of how it impacts your thresholds. This gives you a final opportunity to verify the information before submission.

Behind the Scenes

When your CT600 is submitted to HMRC, the associated companies count is embedded in the CT600 XML in the correct format. The iXBRL accounts filed with Companies House also reflect the adjusted corporation tax figure. The tax computation document, which can be downloaded as a PDF, shows the full calculation including the adjusted thresholds and any marginal relief applied.

The definition of control for associated companies purposes is broad. A person controls a company if they possess or are entitled to acquire:

• The greater part of the share capital or issued share capital of the company
• The greater part of the voting power in the company
• Such rights as would entitle them to the greater part of the income of the company if it were all distributed
• Such rights as would entitle them to the greater part of the assets of the company on a winding up

The rights of connected persons — including spouses, civil partners, relatives, and business partners — are also taken into account. This means that if a husband controls Company A and his wife controls Company B, both companies may be treated as associated.

However, HMRC provides an exception: companies controlled by connected persons (but not by the same individual) are not treated as associated if there is no substantial commercial interdependence between them. Substantial commercial interdependence can be financial (e.g., one company lends to the other), economic (e.g., they share customers), or organisational (e.g., shared management or premises).

When dealing with associated companies, there are several pitfalls that company directors and their advisers should watch out for:

• Forgetting dormant companies: While genuinely dormant companies are typically excluded, a company that has carried on any business activity during the period — even minimal — may still count as associated. Always check the specific circumstances.
• Ignoring connected persons: Many directors do not realise that companies controlled by their spouse or other relatives may be associated. Review the connected persons rules carefully.
• Not updating the count when circumstances change: If you acquire or dispose of a company during the accounting period, the associated companies count may change. The count should reflect the position during the relevant accounting period.
• Confusing associated companies with group relief: Group relief is a separate concept relating to the surrender of losses between group companies. The associated companies rules relate specifically to the corporation tax rate thresholds. A company can be associated without being part of a group, and vice versa.
• Assuming overseas companies don't count: Companies controlled by the same person are associated regardless of where they are incorporated or resident. An overseas company under common control will still reduce your UK thresholds.

Q: Do I need to list my associated companies on the CT600?

A: You need to include the number of associated companies in the relevant box on the CT600 form. HMRC uses this to verify that the correct rate thresholds have been applied. WeFile automatically includes this information in the XML submission.

Q: What if I'm not sure whether a company is associated?

A: If you are uncertain, it is advisable to include the company in your count and consult with a tax professional. Under-declaring associated companies could lead to an incorrect tax computation and potential penalties from HMRC.

Q: Does being a director of another company make them associated?

A: Being a director alone does not create an association. The test is based on control — primarily shareholding and voting rights. A director who holds no shares in a company does not control it for these purposes.

Q: Are companies associated for part of a period?

A: Yes. If a company is associated for any part of the accounting period, it counts as an associated company for the entire period. There is no time-apportioned reduction for partial-period associations.

Q: My company is dormant — do associated companies still matter?

A: If your company is dormant and has no taxable profits, the corporation tax rate thresholds are irrelevant because there is no tax to pay. WeFile correctly skips the associated companies calculation for dormant filings.

The reintroduction of the small profits rate from April 2023 has made the associated companies rules relevant again for hundreds of thousands of UK companies. Whether you control one additional company or several, the impact on your corporation tax rate thresholds can be substantial.

Here are the key takeaways:

• Associated companies are those under common control of the same person or group of persons
• The £50,000 and £250,000 thresholds are divided by (associated companies + 1)
• This can push your company into the marginal relief band or the main rate even with modest profits
• The limits are further pro-rated for short accounting periods and financial year splits
• Connected persons' companies may count unless there is no substantial commercial interdependence
• Dormant companies and non-trading companies are generally excluded

WeFile handles all of this automatically. Simply enter the number of associated companies in Step 1 of the filing wizard, and the system takes care of the rest — adjusting thresholds, calculating marginal relief, generating accurate CT600 XML, and producing compliant iXBRL accounts and tax computations.

If you have questions about whether specific companies in your portfolio are associated, we recommend consulting with a qualified tax adviser. For the filing itself, WeFile is ready to help.