The 'Associated Companies' Trap: How a Second Company Can Quietly Raise Your UK Corporation Tax Bill

The UK has had a two-rate Corporation Tax system since 1 April 2023. Most founders know the headline: 19% if your company is small, 25% if it is large. What far fewer realise is that the line between "small" and "large" can shift the moment you control more than one company — even a company on the other side of the world. This is the "associated companies" rule, and it quietly catches founders who think a second business is a cost-free decision.

How the Two-Rate System Works

For accounting periods from 1 April 2023, Corporation Tax is charged as follows. Profits of £50,000 or less are taxed at the small profits rate of 19%. Profits above £250,000 are taxed at the main rate of 25%. Profits between those two figures are taxed at 25% but reduced by Marginal Relief, which produces a sliding effective rate somewhere between 19% and 25%.

Marginal Relief is calculated using a fraction of 3/200. One consequence worth understanding: within the £50,000–£250,000 band, each extra £1 of profit is effectively taxed at 26.5% — higher than the headline main rate. The thresholds, not just the rate, are what really drive your bill.

Where the Trap Is

Here is the part that surprises people. The £50,000 lower limit and the £250,000 upper limit are not fixed per person. They are divided by the total number of associated companies, including the company you are looking at.

GOV.UK gives a clear example: if your company has three other associated companies, the limits are divided by four. The lower limit drops to £12,500 and the upper limit drops to £62,500. Suddenly profits that would have enjoyed the 19% rate, or generous Marginal Relief, can be pushed straight into the 25% main rate.

What Counts as an "Associated Company"

The definition sits in section 18E of the Corporation Tax Act 2010. A company is associated with another if one controls the other, or if both are under the control of the same person or persons. "Control" here borrows the close-company meaning — broadly, holding more than 50% of the shares, voting power, or rights to income or assets, whether directly or indirectly.

Two points matter enormously for international founders. First, HMRC's own manual is explicit that a company may be an associated company regardless of where it is tax resident. An overseas company you control is counted in the tally even though it pays no UK tax. Second, a company that has not carried on any trade or business at any time during the period — a genuinely dormant company — is disregarded and does not count.

It is also worth knowing that a non-UK resident company cannot itself claim Marginal Relief. So an overseas company can increase the divisor that shrinks your UK company's thresholds, without ever being able to benefit from the lower rates itself.

A Worked Example

Imagine a founder whose UK LTD makes £60,000 of profit, and who controls no other active company. The £60,000 sits between £50,000 and £250,000, so Marginal Relief applies. The relief works out to roughly £2,850, bringing the tax bill to about £12,150 — an effective rate of around 20.25%.

Now imagine the same founder also controls four other active companies anywhere in the world. The limits are divided by five: the lower limit falls to £10,000 and the upper limit falls to £50,000. The £60,000 profit is now above the £50,000 upper limit, so the full 25% main rate applies with no Marginal Relief at all. The bill becomes £15,000 — about £2,850 more on identical profits, purely because of companies that may have nothing to do with the UK business.

How to Stay on the Right Side of It

Before you estimate your Corporation Tax, count every company under common control — including overseas ones, and including companies owned through family or business partners where there is genuine commercial interdependence. Remember that truly dormant companies are excluded, but "dormant" has a strict meaning: no trade or business carried on during the period.

If you are a non-resident founder who owns several entities, do not assume that opening a second company is free. It can raise the effective tax rate on a company you already run. Tell your accountant about every company you control worldwide, not just your UK one, so the thresholds are calculated correctly. Whether a particular foreign structure — for example a US LLC — is treated as a "company" for this test can be fact-specific, so it is worth confirming rather than guessing.

Have Questions About Your Own Situation?

If you own more than one company and you are not sure how the associated companies rule affects your UK Corporation Tax, it is worth talking it through with someone who does this every day. No pressure, no hard sell, just clear answers about your specific setup.

Book a free consultation with our experts →