Accountants for US and UK: Associated Companies & CT Rate

If you control more than one company, the UK corporation tax lower and upper limits are divided by the total number of associated companies, so a founder with four companies is tested against £12,500 and £62,500 rather than £50,000 and £250,000. Dormant-looking and non-UK companies often count. Returns already filed on the wrong count must be corrected.

This is the single most common corporation tax error we see in the accounts of American founders living in Britain, and it is almost always discovered late. As Accountants for US and UK filings, Jungle Tax sees the same pattern repeatedly: a trading company, a property company that holds the London flat, a holding vehicle that was incorporated for a funding round that never closed, and — critically — a Delaware corporation or a US LLC left over from before the move. Each CT600 was filed as though the company stood alone in the small profits band. None of them did.

What does the associated companies rule actually do to your corporation tax rate?

Since 1 April 2023 the UK has operated a two-rate corporation tax system. Profits up to the lower limit are charged at the small profits rate of 19%. Profits above the upper limit are charged at the main rate of 25%. Between the two, the full 25% is charged and then reduced by marginal relief, producing an effective marginal rate of 26.5% on the slice of profit inside the band.

The limits themselves are not fixed. They are divided by the number of associated companies the company has, counting the company itself. This is set out in Part 3A of the Corporation Tax Act 2010, with the associated company definition at sections 18E to 18J, and HMRC's own treatment sits in the Company Taxation Manual from CTM03900 onwards. GOV.UK's public-facing summary of Marginal Relief for Corporation Tax confirms the same mechanic in plainer terms.

How the thresholds divide in practice

Total associated companies	Lower limit (19% ceiling)	Upper limit (25% floor)	Profit of £60,000 is taxed at
1 (standalone)	£50,000	£250,000	Marginal relief band, low 19s% effective
2	£25,000	£125,000	Marginal relief band, higher effective rate
3	£16,666	£83,333	Marginal relief band, close to 25%
4	£12,500	£62,500	Marginal relief band, effectively 25%
5	£10,000	£50,000	Full main rate of 25%

Two further points are routinely missed. First, the limits are also time-apportioned for accounting periods shorter than twelve months, so a stub period after an incorporation or an accounting date change compounds the division. Second, the test is applied to augmented profits — taxable total profits plus certain exempt distributions received from non-group companies — not to the profit figure in the statutory accounts. A founder whose trading company receives dividends from a minority stake can be pushed across a divided threshold by income that is not itself taxable.

What makes two companies associated?

A company is associated with another if, at any time in the accounting period, one controls the other, or both are under the control of the same person or persons. Control is defined by sections 450 and 451 CTA 2010 and is deliberately wide: it covers a person who exercises, is able to exercise, or is entitled to acquire direct or indirect control over the company's affairs. In practice it is tested by reference to share capital, voting power, entitlement to income on a notional distribution, and entitlement to assets on a notional winding up. Any one of those routes is enough.

Three features of the rule cause most of the damage:

• "At any time" is literal. A company sold in month two of a twelve-month accounting period still counts for that period. Founders who restructured mid-year frequently file on the post-restructure count.
• There is no minimum holding period and no de minimis. A company held for a single day is counted the same as one held throughout.
• Worldwide companies count. The rule is not limited to UK-resident companies. A Delaware C corporation, a foreign holding vehicle, or an overseas operating subsidiary you control is an associated company for the purpose of dividing the UK limits, even though it will never file a CT600 and has no UK presence whatsoever.

When are your spouse's or family's companies attributed to you?

This is the part generalist guidance handles worst. The rights and powers of your associates — broadly spouse or civil partner, lineal ancestors and descendants, siblings, business partners, and trustees of certain settlements — can be attributed to you when determining control. But that attribution only applies where there is substantial commercial interdependence between the two companies in question.

Substantial commercial interdependence is assessed across three connections, and one alone can be sufficient:

• Financial — one company gives financial support to the other, or each has an interest in the same financial outcomes. Intercompany loans, cross-guarantees, and a shared overdraft facility are the classic markers.
• Economic — the companies pursue a common economic objective, the activities of one benefit the other, or they have common customers. A property company that exists to house the trading company's premises is squarely in this territory.
• Organisational — common management, common employees, common premises, or common equipment.

For an American founder in Britain the practical sting is that a US spouse's own company — often a consulting S corporation or single-member LLC retained after the move — can be drawn into the count where any of those connections exist. The UK test reaches across borders; the fact that the spouse's company is filing a Form 1120-S in Ohio is irrelevant to whether it divides your London company's £50,000.

Do dormant and non-trading companies count?

Only sometimes, and the distinction is finer than most founders assume. A company is ignored for the count if it has not carried on a trade or business at any time in the accounting period. The words "or business" do a great deal of work. "Business" is materially wider than "trade": holding and letting property is a business, and an investment company managing a portfolio is carrying on a business even though it trades nothing.

There is a separate, narrow let-out for a genuinely passive holding company — broadly one with no assets other than shares in its 51% subsidiaries, no income other than dividends from those subsidiaries which it distributes onward in full, and no chargeable gains or management expenses. That is a demanding specification. A holding company that has retained even a small cash balance earning interest, or that has incurred its own accountancy fees, will struggle to meet it.

The single most expensive misconception here is that Companies House dormancy and corporation tax dormancy are the same thing. Filing dormant accounts under the Companies Act does not establish that the company carried on no business for corporation tax purposes. We routinely see a "dormant" property SPV that in fact holds a let flat, or a "dormant" shelf company that nonetheless holds a deposit and earns interest, counted out of the divisor when it should have been counted in.

What happens to your quarterly instalment payment thresholds?

The same divisor applies to the instalment regime, and because the instalment thresholds are so much larger, the cash-flow consequence usually arrives before the rate consequence does. For accounting periods beginning on or after 1 April 2023 the instalment tests are based on associated companies, replacing the older and much narrower "related 51% group company" test. That change alone dragged a large population of founder-owned groups into instalments for the first time.

Regime	Standalone threshold	With four associated companies	Consequence
Large company	£1.5m augmented profits	£375,000	Four instalments, starting in month 7 of the period
Very large company	£20m augmented profits	£5m	Instalments accelerated into the period itself
Grace-period ceiling	£10m	£2.5m	One-year reprieve lost where profits exceed it

Two mitigations survive. A company with a corporation tax liability below a small monetary de minimis for the period is not required to pay by instalments. And a company that was not large in the previous twelve months gets a one-year grace period, provided its augmented profits do not exceed the (divided) grace-period ceiling. Both are easy to lose once the divisor grows. Note also that the instalment status test looks at the number of associated companies at the start of the accounting period, which is a different measurement point from the rate test — a trap for anyone who incorporates a new vehicle in month one.

How do you correct CT600s already filed on the wrong basis?

Almost every founder who reads the rule properly discovers that at least one submitted return is wrong. The remedy depends entirely on how old the return is.

• Within the amendment window. A Company Tax Return can normally be amended within twelve months of the filing deadline — in practice roughly two years from the end of the accounting period. GOV.UK sets out the mechanics for making changes to a Company Tax Return. The amendment restates the associated company count, recomputes marginal relief, and produces a revised liability. Interest runs from the original due date on the additional tax.
• Outside the window, tax underpaid. The amendment route is closed and disclosure is the correct path. HMRC expects the company to notify and disclose the error, with the penalty position turning on whether the inaccuracy was careless or deliberate, and whether the disclosure was prompted or unprompted. An unprompted disclosure for a careless error can attract a very substantially reduced penalty, sometimes nil.
• Outside the window, tax overpaid. The reverse case is real: founders sometimes count companies that should have been ignored, and overpay. Overpayment relief is generally available for four years from the end of the accounting period.

Correcting one year rarely stands alone. Because the divisor usually persisted across several periods, a correction is normally a multi-year exercise touching each company in the stable, and the instalment position for each year has to be reworked alongside the rate. Sequencing matters: the additional UK tax should be quantified across all companies and all open years before anything is filed, so that interest and penalty exposure is understood as one number rather than discovered piecemeal. Our UK tax services team handles these as a single remediation project rather than a series of ad hoc amendments.

The US overlay: the same companies, tested again on completely different rules

Here is where generalist UK guidance stops and cross-border work begins. The companies you have just counted for the UK divisor are simultaneously being tested under US rules — and the two tests share almost nothing. A company can be associated for UK purposes but not a controlled foreign corporation for US purposes, and vice versa.

	UK: associated companies	US: CFC and Form 5471
Purpose	Divides the CT rate limits and instalment thresholds	Determines income inclusion and information reporting
Control test	Control by the same person or persons; any one of several routes	US shareholders owning 10%+ each, together owning more than 50% by vote or value
Family attribution	Associates' rights attributed, but only with substantial commercial interdependence	Constructive ownership applies mechanically — spouse, children, parents, grandchildren, entities — with no interdependence condition
Dormant companies	Ignored if no trade or business in the period	Dormancy is broadly irrelevant; the filing obligation persists
Non-resident companies	Counted worldwide	Only foreign (non-US) corporations are in scope
Consequence of getting it wrong	Under- or overpaid corporation tax, interest, penalties	Fixed information penalties per form per year, plus an open statute of limitations

The reporting obligation is Form 5471, Information Return of U.S. Persons With Respect To Certain Foreign Corporations, and a US founder with several UK companies generally has a separate Form 5471 for each one. Category 4 captures a US person who had control of a foreign corporation during its tax year. Category 5 captures a US shareholder of a CFC. A founder who owns a trading company, a property company and a holding company outright is typically a Category 4 and Category 5 filer three times over. The schedules required differ by category, and the property company in particular tends to generate passive income that has to be analysed for subpart F.

Two asymmetries are worth internalising. First, the US constructive ownership rules attribute a spouse's or child's shares mechanically, with none of the "substantial commercial interdependence" gating that the UK applies — so the US net can be wider than the UK one on exactly the same facts. Second, a company that is genuinely ignored for the UK divisor because it carried on no trade or business is still a foreign corporation for US purposes, and an unfiled Form 5471 for a dormant vehicle carries the same fixed penalty as one for a live trading company. The statute of limitations on the entire personal return can remain open until the missing form is filed, which converts a paperwork oversight into indefinite exposure on the whole year.

Does a higher UK corporation tax rate help or hurt the US position?

It depends entirely on how the company's profits reach the founder. Where profits are retained and the founder faces a current inclusion on the company's income, the increase in UK corporation tax raises the foreign tax pool that may be creditable — but an individual US shareholder generally cannot access corporate-level foreign taxes without making the relevant election to be taxed as if a domestic corporation, and the credit is subject to a statutory haircut. Where profits are instead extracted as dividends, the extra UK corporation tax simply reduces the distributable reserve, and the US tax on the dividend is computed on the smaller amount. Neither outcome makes the UK error costless; both mean the correction has to be modelled on both sides simultaneously rather than fixed in the UK and reported in the US afterwards.

If earlier US returns were filed without the Forms 5471 at all — a very common position for founders who incorporated UK vehicles before taking US advice — the remediation is a disclosure exercise in its own right. Where the failures were non-wilful, the IRS streamlined filing procedures can be the appropriate route to bring the information returns and the underlying income reporting up to date. That decision has to be made deliberately, and it interacts with the UK correction timeline.

A working method for founders with more than one company

1. Build the complete inventory. Every company you or your associates control, worldwide, with incorporation and disposal dates. Include US entities, dormant shells, and anything acquired or sold mid-period.
2. Test each company against "trade or business", not against Companies House dormancy. Document the reasoning for each exclusion; HMRC will ask.
3. Apply the attribution test properly. For each associate-held company, work through financial, economic and organisational interdependence separately and record the conclusion.
4. Recompute the limits for every open period, including time-apportionment for short periods, and recompute augmented profits rather than reusing accounting profit.
5. Rework the instalment position for each year, remembering that the instalment test is taken at the start of the period.
6. Quantify the whole exposure before filing anything, then choose between amendment, disclosure and overpayment relief per year.
7. Run the US test in parallel, separately, on its own rules — never by translating the UK answer.

Founders approaching a sale should be particularly alert. A prospective buyer's tax due diligence will reconstruct the associated company count from the register of members and the group structure chart, and an unamended CT600 filed on an understated count is a standard warranty and indemnity item. Correcting it before a process begins is materially cheaper than negotiating it during one. If a sale is in contemplation, read this alongside our material on cross-border tax planning and our overview for high net worth individuals.

Speak to us in confidence

If you control more than one company on either side of the Atlantic and you are not certain the divisor on your filed returns is right, the question is answerable quickly and the answer is usually worth having before HMRC raises it. We will reconstruct the count across every open period, quantify the corporation tax and instalment exposure, identify which years can still be amended and which require disclosure, and test the same companies independently against the US CFC and Form 5471 rules. To begin, contact our cross-border team for a confidential consultation.