You've got cash in one company, a project in another, and an inter-company loan looks like the cleanest way to keep the group moving. On paper, it feels efficient. In practice, thin capitalisation rules UK directors face today are less about a neat debt-to-equity ratio and more about whether the funding really looks like something an independent lender would have accepted.
That shift catches people out because the old habit is still alive. Many directors still talk about “safe” debt levels, but HMRC's modern view is different, and the Corporate Interest Restriction now sits alongside transfer pricing as the main commercial test on financing costs. If your group borrows from a connected party, the details matter, the loan amount, the interest rate, the repayment profile, and the commercial logic all come under scrutiny.
Understanding Your Risk with Company Loans
A director sees one company sitting on cash while another needs funding for stock, staff, or a property project. The transfer looks sensible, because the group keeps control of its own money and avoids a bank queue. The problem starts when that internal loan is written on terms no outside lender would accept.
That's the point where the issue stops being ordinary treasury management and becomes a tax question. HMRC looks at whether the borrowing is commercially supportable, and whether the group would have agreed the same deal with an unconnected party. The old idea that there's a fixed ceiling for borrowing just doesn't fit how the UK now approaches thin capitalisation.
A simple inter-company loan can be perfectly legitimate, but only if the terms look real, not improvised.
The practical risk is easy to miss because the money has moved between group companies. Even so, HMRC can still reprice the financing if the deal is too generous to the borrower or too thin on substance. That's why the safer question isn't “How much can we borrow?”, it's “Would an outside lender have said yes on these terms?”
When the answer is shaky, the tax cost lands in the place directors care about most, cash. Interest deductions can be reduced, which raises taxable profit and leaves less group cash available for reinvestment. For a director trying to fund growth, that can turn a tidy internal solution into a recurring profit leak.
A good starting point is to review any related-party funding before money moves, not after the accounts are signed. The connected-party context is usually where the risk begins, and a quick check against commercial reality is far cheaper than defending a weak loan later. See the practical angle on related-party transactions.
What Thin Capitalisation Really Means
Thin capitalisation is often described too narrowly. In practice, it asks a blunt question, would an independent lender have made this loan, on these terms, to this borrower? HMRC frames the issue around the arm's-length principle, which applies to the amount of debt and to the loan terms themselves, including whether the borrowing would have been made at all between independent parties dealing at arm's length, as set out in the legislation notes on connected-company securities.
The bank test directors recognise immediately
A high-street bank does not just ask whether a company wants money. It checks substance, repayment ability, trading history, security, and whether the borrower can service the debt. HMRC applies the same commercial logic when it reviews connected-party funding.
That means the size of the loan matters, but it is only one part of the picture. Interest rate, repayment schedule, covenants, security, and the borrower's overall strength all come into play. A loan that looks fine in a group spreadsheet may still fail if it would never have been offered by an independent bank.
Practical rule: if the lender and borrower are connected, assume HMRC will ask whether the arrangement has enough commercial shape to survive outside the group.
The tax consequence is not always an all-or-nothing denial. HMRC can reprice the loan to a level an independent lender would likely have accepted on the same facts, and that can lead to a proportionate denial of deductions if the debt is viewed as excessive HMRC's guidance on proportionate denial. That distinction matters, because directors cannot treat a loan as safe merely because some interest is being paid.
The loan has to make business sense before it makes tax sense.
The UK also has a long history here. HMRC says the modern framework moved into transfer pricing from 1 April 2004, when the old standalone rules were repealed and the arm's-length review extended to financing arrangements, including UK-to-UK transactions. That is why connected-party borrowing is never just an accounting formality, it is a pricing and evidence exercise as well. For a wider view of the mechanics, see transfer pricing rules in the UK.
If you are comparing your own financing to group norms, the right benchmark is whether a third party would have accepted it. That shift from habit to evidence is where most director mistakes start.
The Modern Framework Corporate Interest Restriction
A director can no longer rely on a simple debt-to-equity check and assume the funding is acceptable. The UK's modern approach is the Corporate Interest Restriction, and it sits alongside the arm's-length test, it does not replace it.
Two tests, not one
The loan still has to make commercial sense at arm's length. Even then, the group's interest deductions can be restricted under the CIR rules if the borrowing costs are too high relative to the group's tax-EBITDA. HMRC does not give companies a fixed safe harbour for thin capitalisation, so there is no single ratio that automatically puts a financing arrangement beyond review.
That is why the old habit of treating debt as acceptable because it is labelled “inter-company loan” gives directors a false sense of comfort. A company can have a genuine borrowing, a real business reason, and an interest rate that looks market-based, then still face a restriction if the wider group interest burden is too heavy. The test is driven by earnings capacity, not just the balance sheet, which is where many groups misjudge the risk.
The modern framework also reflects a wider change in UK tax practice. HMRC moved thin capitalisation into transfer pricing from 1 April 2004, replacing the old standalone rules and extending arm's-length review to financing arrangements, including UK-to-UK transactions. That is why connected-party borrowing is never just a funding decision, it is also a pricing exercise and an evidence exercise. For a practical explanation of the commercial review, see transfer pricing rules in the UK.
The compliance pressure has also stayed live. HMRC statistics show that transfer pricing remains a meaningful part of the department's yield, which tells directors these issues are still being tested in practice HMRC statistics. That does not decide how one loan should be written, but it does show the area is under close scrutiny.
The practical point is simple. Directors need to assess the loan itself and the deductibility of the interest as separate questions. One is about commercial realism, the other is about whether the tax rules allow the cost through.
For a wider technical view of the interest cap rules, the UK's Corporate Interest Restriction regime is set out in the context of interest on business loans.
How the UK Interest Restriction Rules Work in Practice
The CIR regime gives directors a clean place to start, the £2 million de minimis. In general terms, if a group's net interest expense is at or below that level, the CIR rules are not the main issue. Once borrowing costs move beyond that point, the restriction can start to matter quickly.
A simple working example
Take a company with tax-EBITDA of £500,000. Under the fixed ratio method, the deductible interest cap is 30% of tax-EBITDA, which means £150,000 in this example. If the company's net interest expense is higher than that figure, the excess is disallowed for the period and taxable profits go up.
| CIR Calculation Example | Amount | Calculation |
|---|---|---|
| Tax-EBITDA | £500,000 | Given figure |
| Fixed ratio | 30% | CIR standard rate |
| Maximum deductible interest | £150,000 | £500,000 x 30% |
| Net interest expense | Above £150,000 | Amount over the cap is restricted |
That's the kind of calculation directors need to see early, not after year-end. The cap is not there to punish borrowing in itself, it's there to stop a group from pushing too much finance cost into the UK tax base.
The more important point is that this is a two-layer technical test. HMRC says that for businesses with intragroup debt, the borrowing must first be commercially supportable at arm's length, and then even arm's-length interest may still be restricted if group-level borrowing costs exceed the CIR cap HMRC's guidance on the two-layer test. That's where some directors get tripped up, because commercial validity doesn't guarantee full tax relief.
A loan can be perfectly real and still be partly non-deductible.
The structure of the group matters as well. If one company borrows externally and then funds another company internally, the interest trail needs to be checked carefully, because the tax outcome follows the rules, not the cash movement. Once you see the CIR as an earnings-based cap sitting on top of transfer pricing, the core planning question becomes how to keep finance costs efficient without drifting into restriction.
For larger groups, the compliance burden rises further. Some groups also need OECD-format Master File and Local File documentation where the consolidated revenue threshold is EUR 750 million, with those rules effective for corporation tax periods beginning on or after 1 April 2023 and for income tax from 6 April 2024 PwC summary of UK group documentation rules. That doesn't apply to every company, but it shows how quickly financing can move from routine bookkeeping into formal transfer pricing work.
Common Pitfalls and Red Flags for Directors
A loan often becomes a problem long before HMRC raises an enquiry. The warning signs usually sit in the numbers and the paperwork, especially where a company relies heavily on related-party borrowing, has weak profits, and carries little equity. Add loose terms or inconsistent records, and the tax risk increases quickly.
What usually gets attention
- Debt that looks excessive for the business: If the borrower could not realistically have raised the same amount from an outside lender, HMRC is more likely to question the structure.
- Interest terms that look off-market: Zero interest, unusually high interest, or ad hoc changes to terms can suggest the loan was arranged for tax results rather than commercial reasons.
- No repayment discipline: A loan with no real repayment profile can look more like equity presented as debt.
- Multiple intra-group loans moving in circles: Complex funding chains are harder to defend and often make it difficult to show a genuine commercial purpose.
- Weak paperwork: If the company cannot produce a proper agreement, board approval, and a clear funding rationale, the arrangement looks informal rather than commercial.
The technical risk is proportionate denial of interest deductions where debt is viewed as excessive. HMRC can reprice the financing to the level an independent lender would likely have accepted on the same facts and circumstances. Directors often assume the whole loan has to be rejected before tax relief is affected. That is rarely how the issue plays out.
The practical failure point is often timing. Directors inject cash quickly because the business needs it, then leave the structure unchanged for years. By the time the accounts show the weakness, the loan has become routine and nobody wants to disturb it. A review is most useful at that point, because a short-term fix can turn into a recurring tax issue.
Start with the loan amount, the rate, the borrower's earnings, and whether the arrangement would look normal in the market. A genuine commercial case is much easier to defend than a file rebuilt after challenge. A focused review of transfer pricing documentation usually exposes the weak points before they become expensive.
Legitimate Planning and Effective Compliance
A director who treats an inter-company loan as a simple funding shortcut usually creates the biggest tax risk. The better approach is to make the financing easy to defend from the start, because the modern test is not whether the group calls it debt, but whether the terms stand up to arm's-length scrutiny under the Corporate Interest Restriction (CIR) framework.
What good looks like
A formal loan agreement should set out the principal, interest rate, repayment terms, and any security. If the loan is intended to be commercial, the paperwork needs to say that clearly and consistently, because HMRC will compare the document with how the parties behave. Where the paperwork and the conduct line up, the file is much easier to defend.
If the agreement, the bank terms, and the board minutes all tell the same story, the position is much easier to defend.
Benchmarking still matters. A director does not need a perfect market survey to strengthen the file, but the company should be able to explain why the rate and terms were chosen, and how they compare with independent funding for a similar risk profile. That point matters even more where the group is relying on connected-party finance rather than external borrowing, because the commercial logic has to be clear enough to survive challenge.
For larger or more sensitive cases, HMRC can offer more certainty through an Advance Thin Capitalisation Agreement (ATCA), which can give comfort for a defined period if the structure is accepted in advance. That can save time later, especially where the financing supports several entities or a long-running project, and it reduces the risk of revisiting the same argument every year.
HMRC's own statistics show that transfer pricing remains a live compliance area, and that means financing files are not something to leave half-finished. For directors, the practical point is simple. Weak loan files are easier for HMRC to challenge, while clear documentation gives the business a better chance of keeping interest deductions where they belong.
The best position is straightforward. Keep the loan commercial, keep the records clean, and keep the interest profile consistent with what an outside lender would tolerate. That turns compliance from a reactive tax problem into part of sound treasury management.
If you want a clearer view of whether your inter-company loans, interest deductions, and documentation work under the UK's thin capitalisation and CIR rules, Tax Compass can help you review the structure before HMRC does. Visit Tax Compass for practical guidance that helps you spot the gaps, tighten the evidence, and decide whether a more detailed review is worthwhile.