Interest Relief Restrictions: A Guide for Growing UK Companies

What Are the Corporate Interest Restriction Rules?

The Corporate Interest Restriction (CIR) regime, introduced in April 2017, limits the amount of net interest and financing costs that a UK company or group can deduct when calculating its taxable profits. If your company is taking on significant debt — whether through bank loans, director loans, or third-party financing — these rules could affect how much relief you actually receive.

The rules are primarily aimed at larger businesses, but growing companies can fall within scope faster than they expect, particularly when acquiring assets, funding expansion, or operating within a group structure.

The De Minimis Threshold: When the Rules Bite

The good news for smaller companies is that there is a de minimis threshold of £2 million in net interest expense per year. If your company's total net interest costs (interest payable minus interest receivable) are below this figure, the CIR rules do not apply and you can deduct interest in full as a business expense in the normal way.

However, once your net financing costs exceed £2 million — which can happen quickly in a growing company with property debt, acquisition finance, or intercompany loans — you must apply the CIR calculations to determine your allowable deduction.

How the Restriction Is Calculated

Above the de minimis threshold, the CIR rules use two primary tests to determine your maximum interest deduction:

• Fixed Ratio Rule: Your company can deduct net interest up to 30% of its tax-EBITDA (earnings before interest, taxes, depreciation and amortisation, calculated on a tax basis). This is the default rule.
• Group Ratio Rule: If your group has a higher ratio of net interest to EBITDA than 30% — for example, because it is highly leveraged — you can elect to use the group's actual ratio instead, potentially allowing a higher deduction.

The more restrictive of the two rules applies unless you make a specific election. It is worth modelling both scenarios before filing, as the difference can be material.

Carry Forward and Carry Back of Unused Capacity

If your company cannot deduct all of its interest in a given year due to the CIR restriction, the disallowed amount is not lost permanently. Disallowed interest can be carried forward indefinitely and deducted in future periods when the company has sufficient interest capacity. Similarly, unused interest capacity (where your 30% allowance exceeds your actual interest) can be carried forward for up to five years, giving you flexibility as profits fluctuate.

Practical Steps for Directors

• Monitor your net financing costs annually. If you are approaching the £2 million threshold, plan ahead before the year-end rather than being caught out at filing time.
• Review intercompany loans carefully. Interest on loans between connected companies counts within the CIR calculation and is a common area where groups inadvertently trigger restrictions.
• Appoint a reporting company. Groups subject to CIR must nominate a single reporting company to submit the CIR return to HMRC. Missing this step can result in penalties and loss of certain elections.
• Consider the Group Ratio election. If your group is heavily leveraged, the Fixed Ratio Rule may unfairly penalise you. Model the Group Ratio Rule before accepting the default.
• Keep detailed records of tax-EBITDA. The calculation differs from accounting EBITDA, so work with your accountant to ensure the figures are computed correctly on a tax basis.

Common Pitfalls to Avoid

One frequent mistake is assuming that because interest is commercially justified, it is automatically deductible in full. The CIR rules operate independently of the transfer pricing and loan relationship rules — satisfying one set of rules does not guarantee full relief under another. Always review all three frameworks when structuring significant debt.

Another pitfall is failing to file a CIR return on time. Even if no restriction applies, groups with net interest above £2 million may still have reporting obligations. Late or incorrect filings can result in HMRC issuing its own calculations, which may not be in your favour.

Getting It Right

The CIR regime adds genuine complexity for growing companies, but with proper planning it is manageable. If your company is scaling up, taking on acquisition debt, or operating within a group, speak to a qualified UK tax adviser before your next accounting period ends to ensure your interest deductions are maximised and your compliance obligations are fully met.