The corporate interest restriction can become relevant well before a group regards itself as a large multinational. The regime applies by reference to a worldwide group’s UK tax-interest expense and tax-EBITDA, and a worldwide group can consist of a single company. Growing businesses therefore need to monitor the rules when acquisitions, property investment, leveraged expansion or refinancing cause deductible financing costs to increase materially.
The regime does not simply disallow interest above a fixed cash amount. The £2 million threshold is a de minimis, after which the available deduction depends on detailed group calculations.
When the corporate interest restriction needs to be reviewed
A group with less than £2 million of net tax-interest expense for a 12-month period will not ordinarily suffer a restriction. The threshold applies to the worldwide group, not separately to each company.
The corporate interest restriction overview confirms that the regime limits deductions for net interest and economically similar financing costs so that UK deductions remain proportionate to taxable UK activity and the group’s external financing position.
A growing group should therefore begin modelling before net financing expense actually reaches £2 million. An acquisition funded late in one period, a rise in borrowing rates or a new property development facility can move the group across the threshold quickly.
The calculation uses tax-interest, not merely the finance-cost line appearing in the statutory accounts. Loan relationships, derivatives and other financing amounts may require tax adjustments before the threshold can be assessed.
Corporate interest restriction under the fixed ratio method
The default fixed ratio method broadly limits the basic interest allowance to the lower of 30% of the worldwide group’s aggregate UK tax-EBITDA and the fixed ratio debt cap.
Tax-EBITDA is a statutory Corporation Tax concept rather than ordinary management EBITDA. It broadly starts from UK taxable earnings and makes specified adjustments for tax-interest, capital allowances and other items. A group’s bank covenant EBITDA should not therefore be assumed to produce the CIR result.
Where the group has relatively high genuine external borrowing, the alternative group ratio method can sometimes provide a higher allowance. That method uses a ratio derived from qualifying net group-interest expense and group EBITDA based on the consolidated group accounts, subject to its own debt cap and detailed exclusions.
The comparison should be modelled before filing because elections and group data can materially alter the restriction.
Acquisitions can change the worldwide group
CIR operates by reference to the worldwide group headed by the ultimate parent. Acquiring or disposing of subsidiaries can therefore affect group membership, tax-EBITDA and financing amounts during the relevant period.
This is particularly important where a UK business becomes part of an international group or private-equity structure. Financing costs that previously sat below the de minimis cannot simply continue to be reviewed at standalone-company level.
Likewise, a single company can itself constitute a worldwide group where it has no consolidated subsidiaries. The absence of a conventional group structure does not automatically place a heavily financed company outside CIR.
Disallowed interest is not necessarily lost permanently
Where tax-interest exceeds the available allowance, the restricted amount can generally be carried forward indefinitely and potentially reactivated in a later period when sufficient interest allowance is available.
Unused interest allowance can broadly be carried forward for up to five years. This makes record keeping important even in a year where the immediate restriction is manageable. Future profitability, debt repayment or group restructuring may permit previously disallowed deductions to be used.
Groups should therefore maintain a CIR history rather than viewing the calculation as a one-year adjustment.
Administration changed for periods ending from 31 March 2026
Finance Act 2026 altered the administrative position for periods of account ending on or after 31 March 2026.
Under the revised interest restriction return requirements, a reporting company appointed by HMRC remains required to file a return. A reporting company appointed by the group is no longer automatically obliged to submit an interest restriction return for those later periods, although filing can still be beneficial where the group needs to allocate restrictions, preserve unused allowance or reactivate previously restricted interest.
Where a return is required following an HMRC appointment, the filing date is generally the later of 12 months from the end of the worldwide group’s period of account and three months after that appointment.
Financing should be reviewed before year end
The CIR calculation interacts with acquisitions, external and related-party debt, losses and the composition of the worldwide group. Waiting until Corporation Tax returns are being completed can remove opportunities to gather the necessary consolidated information or consider elections properly.
Interest restrictions should ultimately reconcile with each affected company’s Corporation Tax computation. Groups approaching the £2 million net tax-interest threshold, undertaking leveraged acquisitions or carrying historic restricted interest should obtain specialist tax advice while the financing structure and relevant group data remain available.
Disclaimer: This article is provided for general guidance only and does not constitute personal tax, legal or financial advice. The correct tax treatment depends on the specific facts, applicable legislation and HMRC practice at the relevant time. You should contact us for advice tailored to your circumstances before taking or refraining from any action.
