Corporate Interest Restriction can directly affect a company or group’s cash tax, financing decisions and even future acquisitions or restructuring plans as well as reported results.
If your company’s or group’s UK net interest is approaching or above £2m, CIR needs active attention. At that point, interest deductibility is no longer automatic and must be managed. Deductions are capped using either the fixed ratio of 30% of UK tax EBITDA or, where elected, a group ratio based on external gearing. The more favourable method can change year to year as profitability and leverage move.
From a practical perspective, CFOs or Tax leaders should be ensuring CIR is considered in refinancing, acquisitions and wider capital planning. It is equally important that tax EBITDA and tax interest are identified correctly and that disallowed interest and unused allowance are tracked at entity level in case of groups. These are not areas where errors are always visible immediately, but they can have lasting tax consequences.
One point that is often missed is the compliance requirement around the £2m threshold. Even where a group is below £2m and no restriction arises, an interest restriction return still needs to be filed to preserve any unused allowance. If the IRR is not filed, the ability to carry forward that unused capacity for up to five years is lost, which can adversely affect future periods when interest levels increase. Remember it is not your standard corporation tax return but a completely separate notification to HMRC.
Governance is becoming more important as the rules evolve. Upcoming changes prescribed in November 2025 include the removal of automatic reporting company rollover, meaning groups will need to actively maintain a valid appointment, alongside changes to the level of group support required. There is also a greater penalty focus, including risks where returns are filed without a valid reporting company in place. At the same time, refinements to tax EBITDA can reduce available capacity, increasing the likelihood of restriction. These changes apply to periods ending on or after 31 March 2026.
Disallowed interest can be carried forward and reactivated when capacity arises, but it may be lost if the underlying business ceases or becomes negligible. In practice, the biggest risks are often not the technical calculation itself, but missed filings, invalid appointments and weak tracking processes.
If CIR is relevant to your company or group, it needs clear ownership, regular modelling and robust compliance which can avoid a costly surprise tax bill. If you need more clarity or advice around the election of methods or administration and compliance considerations please feel free to reach out.