As tax professionals, we know that the rules never stand still. The Corporate Interest Restriction (CIR) regime is a perfect example: introduced in 2017, it has become a core part of UK corporation tax compliance for any business with significant financing costs. Yet, even eight years on, I see many teams still grappling with the complexity and practicalities of the rules. If you’re responsible for your firm’s technical quality, risk management or client service, you’ll already know that CIR is not something you can afford to leave to chance.

I lead on CIR training at ExtraTax Training, and in this article I want to share why CIR training is still so important, what makes our approach different, and how investing in your team’s CIR knowledge can help you deliver better results for your clients and your firm.

The CIR rules were brought in to stop excessive UK interest deductions from eroding the UK tax base. They limit the amount of interest and similar financing expenses that companies can deduct for corporation tax purposes. The rules are mandatory for any group or standalone company with net financing costs above £2 million. If your clients are anywhere near that threshold – or might be in the next few years due to refinancing or external investment – CIR will be relevant to them.

The regime is complex, and the administration can be just as challenging as the technical calculations. There are forms, deadlines and multiple layers of compliance to navigate. Mistakes don’t just mean penalties – they can result in real tax costs for your clients, and potential claims against your firm if your advice falls short. In my experience, it’s not just the technical specialists who need to understand CIR. Anyone involved in corporation tax compliance, transactions, restructuring or due diligence needs to be able to spot the risks and know when to ask for help.

You might ask: if the rules have been around since 2017, shouldn’t everyone be up to speed by now? The reality is that CIR is still a common source of errors. HMRC continue to highlight mistakes made by both agents and taxpayers.

As the private equity market has grown, more clients are facing complex financing arrangements and encountering CIR for the first time.

The expectation from HMRC is clear: tax teams should now be fully trained and applying the rules correctly. Yet, I regularly reviewed computations where the software had been trusted without question, or where the inputs were wrong because the person preparing the return didn’t really understand the rules.

At ExtraTax Training, we don’t believe in off-the-shelf lectures or “tick-box” CPD. Every CIR training programme I deliver is tailored to the needs and experience of your team. For example, in a recent project with a large firm, we ran two separate sessions:

• one for junior team members focusing on the fundamentals, and
• another for more senior staff covering risk areas, transactions and advanced scenarios.

Both sessions were practical and interactive, with real-life examples and a strong focus on what you actually need to do in practice.

We provide pre-course reading to get everyone up to speed, quizzes to reinforce learning, and case studies that don’t just cover CIR in isolation, but also related financing and tax issues such as hybrid mismatches, transfer pricing, withholding tax and the late paid interest rules. The aim is to give your team the confidence to identify issues, understand the practical steps involved, and know how to research more complex scenarios when they arise.

Our training is never “one size fits all”. I work closely with your senior team before the training to understand your clients, your current knowledge gaps and your business objectives. We review the content together, adapt it as needed, and even tweak sessions after initial feedback to make sure we’re hitting the mark.

In one case, we worked with a client’s own CIR specialist to co-design the content and ensure it was exactly what their team needed. This collaborative approach means your team gets relevant, practical training that genuinely supports their development and your firm’s risk management.

Investing in CIR training isn’t just about technical compliance. It’s about reducing your firm’s risk by ensuring that more people understand the rules and can spot issues before they escalate. It frees up your most expensive specialist resource to focus on high-value, complex work, rather than routine compliance. It also means your team is better equipped to support clients through transactions, refinancing or due diligence – all areas where CIR can have a significant impact and where mistakes can be costly.

Our clients consistently tell us that our training builds confidence, improves team capability and delivers real value – not just for their CPD records, but for their day-to-day work and their client relationships.

CIR training is most relevant for firms with clients whose net finance expenses are – or could be – around the £2 million threshold. This isn’t just the domain of the Big Four or large mid-tier firms. Boutique firms working with scaling businesses, start-ups seeking investment, or any firm whose clients are taking on new external funding will all benefit. Conversely, if your client base is mainly small companies or self-employed individuals, CIR is unlikely to be relevant.

The CIR regime isn’t going away, and neither are the risks that come with it. As financing arrangements become more complex and HMRC’s expectations rise, the need for clear, practical and collaborative training has never been greater. If you’d like to discuss how ExtraTax Training can help your team build CIR expertise and confidence, I’d be delighted to have a conversation.