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An IR35 investigation does not require a contractor to have done anything wrong. It requires HMRC to open a compliance check, and the professional fees, and potential tax liability, that follow apply whether the underlying position was correct or not.

This guide sets out what determines the scale of that exposure: how far back HMRC's lookback window reaches, what a defence typically costs, and what changes if HMRC's challenge succeeds.

The two components of investigation cost

Responding to an HMRC IR35 compliance check has two potential cost components. The first is defence: professional fees for specialist accountants, tax advisers, and, where necessary, legal representation to manage the process. These costs apply regardless of whether HMRC's challenge succeeds. The second is the tax and National Insurance liability itself, together with interest and penalties, if HMRC's challenge succeeds. What does IR35 insurance cover sets out how these two components map onto legal defence cover and tax liability cover as products.

Why the risk is real, and why HMRC's timeline matters

HMRC can open a compliance check into past tax years without needing to give a specific reason. Under normal circumstances, the lookback window is four years. If HMRC suspects carelessness, for example a contract that was clearly inside IR35 but treated as outside, that extends to six years.

This matters because the risk does not disappear when a contract ends. A contractor who operated outside IR35 through a limited company in 2022 and 2023 can still face an investigation into those years in 2026 or 2027. An insurance policy taken out now covers future investigations into past periods, provided the policy was in place for the relevant tax years.

The financial exposure on an outside-IR35 limited company engagement can be substantial. A contractor billing £500 per day across 220 days has annual contract revenue of £110,000. If HMRC successfully argues that engagement was inside IR35, the resulting tax and National Insurance demand, before penalties and interest, can reach tens of thousands of pounds. Add defence costs, and the total exposure easily exceeds £50,000 on a single year.

That is the shape of the exposure: defence costs that apply regardless of outcome, and a five-figure tax liability that applies only if HMRC's challenge succeeds.

The cost of getting it wrong: even when you are right

The case most frequently cited in IR35 discussions is Atholl House Productions Ltd v HMRC [2022] EWCA Civ 501. Presenter Kaye Adams spent nine years and four separate hearings defending her IR35 position. She won. HMRC ultimately declined to pursue the case further after the Upper Tribunal found her outside IR35.

The lesson is not that you need to be a media personality to face this. The lesson is that even a well-founded outside-IR35 position can take years and significant professional fees to defend. Without insurance, those costs fall on the contractor regardless of the outcome.

An IR35 investigation does not require HMRC to be right. It requires you to respond professionally and at length, which costs money whether the enquiry resolves quickly or drags into a tribunal.

What determines whether cover actually protects you

A policy's value is tied to the compliance position underneath it. Where working practices reflect the written contract, with a real substitution position, control that sits with the contractor, and a genuine in-business structure, the risk being insured is a manageable tail risk on an otherwise defensible position.

Where working practices diverge significantly from what the contract says, the picture is different. The Autoclenz Ltd v Belcher [2011] UKSC 41 principle, that tribunals look through written contracts to the reality of the working arrangement, applies equally to the assessment an insurer will make when considering a claim. A policy does not resolve an underlying compliance problem.

This distinction matters in practice: whether or not a policy is in place, a gap between contract and working practice is the same underlying exposure.

For how the main providers compare on cover and cost, see our provider guide.

What about contractors who are already inside IR35?

If you are working inside IR35, either because your client has issued a Status Determination Statement saying so, or because you are working through an umbrella, this exposure is generally not relevant to your current engagement. You are already being taxed as an employee for that work.

The scenario where this exposure remains relevant for inside-IR35 contractors is retrospective: if you previously operated outside IR35 through a limited company and that period falls within HMRC's investigation window, the risk on those past years is unchanged by your current status.

What contractors operating through the small company exemption should know

The raised small company thresholds widen the small company exemption, which means contractors whose end clients come to qualify as small become responsible for their own IR35 status determination again. On HMRC's timetable that starts from 6 April 2027 at the earliest rather than immediately. This increases the number of contractors self-assessing as outside IR35, and increases the number of contractors for whom this investigation risk sits with them directly.

If you are newly self-assessing under the small company rules, the investigation risk described above now sits with you rather than your client. Getting your status right matters more, not less.

How this series fits together

This article covers the scale and mechanics of HMRC's IR35 investigation risk: how far back HMRC can look, and what a defensible position still costs to defend. The next article in the series, how much does IR35 insurance cost, breaks down what drives the premium and how to size cover. The third, what does IR35 insurance cover, sets out exactly what is and is not included in a policy and the conditions that apply to any claim.

The exposure in summary

HMRC can investigate an outside IR35 engagement up to four years after it ends, or six years where carelessness is alleged. The exposure in that window has two parts: the professional fees of responding to and, if necessary, defending an enquiry, and the tax and National Insurance due if HMRC's challenge succeeds. Both apply regardless of whether the underlying IR35 position was correct: defending a position that turns out to be right still costs money to defend.

Frequently asked questions

Is IR35 insurance tax deductible?

Yes. Premiums paid for IR35 insurance through your limited company are generally treated as an allowable business expense, reducing the cost before corporation tax.

Can I get IR35 insurance if HMRC has already started an investigation?

No. Policies must be in place before an investigation begins. Insurers will not cover investigations that are already under way at the point you take out a policy.

Does IR35 insurance cover umbrella company workers?

Not typically. IR35 insurance is designed for limited company contractors operating outside IR35. If you work through an umbrella, you are already inside IR35 for that engagement and have no IR35 liability to insure against.

How far back can HMRC investigate for IR35?

Under standard rules, HMRC can investigate up to four years back. If they believe the error was due to carelessness on your part, this extends to six years.

Does a formal IR35 review affect eligibility for insurance?

You do not need one to obtain a policy, but some insurers may ask whether you have obtained a status assessment when considering a claim. Having a documented understanding of your IR35 position strengthens your position in any dispute.