HMRC Compliance, Tax Investigations, and Dispute Resolution

HMRC compliance activity on UK contractors has materially increased since 2021. The shift of IR35 determination liability to end-clients in the private sector created incentives for HMRC to validate determinations through compliance checks. Nudge letters about offshore income, crypto, and historic IR35 positions have become routine. The penalty differential between unprompted and prompted disclosure, and between careless and deliberate behaviour, can shift the cost of getting it wrong by tens of thousands of pounds.

This guide covers the contractor-specific HMRC compliance and dispute landscape. Each section links to a detailed companion piece.

What triggers an HMRC IR35 enquiry

1. 1Contract review programme: HMRC selects engagements for review based on industry, turnover, and risk profile.
2. 2End-client referral: an end-client failing to issue valid SDSs may trigger HMRC to investigate the contractors involved.
3. 3Self-employment to PSC switch: a former employee who incorporated and immediately contracts back to the same client invites scrutiny.
4. 4High-value or long-duration single-client engagements: the longer and more exclusive an engagement, the higher the inherent IR35 risk profile.
5. 5Random selection: a small percentage of returns each year are reviewed without specific risk flags.

Tax investigation fee protection insurance

Fee protection insurance covers the cost of defending an HMRC enquiry. Typical features:

• Annual premium: £100-£300 for a contractor company.
• Covers professional fees for responding to enquiries, attending meetings, drafting representations.
• Does not cover the underlying tax, interest, or penalties if the enquiry produces an adjustment.
• Typically excludes pre-existing enquiries (active when the policy is taken out) and deliberate fraud.
• Often bundled with accountancy services or available as a standalone product.

For a contractor who is not in active dispute, the policy is usually worth holding: the hourly cost of specialist representation in an enquiry quickly exceeds the annual premium.

Penalty bands: deliberate vs careless

HMRC penalties for inaccurate returns are calculated as a percentage of the tax that would have been due, with the percentage depending on behaviour and disclosure timing:

The behavioural categorisation is consequential. Establishing that errors were careless rather than deliberate can halve the penalty exposure. A specialist negotiator pushing back on HMRC's initial categorisation is materially worth it on contested cases.

Nudge letters

HMRC sends "nudge letters" when their data systems suggest possible undisclosed income or tax position. Common contractor variants:

• Offshore income letters: HMRC has data from foreign bank accounts under the Common Reporting Standard.
• Crypto letters: HMRC has obtained data from major exchanges (Coinbase, Kraken, Binance) on UK customer activity.
• Property/rental letters: data from the Land Registry, letting platforms, or HMO licensing.
• IR35 historic letters: HMRC has data suggesting prior contracts were inside IR35 despite outside treatment.

A nudge letter is not an enquiry but it is a strong indication that HMRC has data on the recipient. Ignoring it typically escalates to formal enquiry. Responding with specialist support produces better outcomes than self-representation.

Engaging a Tax Tribunal specialist

Where a HMRC dispute escalates to the First-tier Tax Tribunal:

• Specialist representation is materially worth it; tribunal procedure is formal and procedural errors can lose otherwise winnable cases.
• Many cases settle before hearing once the formal process is engaged; the threat of tribunal often shifts HMRC's position.
• Tribunal fees are modest relative to the typical amounts in dispute (Default Paper £20-50, Basic £200-400, Standard £400-2,000).
• Cases are decided independently of HMRC; the success rate for well-prepared contractor IR35 appeals at tribunal has been substantial in recent years.