Understanding IR35: What Contractors and Engagers Need to Know

IR35, more formally known as the off-payroll working rules, remain one of the more genuinely confusing areas of UK tax for both contractors operating through their own limited companies and the businesses that engage them. The rules exist to prevent what's sometimes called "disguised employment" - working in a way that's functionally identical to being an employee, while being taxed as if genuinely self-employed through a company structure. Getting the assessment wrong carries real consequences for both sides.

What IR35 Is Actually Trying to Address

The underlying concept is straightforward, even if its application often isn't: if someone would be considered an employee of the engaging business were it not for the fact they're operating through their own limited company, IR35 aims to ensure they're taxed broadly as an employee would be, rather than benefiting from the more favourable tax treatment often available through a company structure. The rules look at the genuine nature of the working relationship - control, substitution, mutuality of obligation, and other factors similar to those used in the broader employment status test - rather than simply accepting the contractual label both parties may have applied to the arrangement.

Who's Responsible for the Assessment

For medium and large private sector businesses, and for all public sector bodies, the responsibility for determining whether a contract falls inside or outside IR35 sits with the engaging organisation, not the contractor's own company. This is a significant responsibility, since getting the determination wrong, particularly if a genuinely inside-IR35 arrangement is incorrectly assessed as outside, can result in the engaging business becoming liable for unpaid tax and National Insurance. Smaller businesses, below the relevant size thresholds, are generally exempt from this responsibility, with the assessment instead falling to the contractor's own company, though it's worth checking carefully whether your business genuinely qualifies for this exemption rather than assuming it by default.

Status Determination Statements

Where the responsibility sits with the engaging business, a formal Status Determination Statement needs to be prepared for each relevant engagement, explaining the reasoning behind the inside or outside IR35 conclusion, and this needs to be communicated properly to both the contractor and any agency involved in the arrangement. A genuine, well-reasoned determination based on the actual working practices, rather than a blanket assessment applied to all contractors regardless of their individual circumstances, is essential - HMRC has specifically criticised blanket determinations that don't reflect genuine, individual assessment of each role.

What "Inside IR35" Actually Means in Practice

If a contract is determined to be inside IR35, the fee payer - typically the engaging business or an agency in the chain - is responsible for deducting tax and National Insurance from payments made to the contractor's company, broadly as though the individual were an employee for tax purposes, even though they remain engaged through their own limited company rather than becoming a direct employee with associated employment rights.

Common Mistakes on Both Sides

Businesses engaging contractors sometimes make the mistake of assuming a role is automatically outside IR35 simply because a contract describes the individual as self-employed, without properly assessing the genuine working practices involved. Contractors, meanwhile, sometimes assume their own view of their status is what matters, without recognising that for larger engaging businesses, the determination genuinely sits with the client, not the contractor's own company. Both sides benefit from taking a proper, documented assessment seriously rather than relying on assumptions from either party.

The Role of Agencies in the Supply Chain

Where a contractor is engaged through a recruitment agency rather than directly, the agency often carries specific obligations within the IR35 framework too, particularly around deducting tax correctly where a role has been determined as inside IR35. Businesses using agencies to source contractors shouldn't assume this removes their own responsibility for the underlying status determination - that obligation generally still sits with the engaging business itself, with the agency simply acting on the determination that's been made and passed down the chain.

Reviewing Ongoing Arrangements, Not Just New Ones

It's worth remembering that a contractor relationship's genuine working practices can evolve over time, even if the original contract hasn't changed - a role that started with genuine flexibility and independence can gradually become more integrated, controlled and employee-like as the relationship matures. Periodically reviewing ongoing contractor arrangements against the genuine current working practices, rather than relying solely on an assessment made when the contract was first agreed, is good practice for businesses that regularly engage contractors on an ongoing basis.

Getting Proper Advice Before You Need It

Given how much genuine judgement is involved in an IR35 assessment, and how significant the financial consequences of getting it wrong can be, this is an area worth getting proper advice on, ideally before a new engagement begins rather than after a determination has already been made informally. We're happy to help both engaging businesses and contractors navigate this properly.