IR35 - Off-Payroll Working Rules (UK)
UK tax legislation determining whether a contractor working through an intermediary (typically a personal service company) should be treated as an employee for income tax and National Insurance purposes, with liability shifting to the end client for medium and large businesses since April 2021.
- What Is IR35?
- How IR35 Status Determination Works
- Off-Payroll Working Rules (Post-April 2021)
- Financial Impact of IR35 Determinations
- Compliance Risks and HMRC Enforcement
- HMRC's CEST Tool: Strengths and Limitations
- IR35 Compliance Best Practices for Employers
- IR35 Statistics [2026]
- Frequently Asked Questions About IR35
What Is IR35?
Key Takeaways
- ✓IR35 (named after the Inland Revenue press release that announced it) is a set of tax rules that target 'disguised employment', where individuals provide services through an intermediary (usually a personal service company or PSC) but would be employees if engaged directly.
- ✓If IR35 applies, income tax and National Insurance contributions (NICs) must be deducted at source as if the contractor were an employee, significantly increasing the tax cost for the worker.
- ✓Since April 2021, medium and large private sector businesses (not the contractor) are responsible for determining whether IR35 applies. Small businesses are exempt, and the contractor retains responsibility.
- ✓HMRC's CEST (Check Employment Status for Tax) tool provides status determinations, but it's been criticised for inconsistency and doesn't account for mutuality of obligation in all scenarios.
- ✓Getting IR35 wrong in either direction creates risk: false determinations inside IR35 lead to contractor disputes and talent loss; false determinations outside IR35 lead to HMRC investigations and back-tax assessments with penalties.
Before IR35, a permanent employee could resign on Friday, set up a limited company over the weekend, and return to the same desk on Monday doing the same job, but paying significantly less tax. The company would pay the PSC a fee. The contractor would take a small salary and extract the remainder as dividends, avoiding both employer and employee NICs on most of their income. IR35 exists to close that gap. It says: if the working relationship looks like employment in substance (regardless of the contractual structure), the tax treatment should reflect that reality. For HR and procurement teams, IR35 affects every contractor engagement. It determines how contracts are structured, how status assessments are conducted, who bears the tax liability, and whether the organisation can attract contractor talent at competitive rates.
How IR35 Status Determination Works
Determining whether IR35 applies requires examining the actual working relationship, not just what the contract says. Three factors carry the most weight in case law and HMRC guidance.
The three key tests
Personal service: Can the contractor send a substitute to do the work in their place? A genuine, unfettered right of substitution (not just a theoretical clause in a contract that's never exercised) is the strongest indicator of self-employment. Control: Does the client control what work is done, when it's done, where it's done, and how it's done? The more control the client exercises, the more the relationship looks like employment. Mutuality of obligation (MOO): Is the client obliged to offer work, and is the contractor obliged to accept it? If the answer is yes to both, there's MOO, which points toward employment. HMRC's CEST tool has been criticised for not adequately assessing MOO.
Additional factors
Beyond the three main tests, tribunals and HMRC consider: whether the contractor provides their own equipment, whether they have financial risk (fixed price vs. time and materials), whether they work for multiple clients, whether they're integrated into the client's organisation (company email, badge, org chart), the length and exclusivity of the engagement, and whether the contractor has the right to profit from sound management. No single factor is decisive. It's the overall picture that matters.
Off-Payroll Working Rules (Post-April 2021)
The off-payroll working rules shifted responsibility for IR35 status determination from the contractor's PSC to the end client. This was the biggest change to contractor taxation in the UK since IR35's introduction.
Who bears responsibility
Medium and large private sector clients must determine the IR35 status of every contractor engagement, issue a Status Determination Statement (SDS) to the contractor and the fee-payer (often an agency), and take reasonable care in making the determination. Small companies (meeting 2 of 3 criteria: turnover not exceeding GBP 10.2M, balance sheet not exceeding GBP 5.1M, not more than 50 employees) are exempt. For small company engagements, the contractor's PSC retains responsibility for determining and applying IR35.
The status determination process
Step 1: Assess the engagement using CEST, external IR35 assessment tools (such as IR35 Shield, Qdos, or Kingsbridge), or specialist legal advice. Step 2: Issue a Status Determination Statement to the contractor and the agency (if one exists). The SDS must include the determination (inside or outside IR35) and the reasons for reaching that conclusion. Step 3: If the contractor disagrees, they have a right to challenge the determination. The client must have a process for considering disagreements and must respond within 45 days. Step 4: If the engagement is inside IR35, the fee-payer (the entity paying the PSC, often an agency) must deduct PAYE tax and NICs before paying the contractor.
Financial Impact of IR35 Determinations
The tax difference between being inside and outside IR35 is significant, which is why both contractors and clients care so much about getting the determination right.
| Component | Outside IR35 (PSC) | Inside IR35 (deemed employment) |
|---|---|---|
| Income tax | Salary taxed at marginal rate; dividends taxed at lower dividend rates | All income taxed at marginal PAYE rates |
| Employee NICs | Paid on small salary only (typically GBP 12,570); none on dividends | Paid on all deemed employment income above the threshold |
| Employer NICs | Paid by PSC on small salary only | Paid by the fee-payer on all deemed employment income |
| Corporation tax | PSC pays 19-25% on retained profits | Not applicable (no PSC profit retention) |
| 5% expense allowance | Not applicable outside IR35 | 5% of gross fees deductible before calculating deemed payment (IR35 only) |
| Approximate take-home difference | Contractor retains more | 20-25% reduction in take-home pay on a typical GBP 500/day contract |
Compliance Risks and HMRC Enforcement
HMRC has significantly increased its IR35 enforcement activity since the off-payroll rules shifted liability to end clients.
HMRC investigation triggers
HMRC targets organisations with high contractor populations, particularly in IT, consulting, financial services, and the public sector. Red flags include: blanket status determinations (determining all contractors as inside or all as outside IR35 without individual assessment), contractors who've been engaged for long periods on rolling contracts, engagements where the contractor works exclusively for one client, CEST results that haven't been reviewed or are inconsistent with the actual working practices, and tip-offs from disgruntled contractors or whistleblowers.
Penalties and back-tax assessments
If HMRC determines that IR35 was wrongly applied and the engagement should have been inside IR35, the fee-payer (or the end client if it failed in its status determination duties) faces: back-payment of PAYE income tax and employee NICs for all affected tax years (up to 6 years, or 20 years if HMRC suspects deliberate non-compliance), employer NICs on all back-dated payments, interest on unpaid tax from the original due date, and penalties of up to 100% of the unpaid tax for careless or deliberate errors. In some cases, HMRC has also pursued directors personally for failing to operate PAYE correctly.
HMRC's CEST Tool: Strengths and Limitations
The Check Employment Status for Tax tool is HMRC's free online tool for making IR35 status determinations.
How CEST works
CEST asks a series of questions about the engagement and provides one of three outcomes: inside IR35 (deemed employed), outside IR35 (self-employed for tax purposes), or unable to determine (further information needed). HMRC states it will stand by CEST results as long as the information entered was accurate and the tool wasn't deliberately manipulated. The tool is free, official, and HMRC-binding, which makes it the default starting point for most status determinations.
Known criticisms
CEST doesn't adequately assess mutuality of obligation, which many tax professionals consider a fundamental flaw. It produces "unable to determine" results in a significant proportion of cases, leaving organisations with no clear answer. Several tribunal decisions have disagreed with CEST outcomes. The questions can be ambiguous, and different users interpreting the same engagement can reach different conclusions. Many organisations use CEST as a starting point but supplement it with specialist legal advice or third-party IR35 assessment tools for high-risk or high-value engagements.
IR35 Compliance Best Practices for Employers
Managing IR35 compliance requires a structured approach that covers every contractor engagement across the organisation.
- Maintain a centralised register of all contractor engagements, including contract terms, status determinations, CEST results, SDS documents, and review dates.
- Assess each engagement individually. Blanket determinations (all inside or all outside) are the fastest way to attract HMRC attention and lose contractor talent simultaneously.
- Ensure working practices match the contract. A contract that says "right of substitution" means nothing if the contractor has never actually been substituted and the client would refuse a substitute in practice.
- Review status determinations whenever the engagement changes: new project, extended duration, change in reporting lines, change in working location, or change in control over how the work is done.
- Establish a documented disagreement process. When a contractor challenges their SDS, the client must respond within 45 days with reasons. Failing to respond shifts liability back to the client.
- Train hiring managers on what creates an employment relationship vs. a genuine contractor arrangement. Most IR35 failures happen at the operational level, not in the contract.
- Budget for the employer NICs cost on inside-IR35 engagements. Many organisations increase day rates by 10-15% to offset the contractor's tax increase and maintain talent attraction.
- Engage specialist IR35 advisers or legal counsel for borderline cases, high-value engagements, and any determination that CEST can't resolve.
IR35 Statistics [2026]
Key data points on IR35 enforcement and contractor engagement in the UK.
Frequently Asked Questions
Does IR35 apply to sole traders who don't have a limited company?
Can a contractor be outside IR35 for one client and inside for another?
What is a Status Determination Statement and who needs one?
What happens if a contractor disagrees with a status determination?
Is my company 'small' for off-payroll purposes?
Can a contractor challenge HMRC's IR35 determination at tribunal?
Written by Adithyan RK