If you're contracting through a limited company in the UK, IR35 isn't something you can afford to ignore. IR35, formally known as the off-payroll working rules, is HMRC legislation that determines whether a contractor working through a personal service company should be taxed as an employee ("inside IR35") or as genuinely self-employed ("outside IR35"), based on factors like control, substitution, and mutuality of obligation. Get your status wrong, and you could face backdated tax bills, National Insurance liabilities, and HMRC penalties of up to 100% of the unpaid tax.
With major changes taking effect from April 2026 — including higher small company thresholds and new liability rules for umbrella companies — this is genuinely one of the most important topics for UK contractors to understand right now. This guide breaks down exactly what IR35 means, how status is determined, what changed this year, and what it all means for your take-home pay.
What Is IR35?
IR35 is HMRC's legislation for the off-payroll working rules in the UK. It applies when a contractor provides services through a personal service company (PSC) or limited company but would have been classed as an employee had they worked directly for the client. The legislation exists to prevent what HMRC calls "disguised employment" — situations where someone works essentially as an employee but structures their arrangement through a company to reduce their tax and National Insurance liability.
IR35 became law in 2000 via the Finance Act, making it one of the longer-standing pieces of UK tax legislation contractors deal with, though it has been significantly reformed since — most notably in 2017 for the public sector and 2021 for the private sector.
Why IR35 Matters So Much Right Now
IR35 doesn't assess your contract on paper — it assesses how you actually work in practice. A contractor working through a limited company who takes direction from one client, works set hours, and cannot genuinely send a substitute is working as an employee in HMRC's view, regardless of what the company structure looks like on paper.
Getting this determination wrong has real financial consequences. Being found inside IR35 triggers Income Tax and National Insurance deductions at source, significantly reducing a contractor's take-home pay. For businesses, an incorrect determination can trigger backdated PAYE, National Insurance liabilities, and HMRC penalties — making accurate status determination essential for both contractors and the companies that engage them.
Inside IR35 vs Outside IR35: What's the Difference?
This is the core distinction every contractor needs to understand clearly.
Inside IR35 — You're treated as an employee for tax purposes. Income Tax and National Insurance are deducted at source, similar to a salaried employee, significantly reducing take-home pay.
Outside IR35 — You're recognized as genuinely self-employed. You pay yourself through a mix of salary and dividends from your limited company, generally resulting in higher take-home pay and more tax efficiency.
Being outside IR35 isn't just a paperwork preference — it reflects a genuinely different working relationship, one where you have real autonomy over how, when, and for whom you work.
The Three Key IR35 Status Tests
HMRC's off-payroll working rules focus heavily on three core factors when determining status:
1. Supervision, Direction, and Control
This test looks at how much control the client has over how you do your work. If the client dictates your working hours, closely supervises your day-to-day tasks, and directs exactly how the work should be completed, this points toward an inside IR35 determination. Genuine contractors typically have much greater autonomy over their methods and schedule.
2. Right of Substitution
Can you send someone else to do the work in your place? A genuine, unrestricted right of substitution — where you could send a qualified substitute and the client would accept them — is one of the strongest indicators of being outside IR35. If the contract effectively requires you personally to perform the work, with no realistic substitution clause, this points toward inside IR35.
3. Mutuality of Obligation
This test examines whether the client is obligated to offer you continued work and whether you're obligated to accept it. In a genuine employment relationship, there's an ongoing expectation of work being offered and accepted. Genuine contractors typically work on a project or task basis, with no obligation on either side beyond the agreed scope.
HMRC's CEST tool (Check Employment Status for Tax) is designed to help assess these factors, though it's worth noting the tool has faced criticism over the years for not always capturing the full nuance of these tests, particularly around mutuality of obligation.
Who Is Responsible for IR35 Status Determination?
Responsibility for determining IR35 status depends on the size and type of the organization engaging the contractor, and this is an area that changed significantly in April 2026.
Medium and large private sector businesses are generally responsible for deciding IR35 employment status under what's known as Chapter 10 of the legislation. These clients must issue a Status Determination Statement (SDS) — a written document stating whether the engagement is inside or outside IR35, along with the reasoning behind that decision.
Small companies, along with PSCs engaged by overseas-based clients, fall under Chapter 8, meaning the contractor is responsible for making their own IR35 determination.
If you're setting up your own limited company as a contractor, understanding which chapter applies to your engagements is a foundational part of getting your tax position right. Our guide on sole trader vs limited company tax covers the broader considerations of operating through a company structure in the UK.
What Changed with IR35 in April 2026?
This year brought the most significant shift to off-payroll working since the 2021 private sector reform. Two major changes took effect from 6 April 2026:
1. Increased Small Company Thresholds
The financial thresholds used to determine whether an end client counts as "small" — and is therefore exempt from having to make IR35 determinations itself — increased substantially:
Turnover Threshold — Increased from £10.2 million to £15 million
Balance Sheet Threshold — Increased from £5.1 million to £7.5 million
Employee Threshold — Remained unchanged at 50 employees
A company qualifies as small if it meets at least two of these three criteria. As a result of this change, around 14,000 companies moved from medium to small classification, shifting IR35 determination responsibility away from these businesses and back onto the contractor's own personal service company.
2. Joint and Several Liability for Umbrella Companies
Perhaps the more consequential change: from 6 April 2026, agencies and end clients can become liable for PAYE underpayments where workers are supplied through umbrella companies, even when the umbrella company runs payroll correctly on its own end. This new Joint and Several Liability (JSL) framework means that where PAYE isn't correctly operated somewhere in the labour supply chain, HMRC can recover unpaid tax from multiple parties — not just the umbrella company itself, but potentially the agency and end client as well.
This represents a major shift in supply chain compliance responsibility, and businesses engaging contractors through umbrella arrangements now have a much stronger incentive to scrutinize those arrangements closely.
What Happens If You're Found Inside IR35?
If HMRC or your engaging client determines that your contract falls inside IR35, several things follow:
- Income Tax and National Insurance are deducted at source, similar to standard PAYE employment, before you receive payment.
- Your take-home pay decreases, since you lose the tax efficiency that comes from paying yourself through a combination of salary and dividends.
- You may still operate through your limited company, but the specific engagement is taxed as if you were an employee for that contract.
- Backdated liabilities can apply if HMRC determines a previous engagement should have been treated as inside IR35, potentially resulting in significant unexpected tax bills.
If you believe a Status Determination Statement is incorrect, you do have recourse: you can formally request written confirmation of your client's size and challenge an SDS in writing. The client then has 45 days to respond with reasoned grounds — and if they fail to respond within that window, liability for any underpaid tax can shift back to them.
Does IR35 Apply to Sole Traders?
No — this is a common point of confusion. IR35 specifically applies where a worker provides services through an intermediary, most commonly a personal service company or limited company. If you operate purely as a sole trader, without an intermediary company structure, IR35 doesn't apply to you directly, since the legislation is specifically designed to address arrangements involving an intermediary entity.
That said, sole traders still need to ensure their working arrangements genuinely reflect self-employment rather than disguised employment under separate employment status rules, so it's worth understanding how HMRC assesses self-employment status more broadly, even outside the IR35 framework specifically.
IR35 for Umbrella Company Workers
Many contractors, particularly those working on inside-IR35 engagements, choose to work through an umbrella company rather than their own limited company. An umbrella company essentially acts as your employer for tax purposes, handling PAYE, National Insurance, and payslip administration on your behalf, while you continue working on a contract basis for your actual client.
With the new Joint and Several Liability rules from April 2026, it's more important than ever to work only with reputable, compliant umbrella companies, since payroll failures anywhere in the supply chain can now create liability exposure for the agencies and clients involved — which in turn increases scrutiny on which umbrella providers they're willing to work with.
Practical Tips for Contractors Navigating IR35
- Review your contract terms carefully, not just the written wording but how the engagement actually operates in practice — HMRC looks at the reality of the working relationship, not just the paperwork.
- Keep documentation of your working arrangement, including evidence of autonomy, substitution rights used or offered, and the absence of mutual obligation beyond agreed project scope.
- Understand your client's size classification, since the April 2026 threshold changes may have shifted your responsibility for IR35 determination back to you if your client moved from medium to small.
- Don't rely solely on the CEST tool, given its known limitations around nuanced cases — professional advice is often worthwhile for borderline determinations.
- Challenge incorrect Status Determination Statements formally, using the 45-day response window to your advantage if you believe your status has been wrongly assessed.
If you're weighing whether operating through a limited company remains the right structure for your contracting work given these changes, our comparison of sole trader vs limited company tax and guide to Companies House compliance requirements are useful starting points for reassessing your setup.
Frequently Asked Questions
What is IR35 in simple terms? IR35 is UK legislation that determines whether a contractor working through a limited company should be taxed as an employee ("inside IR35") or as genuinely self-employed ("outside IR35"), based on how the working relationship actually operates in practice.
What is the difference between inside and outside IR35? Inside IR35 means Income Tax and National Insurance are deducted at source, similar to employment, reducing take-home pay. Outside IR35 means you're recognized as genuinely self-employed, generally allowing more tax-efficient pay through a mix of salary and dividends.
Who decides if a contractor is inside or outside IR35? Medium and large private sector clients are responsible for making this determination and must issue a Status Determination Statement. Contractors working with small companies or overseas clients are responsible for determining their own status.
What are the IR35 status tests? The three key tests are supervision/direction/control (how much the client controls your work), right of substitution (whether you can send someone else to do the work), and mutuality of obligation (whether there's an ongoing expectation of work being offered and accepted).
What changed with IR35 in April 2026? Small company thresholds increased (turnover from £10.2m to £15m, balance sheet from £5.1m to £7.5m), shifting around 14,000 companies from medium to small. Additionally, new Joint and Several Liability rules now make agencies and end clients potentially liable for PAYE underpayments in umbrella company arrangements.
Does IR35 apply to sole traders and self-employed individuals? No. IR35 specifically applies to workers providing services through an intermediary, such as a personal service company or limited company. Sole traders operating without such an intermediary structure fall outside IR35's scope, though separate employment status rules still apply.
Final Thoughts
IR35 remains one of the most consequential pieces of legislation for UK contractors, and the April 2026 changes — particularly the threshold increases and new umbrella company liability rules — make it more important than ever to understand exactly where you stand. Whether you're newly contracting or have operated through a limited company for years, taking the time to genuinely assess your working arrangement against the three key status tests, rather than relying purely on contract wording, is the safest path to compliance.
If you're setting up your contracting structure or reassessing it in light of these changes, our guides on sole trader vs limited company tax and Companies House compliance requirements offer further groundwork for getting your setup right. For authoritative, up-to-date guidance directly from HMRC, you can also refer to GOV.UK's official off-payroll working rules page.
Not sure whether your current contract puts you inside or outside IR35, or need help reviewing your setup after the April 2026 changes? Explore Advanced Services at Brandora Services to get expert, personalized guidance on staying compliant and tax-efficient as a UK contractor.