IR35 Explained: A Practical Legal Guide for UK Contractors and Businesses

If you work as a contractor in the UK - or hire contractors for your business - you’ll probably have come across “IR35” before. Even so, this HMRC regulation causes plenty of confusion and headaches, especially if you’re not a tax or employment law expert.

Maybe you’re wondering if you’re inside or outside IR35, if the new rules mean you have to budget for extra tax, or how to handle the risks for your company. You’re not alone. Every year, thousands of contractors and businesses get caught out by misunderstanding IR35, leading to hefty tax bills and even HMRC investigations.

But don’t worry - we’re here to help you navigate IR35 in plain English. This guide breaks down IR35 explained simply, including how the rules work, what counts as “employment” for tax purposes, and the key steps you need to take to protect your business or contracting career from day one.

What Is IR35? Understanding The Basics

Let’s start with the essentials: IR35 is a piece of tax legislation in the UK (officially known as “off-payroll working rules”) designed to clamp down on so-called “disguised employment.” In simple terms, it aims to ensure people who work like employees - but invoice through their own limited companies (called Personal Service Companies or PSCs) - pay broadly the same tax and National Insurance as regular employees.

The idea is to close a loophole. Without IR35, you could (in theory) stop being an employee one day, set up a PSC the next, and do exactly the same job for your former employer - but pay less tax and NI, and the business pays less as well. HMRC sees this as unfair and wants to catch it.

The catch? IR35 law doesn’t just affect big businesses or high-profile contractors - it potentially impacts anyone using a limited company to provide personal services. And getting the rules wrong can result in significant tax liabilities, penalties, and extra compliance work.

Who Does IR35 Apply To?

IR35 can impact you if:

• You are a contractor who supplies services personally (not via a large workforce or genuine company), usually via a PSC, partnership, or intermediary;
• You are a business (sometimes called the “end client” or “hirer”) that engages such contractors via intermediaries; or
• You are an agency involved in placing contractors with clients.

Since April 2021, the party responsible for determining IR35 status - and for accounting for tax and National Insurance - usually depends on whether your client is in the public or private sector, and their size. Read more about the contractor vs employee distinction in our dedicated guide.

What Are The IR35 Requirements? The Rules Explained

IR35 essentially asks: if you (the contractor) were providing services directly to the client - without your company in the middle - would you really be an employee? If so, your contract is “inside IR35”. If not, you’re “outside IR35”.

But how do you work this out? Here are the key factors HMRC (and tribunals) look at:

• Personal service: Are you required to do the work yourself, or can you send a substitute?
• Control: Does the client decide how, when, and where you do the work?
• Mutuality of obligation: Is the client obliged to offer you work, and are you obliged to accept it?
• Financial risk: Do you provide your own equipment? Are you responsible for correcting mistakes at your own cost - as a true business would?
• Part of organisation: Do you act like other employees (e.g., attend staff meetings, get staff perks), or are you truly independent?

IR35 status is determined by the actual working practices, not just the written contract. Even if your contract says you’re “outside IR35”, if the day-to-day work is controlled and managed like an employee, you could fall foul of the rules.

Who Is Responsible For Making The IR35 Determination?

The rules about who makes the decision (“status determination”) changed in 2021. Here’s how it works:

• Small private sector clients: Contractors (via their PSC) are responsible for deciding IR35 status and handling tax/NICs.
• Medium or large private sector clients, and public sector clients: The end client is responsible for making the IR35 status determination and informing both the contractor and any agencies involved.

This means that, in most cases involving larger businesses, it is now the client who faces risk if they get the IR35 determination wrong. For smaller companies, the onus still falls mainly on the contractor’s company.

How Is IR35 Status Determined In Practice?

HMRC uses multiple factors to assess employment status for IR35 regulation. This is a “whole picture” approach, so there’s no single test. Contracts, actual working practices, and historic arrangements all matter.

The main test areas are:

• Substitution: Can the contractor genuinely send someone else to do their work?
• Supervision, direction, control: Is the contractor managed like a staff member, or are they free to do the work as they see fit?
• Other factors: Who provides equipment, who bears financial risk, and whether the contractor is “part and parcel” of the organisation.

HMRC’s CEST tool (“Check Employment Status for Tax”) can provide an indication, but its results are not guaranteed to stand if HMRC investigates and finds the reality doesn’t match the paperwork.

Confused? It’s very common - many businesses seek legal advice on contractor status to be sure.

What Happens If You’re Inside IR35?

If you’re inside IR35, this means that - for tax purposes - you’re classed as an “employee”. Your clients (if they’re medium or large businesses) will deduct income tax and employee National Insurance at source, just like payroll staff. If you’re responsible (for example, working for a small client), your PSC must pay a “deemed payment” that covers what you would owe if you were directly employed.

Some consequences of being inside IR35:

• Loss of tax-efficient income via dividends - you pay tax/NI as if employed
• Employer’s National Insurance may be deducted from what clients pay you
• Increased admin and paperwork, especially for companies hiring contractors
• Potential back taxes, penalties and interest if HMRC later decides you got the status wrong

If you’re found outside IR35, you can pay yourself via salary and dividends (through your PSC) and use more flexible business expenses.

What Are The Main IR35 Risks For Businesses?

If you’re an organisation engaging contractors, IR35 matters - not just for legal compliance, but to avoid unexpected bills and disputes. Here’s why:

• For medium and large companies, the legal responsibility is now on you to assess and communicate IR35 status correctly.
• If HMRC disagrees with your determination, the business may be liable for unpaid tax and national insurance, plus penalties and interest.
• Disputes can also arise with contractors who disagree with your status determination.
• There’s a reputational risk if you consistently get IR35 wrong.

This is why you should have robust contracts, get professional guidance, and regularly review your arrangements. Our guide to contractor agreements can help you understand the importance of legally sound paperwork here.

What Should Contractors Do To Stay Compliant With IR35?

As a contractor, the best approach is to treat IR35 compliance as a core part of your business risk management from the outset. Here’s how:

• Check your contracts: Make sure your written terms are consistent with being “outside IR35” (right to substitute, limited client control, no ongoing mutual obligation).
• Review your working practices: Actual day-to-day work arrangements must match your contract. HMRC will look at what happens in reality.
• Communicate with clients: Especially if you work with larger businesses who make the IR35 determination, keep clear records and maintain open dialogue.
• Seek expert advice: If in doubt, get a proper IR35 status review. Don’t rely on generic templates or assumptions.
• Consider insurance: Specialist insurance can provide cover for costs if you’re investigated by HMRC for your IR35 status.

How Should Businesses Manage IR35 With Their Contractors?

If you hire contractors via PSCs, agencies, or intermediaries, here are practical ways to manage IR35 compliance:

1. Audit your contractor arrangements. Review all contracts and working practices. Identify where IR35 applies.
2. Make and document IR35 status determinations. For each engagement, decide if it’s inside or outside IR35 and issue a Status Determination Statement (SDS).
3. Communicate IR35 decisions to contractors and agencies. HMRC wants you to justify your reasoning and share decisions with affected parties. Maintain strong records.
4. Update contracts and practices. Ensure contracts align with reality, and adjust roles if needed.
5. Build compliance into onboarding and HR processes. Invest in training for your team and regularly review your approach with legal advisors, especially as regulations develop.

For tailored assistance drafting or reviewing business contracts, you may want to explore our contract law solicitor services.

IR35 And Different Business Structures: What Should You Know?

IR35 mainly applies to so-called “intermediaries”: contractors providing personal services via their own limited companies (PSCs), or sometimes via partnerships. If you contract as a sole trader without a company in the middle, IR35 doesn’t apply - but your client may be at risk of PAYE obligations.

If you’re considering starting out (or switching status), you should review which business structure fits your work and risk profile. Our guides on company vs partnership and sole trader vs limited company can help you weigh up your best route - but remember, IR35 is only part of the picture.

What Are The Penalties For IR35 Non-Compliance?

HMRC is serious about enforcing IR35: get it wrong, and the financial consequences can be significant.

• Backdated tax/NIC: You or your client could be liable for all tax/NIC that should have been paid, possibly for years of contracts (plus “employer” contributions, too).
• Interest: Additional interest charged on unpaid amounts.
• Penalties: These can be stiff, especially for “careless” or “deliberate” errors - up to 100% of unpaid tax in serious cases.
• Loss of goodwill: Clients and agencies may drop you if your compliance is questioned.

It’s worth taking IR35 requirements seriously and investing in robust documentation and processes. For more on this, see our guide to breach of contract risks.

Do I Need Professional IR35 Advice?

In many cases, yes - especially if you are:

• Unsure about the IR35 status of a particular engagement
• Dealing with disputes or disagreements about IR35 status with a client or contractor
• Setting up new business structures or moving between self-employment and contracting
• Drafting or reviewing contracts to reflect “outside IR35” arrangements

As the rules are complex and the risks of getting it wrong are high, seeking advice from a legal expert who understands employment law and tax is always a sound move. You can also explore our contract review services for specific contract queries and compliance support.

Key Takeaways

• IR35 (off-payroll working rules) determine whether contractors should be classed as “employees” for tax purposes, and have major implications for both contractors and businesses hiring them.
• Your IR35 status is based on both your contract and the actual working relationship - substitution, control, and mutual obligation are key factors.
• Since 2021, most medium and large businesses are responsible for assessing and communicating IR35 status - and liable if they get it wrong.
• Being inside IR35 means tax and NIC are deducted like regular payroll - often impacting take-home pay and your company’s profit.
• Contractors and businesses should regularly review contracts and working practices to ensure IR35 compliance (it’s not a one-off check!).
• Getting IR35 wrong can lead to costly back taxes, penalties, interest, and even reputational harm - but good record-keeping, proper contracts, and legal advice will protect you from day one.
• If in doubt about IR35, don’t go it alone - get expert legal help to assess your situation, draft robust contracts, and keep your business protected as you grow.

If you’d like specific IR35 guidance or support for your business or contracting setup, reach out to us anytime at 08081347754 or team@sprintlaw.co.uk for a free, no-obligations chat with our friendly team of legal experts. We’re here to help you get your legal foundations right and focus on growing your business with confidence!