IR35. Two letters and two numbers that strike fear into the heart of every contractor in the UK. In plain English, IR35 is the set of off-payroll working rules that have been around since April 2000 (formally sitting in Chapter 8, Part 2 of ITEPA 2003). What it boils down to is this: should you be taxed like an employee, or can you legitimately pay yourself through your limited company using the classic salary-and-dividends structure? Get this wrong, and you could be looking at tens of thousands in back-taxes, penalties, and interest. That’s not a typo.
Inside vs Outside IR35 — What’s the Actual Difference?
Outside IR35 is where every contractor wants to be. It means HMRC accepts your engagement as genuinely self-employed. You invoice your client through your limited company, pay yourself a modest salary, and take the rest as dividends. Tax-efficient. Legal. Lovely.
Inside IR35 is the opposite. HMRC reckons you’re basically a permanent employee who’s routing payments through a company to dodge tax. If your engagement falls inside IR35, someone has to deduct PAYE income tax, employee National Insurance, and employer National Insurance before you see a penny. For medium and large businesses, that someone is the end client. For small companies, it’s still on you.
Who Actually Decides Your IR35 Status?
This changed in April 2021, and it caught a lot of people off guard. Before that, contractors self-assessed. Now, for medium and large private-sector businesses, the end client makes the call. They have to issue something called a Status Determination Statement (SDS) and take “reasonable care” in reaching that decision. If they don’t bother with reasonable care, liability can bounce right back to them.
Small companies — that’s fewer than 50 employees, turnover under £10.2m, or balance sheet under £5.1m under the Companies Act definition — are exempt. If you’re contracting for a small company, you still self-assess. Worth checking before you panic.
The Three Tests That Actually Matter
HMRC and the courts lean on three tests that have been built up from decades of employment case law:
- Control: Does the client tell you how, when, and where to do the work? If you genuinely choose your own methods, set your own hours, and work from wherever you like, that points outside IR35. If your client is dictating your schedule and sitting you at a desk in their office five days a week? That’s looking inside.
- Substitution: Could you send someone else to do the work instead of you? This is one of the strongest indicators. A genuine, unfettered right of substitution — where you could actually send a qualified replacement and the client couldn’t refuse — screams self-employment. If the client would never accept anyone but you, you look like an employee.
- Mutuality of Obligation (MOO): Is the client obliged to give you work, and are you obliged to accept it? In a real business-to-business relationship, either side can walk away between projects. A rolling, open-ended engagement with guaranteed hours? That looks like employment.
Other Factors That Come Into Play
Beyond the big three, tribunals also look at:
- Financial risk: Do you have genuine skin in the game? Your own equipment, professional indemnity insurance, the risk of not getting paid if a deliverable fails?
- Part and parcel: Are you embedded in the client’s organisation? Staff meetings, company email address, appearing on the org chart — all bad signs.
- Exclusivity: Can you work for other clients at the same time? If yes, that’s a good indicator of genuine self-employment.
- Business on own account: Do you have your own website, multiple clients, and actively market your services? That’s what a real business looks like.
What Happens If You Get It Wrong
Let’s not sugar-coat this. If HMRC investigates and decides your engagement is inside IR35, the bill can be brutal. You’ll owe the difference between what you actually paid and what you should have paid as an employee — potentially going back several years. On top of that, HMRC adds interest (roughly 7.5% per year right now) and penalties of up to 100% of the tax owed if they think you were careless or deliberately bending the rules.
Take Dave, an IT contractor in Manchester earning £500/day. Over three years outside IR35, the difference between what he paid and what HMRC says he should have paid could easily hit £40,000–£60,000. That’s a house deposit. Gone.
How to Protect Yourself
First up: make sure your contract matches reality. If it says you have a right of substitution, that right needs to be genuine — not just words on paper that would never actually happen.
Second, use HMRC’s Check Employment Status for Tax (CEST) tool. It’s not perfect — it gets criticised for spitting out “unable to determine” way too often — but HMRC will generally stand by the result if you entered accurate information. It’s free, so there’s no excuse not to use it.
Third, consider getting a professional IR35 status review from a specialist like QDOS, IR35 Shield, or Kingsbridge. They charge £50–£200 per contract, but most come with tax liability insurance that covers the cost of a defence if HMRC comes knocking. Cheap peace of mind, honestly.
Fourth, keep evidence of your working practices: emails showing you chose your own hours, records of substitution discussions, proof you use your own equipment, and evidence of other clients. Build this file as you go — don’t try to reconstruct it three years later when HMRC writes to you.
Recent Case Law Worth Knowing
Two cases to have on your radar: Atholl House Productions Ltd v HMRC (2022) clarified how important the “business on own account” test is, and HMRC v Professional Game Match Officials Ltd (PGMOL) reinforced that mutuality of obligation must exist at the individual engagement level, not just in some overarching framework agreement.
The courts have also been looking more closely at the “hypothetical contract” — what the actual working relationship looks like when you strip away the written contract. So it really matters that your day-to-day reality matches your paperwork.
Check Your Status
Not sure where you stand? Our free IR35 assessment walks you through the key factors and gives you a plain-English indication of your likely status. Takes about five minutes.