IR35 Explained: How It Affects Contractors and Freelancers

IR35 determines whether you're taxed as an employee or self-employed. It's one of the most misunderstood rules in UK tax, and getting it wrong can cost thousands. This guide explains what IR35 is, how it affects contractors and freelancers, and how to check your status using HMRC's official tool.

What Is IR35?

IR35 (the Income Tax (Earnings and Pensions) Act 2003) is HMRC's way of stopping contractors from gaining unfair tax advantages by calling themselves self-employed when they're really employees in all but name. If you work the same shifts as staff, have a manager, and can't choose your hours, you're probably inside IR35 — meaning you get taxed like an employee, even though you invoice for your work.

The rule exists because traditional employees pay income tax and National Insurance on their salary. A contractor doing the same job for the same employer, but classified as "self-employed", could claim business expenses and pay less tax overall. HMRC closed that gap in 2000 (hence IR35), and made it even tighter in 2021 by shifting the burden of proof: your client now decides if you're inside or outside IR35, not you.

Inside IR35 vs Outside IR35: The Tax Difference

This is where it gets real. Your tax bill swings dramatically depending on which side of the line you fall.

Outside IR35 (genuinely self-employed):

You invoice your client for your daily rate (say, £350/day).
You're responsible for your own income tax and National Insurance.
You can claim business expenses: home office, software, equipment, mileage.
You pay Class 2 NI (£163.80/year in 2026) plus Class 4 NI (8% on profits between £11,908–£50,270).
If you take a profit of £30,000, you pay ~£6,000 in income tax and NI, leaving you ~£24,000.

Inside IR35 (treated as an employee for tax):

You invoice your client, but they treat you like an employee for tax purposes.
They deduct income tax and National Insurance from your invoice payment (usually via a settlement company or umbrella company).
You lose the ability to claim most business expenses.
You effectively pay the same tax as a permanent employee, but without sick pay, pension contributions, or holiday pay.
If you invoice £30,000, you might take home only £20,000 once they've deducted tax and NI.

The difference: £24,000 take-home (outside) vs £20,000 (inside) on the same gross invoice value. That's £4,000 a year, or roughly 20% less in your pocket.

For a longer-term engagement, this compounds. Six months inside IR35 when you should be outside costs you £2,000. A year costs £4,000. This is why contractors argue so hard about their IR35 status — it's not pedantic tax code; it's direct money in their pocket.

How to Check Your IR35 Status

HMRC provides the Check Employment Status for Tax (CEST) tool — a questionnaire that asks about control, substitution, mutual obligation, and other factors. You answer honestly, and CEST gives you a status: "employed" (inside IR35) or "self-employed" (outside IR35).

Important caveat: CEST is a snapshot based on the information you give it. HMRC can still challenge the result later if circumstances change or if they think you've answered dishonestly. But it's your best defence — if you follow CEST's guidance and HMRC disagrees, you have a documented opinion to point to.

Key questions CEST asks:

Does your client control how, when, and where you work?
Can you send a substitute to do the work instead of you?
Do you have to do the work yourself, or can you delegate?
Is there mutual obligation (they have to offer work, you have to accept)?
Do you provide your own equipment and tools?
Do you invoice for your time, or receive a regular payment?

If the answers point to "you're supervised like an employee", you're inside IR35. If you have genuine freedom to work for other clients, choose your hours, and bring in a substitute, you're probably outside.

Run CEST with your accountant or via an umbrella company — they usually do it for free as part of their service. Never ignore IR35 or guess; the penalty for misclassification is substantial.

Tax Worked Example: Inside vs Outside

Scenario: You're a contractor earning £400/day, working 220 days a year for one client in London.

Your gross invoice: £88,000 (£400 × 220 days)

Outside IR35 (self-employed):

Income tax on £88,000 minus £12,570 personal allowance = £75,430 × 20% = £15,086
Class 2 NI: £164
Class 4 NI: 8% on profit between £11,908–£50,270 (£38,362 × 8%) = £3,069
Class 4 NI: 3% on profit above £50,270 (£36,738 × 3%) = £1,102
Total tax/NI: £19,421
Take-home: £68,579

Inside IR35 (via umbrella company):

Umbrella company takes ~£3,000 for running payroll, employer NI, pension admin
Remaining to pay you: £85,000
Your income tax on £85,000 minus £12,570 = £72,430 × 20% = £14,486
Your employee NI: 8% on £85,000 minus £12,570 = £5,794
Their employer NI: 15% on (£85,000 - £12,570 - some allowance) = ~£10,850 (but they deduct this from your invoice)
Take-home: ~£53,870 (after all deductions)

The gap: £14,709 per year — or 22% less take-home inside IR35 doing the same work for the same client.

This is why contractors outside IR35 often charge a higher daily rate than their permanent-employed counterparts. They need to cover their own tax, NI, pension, and holiday pay.

Planning Tips for Contractors

Understand your contract terms. Before you accept a contract, ask explicitly: "Am I being engaged inside or outside IR35?" If they're unsure, run CEST together. If they say "we've determined you're inside IR35", ask to see their assessment. If they refuse, walk away — it's a red flag.

Set up properly. If you're outside IR35, consider whether a limited company, sole trader structure, or umbrella company makes sense. Each has different tax efficiency and admin overhead. An accountant can model this for your specific situation.

Track your expenses. Outside IR35, you can claim genuine business expenses: home office use (£12/week is a standard claim), software subscriptions, professional fees, mileage, equipment. These reduce your taxable profit. Inside IR35, you can't — your expenses are already priced into the umbrella company's deduction. So the tax advantage evaporates.

Watch the National Insurance cliff at £12,570. Both inside and outside IR35, you don't pay NI on your first £12,570 of profit (2026 threshold). Outside IR35, consider whether taking some profit as a dividend on a limited company saves NI (dividends have different treatment). Inside IR35, you have no choice — you're taxed as an employee all the way.

Plan for gaps between contracts. Permanent employees get sick pay and holiday pay; contractors don't. Budget for 20–30 days of unpaid time per year (holiday, sickness, gaps between gigs). If you're outside IR35, you should be charging more per day to cover this. If you're inside IR35, you lose the benefit of flexibility without gaining the security.

Use the salary calculator to model your take-home. Input your expected gross income and check whether pension contributions, student loans, or other deductions affect your take-home. Then compare the result to what an umbrella company quotes you — they should roughly match.

Common IR35 Misconceptions

"If I work via a limited company, I'm automatically outside IR35." False. Your structure (limited company, sole trader, umbrella) is just the vehicle. CEST judges the engagement — how you work, not what paperwork you filed. You can be inside IR35 working through a limited company just as easily as through a sole trader setup. The structure affects tax efficiency once your IR35 status is determined, but doesn't determine the status itself.

"If my client says I'm outside IR35, I'm safe." Not entirely. The client makes the determination (as of 2021), but if HMRC audits you later and disagrees, the burden is on you. Keep records: your contract, any emails discussing working arrangements, timesheets, CEST results. If you followed CEST in good faith, you have some protection, but it's not absolute.

"I can just claim the tax back later if I get it wrong." You might be able to, but you'll face penalties and interest. Late payment interest runs at 2.5% per quarter. A £4,000 mistake inside a year compounds quickly. Prevention (getting CEST right upfront) is far cheaper than cure.

Frequently Asked Questions

Q: Can I appeal if my client determines I'm inside IR35?
A: Your client makes the determination based on the engagement rules. If you disagree, you can ask them to reconsider, but ultimately they decide. If you believe their decision is wrong, you can report it to HMRC and ask for a formal determination. HMRC can take months to respond, so this isn't a fast option. It's better to settle status before accepting the contract.

Q: What if I work for multiple clients?
A: IR35 status is determined per engagement, not per person. You might be outside IR35 with Client A (you control your hours, can substitute) and inside IR35 with Client B (they control your schedule, you can't substitute). Each contract is assessed separately. This can make tax planning complex if you're juggling multiple clients with different statuses.

Q: Does holiday pay change if I'm inside IR35?
A: Inside IR35, you're supposed to get holiday pay like a permanent employee — roughly 5.6 weeks per year. However, umbrella companies often don't pay this unless you explicitly negotiate it. It's one of the hidden costs of IR35. Check your umbrella company's terms — do they add 5.6 weeks of holiday pay into your rate, or do they charge it separately?

Q: If I'm outside IR35 and my client goes bust, am I protected?
A: No. Self-employed contractors have no statutory redundancy rights and no claim on the National Insurance fund if a client stops paying you. Permanent employees, and contractors inside IR35, have statutory protections. If you're outside IR35, build up a cash buffer (aim for 3 months of expenses) to cover gaps.

Q: Can I negotiate a higher rate if I'm inside IR35?
A: You can ask, but clients usually won't budge — they've already factored in the umbrella company deductions. What you can do is ask whether they'll cover any of the umbrella costs, or whether they'll agree to re-assess your status if circumstances change. Negotiate before you start, not after.

Q: How often can my IR35 status change?
A: If the terms of your engagement change, your status can change. If your client stops giving you supervised training, or lets you work with other clients, or drops the requirement to work certain hours, you might flip from inside to outside IR35. Conversely, if they increase control, you might flip the other way. Run CEST again if material terms change.

Q: Should I use an accountant for IR35?
A: Highly recommended. A good accountant costs £600–£1,500 per year for a sole trader, or £1,500–£3,000 for a limited company. They'll run CEST with you, help you structure your contract, claim all your expenses, and keep records in case HMRC audits. Given the stakes (£4,000+ per year), the cost is small insurance.

Next Steps

Check your current status. Go to CEST and answer the questions honestly based on your actual working arrangement.
Understand your contract. Ask your client or umbrella company explicitly whether you're inside or outside IR35. Get it in writing.
Model your take-home. Use our salary calculator or ask your accountant to show you the tax and NI breakdown for your expected income.
Compare structures. If you're outside IR35, consider whether a limited company, sole trader, or umbrella company makes sense for your situation. Learn more about umbrella companies vs limited companies.
Keep records. Hold onto your contract, CEST result, and any correspondence about your IR35 status. If HMRC asks, you'll be grateful.

For more on how your pay is constructed, see how to read your payslip and how tax codes affect your take-home pay.