What is IR35? A plain-English guide for contractors

What is IR35?

IR35 is the UK tax rule that asks one question: is the contractor working for this client REALLY self-employed, or are they functionally an employee disguising themselves as self-employed for the tax break?

If HMRC concludes the engagement IS disguised employment (“inside IR35”), the contractor, or, since 2017 in the public sector and 2021 in medium/large private sector, the engager, has to operate PAYE on the contract value as if it were salary. Income tax + employee NI + employer NI on the deemed payment.

If the engagement IS genuine self-employment (“outside IR35”), the contractor invoices via their Ltd company, takes a small salary plus dividends, and saves a meaningful chunk of tax compared to the equivalent employed salary. At £500/day, the difference is roughly £3,500–£4,000 per year.

That difference, and the question of who decides which side of the line you're on, is the entire IR35 story.

Three regimes: Chapter 8, 2017, 2021

IR35 has actually changed three times. Knowing which regime applies to your engagement is the first thing to check.

Chapter 8 (April 2000 onward)

The original regime, named after Chapter 8 of the Income Tax (Earnings and Pensions) Act 2003. The contractor's personal service company (PSC, i.e. their Ltd) is responsible for assessing each engagement and operating deemed PAYE on the income if it's inside IR35. HMRC enforces by investigating individual contractors. Still applies to engagements with small private-sector clients today (see the small-client exemption section).

Chapter 10, public sector (April 2017)

HMRC moved the determination responsibility from the contractor to the engager (the public-sector body) and shifted the PAYE liability to the fee-payer (usually an agency). The reform was a response to widespread Chapter 8 non-compliance in the public sector. Public bodies became personally liable for incorrect determinations, so most defaulted to “inside” on anything borderline.

Chapter 10, medium/large private sector (April 2021)

Same reform extended to medium and large private-sector clients. The contractor still has the right to disagree with the determination via a Status Disagreement Process, but the determination itself is now made by the client. This caused a significant market disruption, many outside-IR35 contractors found their engagements blanket- redetermined as inside, and many shifted to umbrella employment as a result. Small private-sector clients are exempt, they remain under Chapter 8.

The three principal tests

The legal test for IR35 status comes from a 1968 case (Ready Mixed Concrete v Minister of Pensions). Three factors are decisive; everything else is supporting evidence.

1. Control

Who decides what work is done, how it's done, where it's done, and when? An employee is told what to do by their manager. A genuine contractor is given a deliverable and decides their own approach. Heavy day-to-day direction (“join this stand-up at 9am every day, work on the tickets I assign you”) is employee-shaped. Outcome-based engagement (“deliver this feature by end of Q1, however you want to do it”) is contractor-shaped. Most software contracts in big companies fall somewhere in the middle.

2. Substitution

Could you send a different qualified person to do the work in your place, and would the client be required to accept them? An employee can't substitute themselves, their employer wants THEM. A genuine contractor provides a service that someone else of the right skill could theoretically deliver. A clear, unfettered substitution clause in the contract is necessary; actually exercising substitution at any point during the engagement is the gold-standard evidence.

3. Mutuality of obligation (MOO)

Does the client have to offer you work, and do you have to accept it? An employment relationship has continuous mutual obligation, your employer keeps paying you and you keep showing up. A genuine contractor relationship is engagement-by-engagement, with the right on both sides to walk away when the current piece of work ends. Long rolling contracts with auto-renewal look more like employment; project-based engagements with defined start and end dates look more like genuine self-employment. HMRC's position has historically been that MOO is automatic in any paid engagement; the courts have repeatedly disagreed.

Other factors HMRC weighs

None of these on their own decides status, but they all tip the scales when the three principal tests are ambiguous. A contractor who looks “in business on their own account” in these supporting ways is more likely to be ruled outside.

• Financial risk. Do you bear loss if the project fails? Have you fixed your price and now have to eat any cost overrun? An employee never bears business risk.
• Equipment.Do you bring your own laptop, tools, software licences? Or does the client issue you their kit? Bringing your own is contractor- shaped (though at most software clients you'll be forced to use their VPN-locked machine, partial credit).
• Integration.Are you on the client's org chart? Do you have a client email address with their branding? Are you in their internal directories? Do you attend the team Christmas party? All employee signals.
• Exclusivity. Do you work for multiple clients in parallel, or just this one? An employee has one employer; a genuine contractor has a portfolio (or at least the right to).
• Length of engagement.A 4-week project looks more like a contractor; a 3-year rolling engagement at the same client looks more like an employee. There's no hard rule but multi-year engagements at one client raise scrutiny.
• Method of payment. Hourly billing with timesheets looks more employee-shaped; fixed-price milestones look more contractor-shaped.
• Right to engage helpers.Can you subcontract part of the work to your own staff or another contractor? Employees can't.

How status is actually determined

Two things matter: the contract WORDING and the actual WORKING PRACTICES. HMRC has long held that working practices override contract wording when they conflict (and the courts agree). A contract with a great substitution clause that's never used and would never be accepted is worth less than a contract with a mediocre clause that's actually been exercised.

CEST: HMRC's self-service tool

Check Employment Status for Tax (gov.uk/check-employment- status-for-tax) is HMRC's 16-question online tool. It's officially binding ONLY when the answers reflect the actual engagement. Heavily criticised for under-weighting mutuality of obligation, for years HMRC's default position was that MOO exists in any paid engagement, which courts have repeatedly rejected. CEST also returns “unable to determine” in a meaningful percentage of cases. Use it as a starting point but not as definitive evidence.

Specialist contract reviews

For £200–£300 you can get a contract reviewed by a specialist (Qdos, Kingsbridge, IR35 Shield, Bauer & Cottrell). They'll flag clauses that fail the IR35 tests, suggest amendments, and give a risk rating. Worth doing for any new engagement valued above £30,000, the review cost is rounding error compared to a back-tax bill if you're wrong.

Status Determination Statement (Chapter 10 only)

Under the off-payroll rules, the engager has to give you a written Status Determination Statement (SDS) setting out their decision and the reasons. You can disagree via the Status Disagreement Process, the engager has 45 days to respond. In practice the disagreement process has low success rates because most engagers default to risk-averse “inside” determinations and won't budge.

The small-client exemption

The off-payroll rules (Chapter 10) only apply to medium and large private-sector clients. Small clients are exempt, their engagements remain under Chapter 8, where the contractor's PSC determines status.

A client is “small” (Companies Act 2006 definition) if they meet at least 2 of:

• Annual turnover not more than £10.2 million
• Balance sheet total not more than £5.1 million
• 50 employees or fewer

(These thresholds rose in April 2025 under the Companies (Accounts and Reports) (Amendment) Regulations 2024 to £15m / £7.5m / 50, check the current applicable threshold for your engagement's tax year.)

The practical effect: if you contract for a small consultancy or a startup that meets the small-client test, YOU determine your IR35 status and operate deemed PAYE if inside. If you contract for any FTSE company, bank, BigCo, or public-sector body, the engager determines status under Chapter 10.

What happens if HMRC says you're wrong

HMRC opens an enquiry into one or more of your past engagements (typically going back 4 years for carelessness, 6 for negligence, up to 20 for deliberate evasion). The investigation involves correspondence, document requests (contracts, emails, invoices), and sometimes interviews. It can last 12–36 months.

If HMRC concludes the engagement was inside IR35 when you treated it as outside, the bill is:

• Back tax: the difference between deemed-PAYE liability and what you actually paid. For a £500/day contractor that's ~£10,000–£15,000 per year of disputed engagement.
• Interest: ~7-8% per annum (HMRC's late-payment rate, currently 7.25%).
• Penalties: 0% (innocent error and reasonable care), 10–30% (carelessness), 20–70% (deliberate but not concealed), 30–100% (deliberate and concealed).

A typical multi-year case lands at £20,000–£80,000 of total liability. Hot cases, usually involving multiple engagements at the same end client over years — occasionally exceed £200,000.

October 2024 off-set provisions:if you've already paid corporation tax + dividend tax on the income via your Ltd, HMRC now has the power to set that off against the deemed PAYE liability of the engager (under Chapter 10). This stops double-taxation when an engager is found liable. It doesn't help much under Chapter 8 where the contractor IS the liable party.

IR35 insurance economics

For £200–£500/year you can buy IR35 insurance from Kingsbridge, Qdos, IR35 Shield, or QDosContractor. Standard policies cover:

• Investigation defence costs (representation by an IR35 specialist solicitor or accountant)
• The tax liability itself if HMRC wins, including interest and penalties (subject to policy limit, often £100,000)
• In some cases, the legal costs of appealing to a First-tier Tribunal

The maths: at £400/year of premium and a 5% chance of an investigation in any given year (typical for outside-IR35 contractors), the expected loss without insurance is £40,000 × 0.05 = £2,000. Pay £400 to cover a £2,000 expected liability, the insurance is worth more than its premium.

Worth it if your engagement has any IR35 risk. Skip only if you're working for a small private-sector client where YOU determined status (Chapter 8) AND the engagement is genuinely outside on the principal tests AND you have a specialist contract review on file backing up your determination. Even then, £200–£400/year is cheap peace of mind.

Practical takeaways

1. Check whether your engagement is under Chapter 8 (small client → contractor responsible) or Chapter 10 (medium/ large client → engager responsible). This dictates the entire workflow.
2. For Chapter 10, demand the Status Determination Statement up front. Read it. If you disagree, use the Status Disagreement Process within 45 days.
3. For any outside-IR35 engagement worth more than £30,000/year: get a specialist contract review. £200–£300 is cheap insurance against a £20,000+ back-tax bill.
4. Buy IR35 insurance from a reputable specialist for £200–£500/year. The expected loss without it is greater than the premium for almost all outside contractors.
5. Match your working practices to your contract. The two must align, if your contract says you can substitute but your working practices show you never could, the substitution clause is worthless.
6. Maintain your “in business on your own account” signals: own equipment, multiple clients over time, professional indemnity insurance, Ltd company branding, no client-branded email signature.
7. If your engagement is genuinely inside IR35, use an FCSA-accredited umbrella. Don't try to engineer outside-IR35 status into a role that isn't, HMRC will catch up eventually.

Frequently asked questions

Why does IR35 exist at all?

Originally to stop the trick of being a permanent employee on Friday and a contractor for the same employer on Monday, same desk, same boss, same work, but charging via a Ltd company to take dividends instead of PAYE salary. HMRC argued that was disguised employment for tax avoidance. The 1999 budget (Inland Revenue Press Release 35, hence the name) introduced rules requiring the contractor's PSC to determine whether each engagement was truly self-employed, and to operate deemed PAYE if it wasn't. The framing has held up since: the test is whether the engagement actually looks like self-employment, not whether the contractor calls it self-employment.

If I'm inside IR35 do I have to use an umbrella?

No, but it's almost always the cleanest option. Alternatives are (1) work via your Ltd with deemed-payment computation, legally permitted but loses the dividend advantage AND keeps the admin overhead, so almost always worse than umbrella; (2) be paid PAYE directly by the agency or end client; (3) work via the engager's preferred umbrella product. For most inside-IR35 engagements, picking your own FCSA-accredited umbrella is the standard answer. See our umbrella explainer for the mechanics.

What's the difference between Chapter 8 and Chapter 10?

Chapter 8 is the original IR35 regime, in force from April 2000, the contractor's PSC determines status and operates deemed PAYE if inside. Chapter 10 is the off-payroll regime, the END CLIENT determines status and the fee-payer (usually an agency) operates PAYE. Chapter 10 applies to public sector from April 2017 and to medium/large private sector from April 2021. Chapter 8 still applies to engagements with small private-sector clients (the small-client exemption). Most contractors today are governed by Chapter 10 because they work for medium/large clients.

What does 'genuine right of substitution' mean?

You can send a different qualified person to do the work in your place, and the client has to accept them (provided they meet the skills required). It's the single strongest indicator of genuine self-employment. The catch: you have to actually have this right in practice, not just on paper. If your contract has a substitution clause but the client would never accept anyone except you (because they hired you specifically), it's a 'fettered' substitution clause and HMRC won't credit it. If you've actually substituted at any point in the engagement, that's gold-standard evidence. Most contracts have unfettered substitution clauses but few contractors ever exercise them, which is why HMRC scrutinises whether the right is real.

What's CEST and should I trust it?

CEST (Check Employment Status for Tax) is HMRC's online status tool. It asks ~16 questions about the engagement and returns 'inside IR35' / 'outside IR35' / 'unable to determine'. It's officially binding ONLY if the answers are accurate and reflect the actual working practices. Two big problems: (1) CEST is widely criticised for under-weighting mutuality of obligation, for years HMRC's position was that MOO is automatic in any contract, which the courts repeatedly disagreed with; (2) the result reflects what you tell it about the engagement, so two contractors at the same client can get opposite results depending on how they answer. Use CEST as a starting point, not as definitive evidence. Pay £200 to a specialist (Qdos, Kingsbridge, IR35 Shield) for a contract review if your engagement is borderline.

Does my LinkedIn job title matter for IR35?

Yes, more than you'd think. If your LinkedIn says you're a 'Senior Software Engineer at BigCorp' (vs 'Independent Software Consultant') and your email signature uses BigCorp's branding, HMRC will use that as evidence you're functionally part of the client's organisation, which counts toward 'integration', a non-principal but meaningful IR35 factor. Use 'Independent contractor' or 'Director, [Your Ltd Name]' as your actual job title across all professional channels while you're contracting. It's a small thing but it adds up over an investigation.

What's the typical cost of getting IR35 wrong?

For a £500/day contractor outside IR35 found by HMRC to actually be inside, the back tax is roughly the difference between what you paid (salary + dividends + corporation tax via Ltd) and what you should have paid (full PAYE on the deemed payment). For a typical year that's £10,000–£20,000. HMRC adds interest (~7-8% currently) plus penalties (10-30% for careless behaviour, up to 100% for deliberate concealment). For a multi-year engagement HMRC can go back 4 years (carelessness) or 6 years (negligence) or 20 years (deliberate). A typical investigation lands somewhere between £20,000 and £80,000 of back tax + interest + penalties. IR35 insurance from Kingsbridge, Qdos, or QDosContractor covers investigation cost AND the tax bill itself for £200–£500/year of premium, easily worth it if your engagement has any IR35 risk.

What about working for the public sector?

Public-sector engagements have been under Chapter 10 (off-payroll) since April 2017, the public-sector body determines your status and the fee-payer operates PAYE. There's no small-client exemption in the public sector regardless of the public body's size. Most public-sector engagements default to inside IR35 because public bodies are risk-averse about getting determinations wrong (they're personally liable). Some specialist roles (true consultancy with deliverables, not bums-on-seats) get outside-IR35 determinations but the bar is high.