What Is IR35 and Why It Matters for UK Contractors

What is IR35? It is the UK tax legislation that decides whether a contractor working through a limited company is taxed as a genuine business or as a disguised employee. The label sounds technical but the financial consequence is direct: get classified outside IR35 and you keep more of your day rate; get classified inside and HMRC treats you the same as a PAYE employee, minus the rights.

For anyone contracting through their own Ltd company in the UK, IR35 is the single biggest tax variable in the entire job. It can move take-home by ten to twenty thousand pounds a year on the same contract. This article walks through what IR35 actually is, why it matters, the changes that landed in 2017 and 2021, and the practical playbook for setting up a limited company in a way that pays you properly when you are outside.

Contents

• What Is IR35? A Plain English Definition
• Inside IR35 vs Outside IR35
• How the 2017 and 2021 Off-Payroll Rules Changed the Game
• Why IR35 Matters Financially
• How to Pay Yourself from a UK Limited Company
• Common IR35 Mistakes That Cost You
• Frequently Asked Questions

What Is IR35? A Plain English Definition

IR35 is short for Inland Revenue press release 35, issued in March 1999 and turned into law as the intermediaries legislation in April 2000 by Gordon Brown. The name stuck. The legislation now lives in Chapter 8 (for engagements with small private clients) and Chapter 10 (for public sector and large private clients) of the Income Tax (Earnings and Pensions) Act 2003. The contractor's own company is commonly called a Personal Service Company (PSC) - the intermediary IR35 was written to address.

The legislation has one purpose: to stop people who would otherwise be employees from interposing a limited company between themselves and the end client purely to access company-level tax rates. HMRC's argument is that if you would be an employee but for the contract, you should be taxed as an employee.

A contractor is inside IR35 when their working arrangement is substantively the same as employment - same hours, same office, same boss, same exclusivity, same kit. They are outside IR35 when the arrangement is genuinely a business-to-business engagement - the contractor can substitute someone else, sets their own hours, uses their own equipment, takes financial risk, and works on a defined project rather than open-ended employment.

The boundary is fuzzy on purpose. HMRC and the courts use a handful of factors built up over decades of case law: control, substitution, mutuality of obligation, financial risk, equipment, integration into the client's business, and exclusivity. No single factor decides it; the overall picture does.

Inside IR35 vs Outside IR35

The practical difference between the two:

On the same gross contract revenue, outside IR35 typically nets the contractor 10-20% more take-home than inside. The gap widens at higher day rates (more profit sits inside the dividend tax brackets, which are lower than equivalent income tax bands) and narrows when employer NI eats into the inside-IR35 deemed payment from the top.

How the 2017 and 2021 Off-Payroll Rules Changed the Game

Until 2017, the contractor's own Ltd company decided its IR35 status, paid the tax accordingly, and dealt with HMRC if challenged. The system was unenforceable at scale - tens of thousands of contractors, each making their own determination, with HMRC investigating maybe a few hundred per year.

The 2017 public sector reform changed who decides. From April 2017, public-sector end clients (the NHS, councils, the BBC) became responsible for determining the IR35 status of contractors engaged through their own limited companies. Get it wrong as the end client and you become liable for the unpaid tax.

The April 2021 private sector reform extended this to large and medium-sized private-sector clients (those exceeding two of: £10.2m turnover, £5.1m balance sheet, 50 employees). The contractor's own Ltd company kept the status determination only when working for small private clients.

The practical effect was a wave of "inside-IR35" determinations issued by risk-averse end clients in 2020-2021. Many banks and large tech firms blanket-banned outside-IR35 engagements rather than do the case-by-case work. The market is more mixed now, with specialised firms (CFD Action, Markel, QDOS) reviewing contracts and engagements to support outside determinations.

Why IR35 Matters Financially

On a £500/day, five days a week, 46 weeks of the year, the gross contract revenue is £115,000. Run that through the IR35 calculator and you see roughly:

• Outside IR35: salary £12,570, employer NI £1,135, corporation tax around £19,400, dividends about £82,000, dividend tax about £15,000, employee NI 0, take-home around £79,000.
• Inside IR35: revenue £115,000, employer NI deducted at source about £13,500, taxable salary about £101,500, income tax about £29,500, employee NI about £4,200, take-home around £67,500.

The gap is around £11,500 a year on the same gross revenue. At higher day rates the gap grows: £750/day pushes the outside advantage past £15,000; £1,000/day past £20,000. None of this is exotic optimisation - it is the standard tax treatment of a small limited company.

The other piece worth flagging: an employer pension contribution from your own Ltd company is the most tax-efficient money in the entire UK system. The Ltd pays your pension before corporation tax (saving 19-26.5% CT), there is no employer NI on pension contributions, and you save the income tax and employee NI on the contribution too. A £20,000 pension payment from a Ltd outside IR35 effectively costs the contractor around £8,000 of foregone take-home for £20,000 in the SIPP. Few financial moves available to UK individuals beat that.

How to Pay Yourself from a UK Limited Company

The textbook setup for a sole-director Ltd outside IR35 in 2025/26:

Step 1: Pay yourself a director salary at the Personal Allowance (£12,570). This sits within the personal allowance so attracts no income tax, and within the National Insurance threshold so attracts no employee NI. The company pays employer NI of 15% on the slice above the £5,000 secondary threshold (about £1,135 of NI for the year), but the salary itself is deductible from corporation tax, so the net cost of that £12,570 salary is much lower than it looks. For a sole director who cannot claim the Employment Allowance (an HMRC rule since 2016), this is generally the optimal salary.

Step 2: Pay an employer pension contribution from the company. No upper limit other than the annual allowance (£60,000 in 2025/26, plus any carry-forward from the last three tax years). Deductible from corporation tax. No employer NI. The single most efficient pound a Ltd company can spend.

Step 3: Claim legitimate business expenses. Accountancy fees, software subscriptions, training courses, business travel, equipment, professional indemnity insurance, mobile phone if used for business, home office rent under HMRC's flat-rate scheme. Keep receipts and a defensible reason for each.

Step 4: Pay the remaining profit out as dividends, in the tax year that suits you. Each tax year you get a £500 dividend allowance (taxed at 0%) and then bands of 8.75% (basic rate), 33.75% (higher rate), 39.35% (additional rate) - covered in detail in our UK dividend tax guide. Dividends stack on top of your salary for band purposes. Crucially: dividends can only be paid from post-tax profit, so corporation tax (19% small profits rate up to £50,000, 25% main rate above £250,000, marginal relief between) comes off first.

Step 5: Time dividends for your circumstances. If you have a spouse working in the company, splitting dividends across two basic-rate taxpayers is far better than concentrating them in one. If you have a high-income year approaching, retain profit in the company and pay it out in a future year. The Ltd is its own tax entity - it does not have to distribute every penny each March.

The IR35 calculator handles all of this end-to-end and shows the take-home difference vs inside IR35.

Common IR35 Mistakes That Cost You

The mistakes that get contractors caught inside IR35 (or hit with retrospective bills) are predictable:

• Working through a single end client for years on rolling contracts. This is the most common reason HMRC challenges a status determination. A two-year engagement that gets quietly extended into year five looks like employment to a tribunal.
• No genuine substitution clause, or one the client would never honour. If you cannot send someone else in your place, you are not a business providing a service.
• Sitting next to the same team, working the same hours, going to the same Christmas party. Integration into the client's business is heavily weighted.
• Letting the client direct how you do the work, not just what. Outside IR35 means the contractor decides the method; inside IR35 means the client tells you.
• Skipping a contract review. A specialist review (Markel, QDOS, CFD Action) costs £100-£200 and finds the clauses that would sink the contract in a tribunal.
• Confusing inside-IR35 with self-employment freedom. Inside IR35 you pay employee taxes without employee rights. Sometimes the right answer is to take a permanent role and stop pretending.

For sole-director Ltds operating outside IR35, three further setup mistakes cost real money:

• Drawing a director salary above £12,570. Crosses into 20% income tax and 8% employee NI, both of which are worse than the 8.75% dividend tax on the same money.
• Skipping employer pension contributions because the cash flow feels tight. The Ltd-to-pension route is the cheapest pension money available in the UK. Even small contributions compound disproportionately.
• Forgetting the £500 dividend allowance every year. It is small but it is free.

Frequently Asked Questions

What is IR35 in simple terms?

IR35 is the UK tax rule that decides whether a contractor working through their own limited company is treated as a genuine business (outside IR35) or as a deemed employee for tax purposes (inside IR35). It exists to stop people who would otherwise be employees from using a Ltd company purely to access company-level tax rates.

How do I know if I am inside or outside IR35?

Status depends on factors built up through decades of case law: control over how you work, the right to send a substitute, mutuality of obligation, financial risk, equipment ownership, and integration into the client's business. Since April 2021, for most private-sector engagements, the end client makes the determination, not you. HMRC's own CEST tool gives a rough indicator but specialist contract reviews from firms like Markel or QDOS are far more reliable.

What is the optimal director salary for a sole-director Ltd in 2025/26?

Almost always £12,570 - the Personal Allowance. It sits below the income tax and employee NI thresholds (both at £12,570) and the company gets a corporation tax deduction on the salary plus the employer NI it pays. £5,000 (the Secondary Threshold, where employer NI starts) is the only other defensible alternative and produces slightly lower take-home in most scenarios.

Can I claim expenses inside IR35?

Almost none. Under the off-payroll rules, the deemed payment is treated as employment income and is subject to the same expense restrictions as a normal PAYE employee. Pension contributions remain possible via salary sacrifice but the broad expense deductibility that applies outside IR35 does not.

What happens if HMRC reclassifies my contract from outside to inside?

HMRC can go back six years (or twenty if they allege deliberate non-compliance) and reassess the income as if it had been taxed inside IR35 all along. The bill includes the unpaid income tax, employee NI, employer NI, and interest. The numbers can run into six figures. Since 2021, the liability often sits with the end client or fee-payer rather than the contractor's Ltd, but the contractor still loses the engagement.

Is being inside IR35 always bad?

No - inside IR35 is just employment, taxed as employment. If the engagement is genuinely employment-like (long-term, integrated, no substitution), inside IR35 is the correct legal answer. The mistake is paying inside-IR35 tax while trying to operate as if you are outside. If you are inside, take the permanent role with the holiday pay and sick pay, or negotiate a higher day rate to compensate for the deemed-employment treatment.

Read Next

• IR35 Calculator - the worked numbers for any day rate
• Salary Sacrifice Pension UK - the parallel pension move for PAYE earners
• SIPP vs Workplace Pension - where Ltd-company pension contributions land
• ISA vs Pension UK - the broader wrapper decision

Further Reading:

I Will Teach You To Be Rich - Ramit Sethi - Not IR35-specific, but the underlying principle (automate your finances, ruthlessly cut what does not serve you, optimise the system not the individual transactions) applies cleanly to running a UK Ltd company tax efficiently. (Affiliate link.)