IR35 Explained: What Contractors Need to Know in 2026/27

IR35 is the shorthand for a set of tax rules designed to prevent what HMRC calls "disguised employment" — situations where someone works as an employee in all practical respects but operates through a personal service company (PSC) to pay less tax and National Insurance than they would as an employee.

The term "IR35" comes from the 2000 Inland Revenue press release that introduced the rules. Two decades later, the rules have been substantially expanded, and IR35 has become one of the most contentious and complex areas of UK tax law for contractors.

The core concept: employment in disguise

The problem IR35 addresses is straightforward in theory. A contractor who works exclusively for one client, on-site, under the client's direction, using the client's equipment, with no financial risk, and no right to send a substitute — is, functionally, an employee. Running that arrangement through a limited company to take a small salary and extract the rest as dividends, saving NI and paying lower dividend rates, was the arrangement IR35 was designed to disrupt.

The difficulty is that genuine contractors — people who truly are in business for themselves, running genuine commercial risk, working with multiple clients, substituting their own labour — were always meant to be outside IR35. The legislation has always intended to distinguish genuine self-employment from disguised employment, not to eliminate contracting as a commercial model.

In practice, the line between the two is genuinely ambiguous for many contractors, and the tests applied are fact-specific and judgment-dependent.

The status tests

Whether you are inside or outside IR35 is determined by the same employment status tests that the courts use in employment law. The key factors are:

Substitution: Can you send a qualified substitute to do the work without the end client's approval? A genuine right of substitution (which the client cannot veto purely on preference grounds) points strongly toward outside IR35. Personal service — an expectation that you specifically will do the work — points inside. Note: the right must be genuine, not theoretical; if you've never exercised it and the client clearly expects you personally, it won't carry much weight.

Control: Who controls how, when, and where the work is done? An outside-IR35 contractor controls their own working method and time. An inside-IR35 worker operates under the client's direction and supervision, much like an employee.

Mutuality of obligation: Is the client obliged to offer you work, and are you obliged to accept it? Employees have mutuality of obligation — the employer provides work and the employee works. Genuine contractors have no guaranteed work and no obligation to accept individual engagements.

Other factors include financial risk (do you bear the risk of a bad outcome?), provision of equipment, integration into the client's organisation, and whether you work for multiple clients simultaneously or serially.

No single factor is determinative. Status is assessed by looking at the full picture of the working arrangement, not just what the contract says. HMRC has been clear that a contract that describes you as self-employed doesn't make you self-employed — the reality of how you work is what counts.

Who decides — before and after April 2021

The off-payroll working rules (Chapter 10 ITEPA 2003, often called the reformed IR35 or "Chapter 10 rules") changed who determines status depending on the size of the end client.

For contracts with large and medium-sized private sector clients and all public sector bodies, the end client is responsible for determining your IR35 status and issuing a Status Determination Statement (SDS). If they determine you're inside IR35, the fee-payer (usually the agency) deducts income tax and NIC from your fees before paying your company. Your company receives payment net of these deductions.

For contracts with small private sector clients (meeting at least two of: fewer than 50 employees, turnover below £10.2m, balance sheet below £5.1m), the contractor's own PSC is still responsible for determining IR35 status under the original Chapter 8 rules.

If you're unsure what size your end client is, ask — the client has an obligation to tell you if they are small.

Inside IR35: what it actually means financially

If a determination places you inside IR35, the financial impact is significant. The fee-payer deducts income tax and NICs (both employer and employee) as if you were employed, before paying your company. Your company receives the net amount, and you can pay yourself a salary from that — but the tax has already been collected. The key difference from genuine employment: your company still pays employer NIC (13.8% above the Secondary Threshold), making the inside-IR35 arrangement more expensive than direct employment.

The practical take-home on a £500/day rate inside IR35 is substantially lower than the same rate outside. Use the IR35 Calculator to model the exact numbers for your day rate and circumstances — the difference is often £15,000–£25,000 per year at typical contractor income levels.

Outside IR35: how the tax advantage works

When you're genuinely outside IR35, you operate your limited company as a legitimate business. You pay yourself a modest salary (typically up to the Primary Threshold or Personal Allowance to minimise NI) and extract remaining profits as dividends after corporation tax. The advantages:

• Dividends are not subject to National Insurance (neither employer nor employee)
• Dividend tax rates (8.75%, 33.75%, 39.35%) are lower than income tax rates
• Corporation tax at 19% (small profits rate) is lower than higher-rate income tax at 40%
• Retained profits inside the company can be timed for extraction strategically

This is why outside IR35 status is so valuable. It's not tax evasion — it's the legal consequence of genuinely operating as an independent business rather than as a disguised employee.

CEST and the challenge of getting determinations right

HMRC's Check Employment Status for Tax (CEST) tool is the official tool for determining IR35 status. It asks about the key factors and produces an "employed", "self-employed", or "cannot determine" result. HMRC has stated it will stand behind CEST results, provided the information entered is accurate.

In practice, CEST is controversial. Contractors and advisers have consistently noted that it does not adequately weight certain factors (particularly the lack of mutuality of obligation, which many employment lawyers regard as fundamental), and that it returns "cannot determine" in a large proportion of cases. For clear-cut situations, it works. For borderline arrangements, it often provides little comfort.

Protecting your outside-IR35 position

If you're outside IR35, protecting that status requires active management:

• Ensure your contract reflects reality. The contract should describe the actual working arrangement — substitution rights, lack of mutuality, control over working methods. If the reality is different from the contract, the reality governs.
• Exercise the substitution right where possible. Even one genuine substitution in an engagement strongly evidences the right's existence.
• Work with multiple clients. A contractor who has worked for one client for five years, exclusively, full-time, is in a more difficult position than one who has three current clients.
• Don't integrate more than necessary. Avoid having a line manager, attending internal all-hands meetings, getting a company laptop and an internal email address if it can be avoided — these facts look more like employment.
• Keep a working practices record. A short document recording how each engagement actually operates — written at the time, not retrospectively — is valuable evidence if HMRC ever challenges your status.

Blanket determinations and unfair treatment

One of the practical problems since April 2021 is that some large clients have issued blanket "inside IR35" determinations for all their contractor engagements, regardless of individual circumstances. This was partly a risk-aversion response to the legislation, and partly an administrative shortcut.

If you receive an inside IR35 determination you believe is wrong, you can challenge it through the client's status disagreement process (which they are legally required to have). If that fails, you can seek an independent opinion from an employment status specialist. Determinations can be overturned, but it requires evidence and persistence.

The financial decision

For some contractors, the difference between inside and outside IR35 is the difference between contractor rates making financial sense and not. If you're inside IR35 at your current rate, you're effectively being paid as an employee but without employment benefits (holiday pay, sick pay, pension employer contributions, employment rights). This is worth negotiating — contractors inside IR35 often seek a rate uplift to compensate.

The IR35 Calculator models both inside and outside scenarios at your rate so you can see the actual take-home difference and make an informed case for rate adjustment if needed.