What IR35 actually is, and why it matters so much
IR35 is a set of HMRC tax rules designed to determine whether a contractor working through a limited company should be taxed as an employee. The core idea is simple: if your working arrangement looks like employment in all but name, HMRC believes you should pay employment taxes rather than the more favourable limited-company tax structure.
The financial stakes are significant. An outside-IR35 contract at £650/day can yield around £108,000 net per year through efficient salary and dividend structures. The same £650/day contract inside IR35, processed through an umbrella company, nets closer to £84,000. That is a £24,000 gap on the same day rate, and HMRC can pursue unpaid tax for up to six years if a determination is wrong.
Take-home comparison: outside IR35 vs inside IR35 vs permanent
Estimated figures at £650/day · 230 working days · 2026 UK tax rates
| Arrangement | Est. annual gross | Est. net take-home |
|---|---|---|
Outside IR35 (via Ltd company) Salary + dividends structure, assuming 230 working days | £162,500 | £108,000 |
Inside IR35 (via umbrella) Umbrella margin and employer NI deducted from rate | £149,500 | £84,200 |
Permanent employee equivalent £95K PAYE, standard 2026 tax codes | £95,000 | £64,000 |
The three tests HMRC uses to determine IR35 status
IR35 status is not a checklist. It is a holistic assessment. But three factors carry the most weight in any determination, whether made by the client (for medium or large private-sector engagements post-April 2021) or assessed by HMRC directly.
The three key IR35 status tests
Substitution
Outside IR35
You have a genuine right to send a substitute to do the work if you are unavailable, and the client would accept them.
Inside IR35
The client specifically requires you as an individual. Your personal service is what they are paying for.
Control
Outside IR35
You decide how the work is done, when you do it, and often where. You have genuine independence over method and schedule.
Inside IR35
The client directs how, when, and where you work to a degree that resembles an employment relationship.
Mutuality of obligation
Outside IR35
There is no obligation on the client to offer you further work, and no obligation on you to accept it. Each engagement is genuinely discrete.
Inside IR35
There is an expectation of ongoing work and a de facto obligation to turn up and continue. The relationship looks like employment.
Who determines IR35 status now? (It's not always you)
Since April 2021, the off-payroll working rules (IR35 reform) shifted responsibility for determination from the contractor to the client for medium and large private sector engagements. This is arguably the most important practical change in contracting in a decade.
- Small private sector clients: The contractor's limited company remains responsible for determining status. This applies to clients with fewer than 50 employees, or annual turnover under £15M, or a balance sheet under £7.5M. Two of three criteria must apply. These thresholds were raised from £10.2M / £5.1M on 6 April 2026. Around 14,000 companies are newly classified as small.
- Medium and large private sector clients: The client must make a Status Determination Statement (SDS) before the contract starts. They must take reasonable care in making that determination. If they fail to do so, responsibility shifts back to them, not to you.
- Public sector clients: Public sector clients have been required to determine IR35 status since 2017. HMRC's CEST tool is commonly used, but CEST has well-documented limitations and does not cover mutuality of obligation adequately.
Inside IR35: working through an umbrella company
If your contract is inside IR35, you will almost always be paid via an umbrella company. The umbrella acts as your employer, processes your pay as PAYE, and deducts Income Tax, Employee's National Insurance, and Employer's National Insurance from your contract rate before paying you. This last point is important: Employer's NI (13.8% in 2026) is taken from your day rate, not added on top of it. This is why inside-IR35 take-home is lower than people expect even at high day rates.
Not all umbrella companies are equal. Legitimate umbrellas are compliant, transparent about their margin (typically £15–£30/week), and FCSA-accredited. Be extremely cautious of umbrellas offering "tax efficiency schemes," loan arrangements, or suspiciously high take-home percentages. HMRC has aggressively targeted disguised remuneration schemes, and liability for unpaid tax flows through to the worker, not the scheme operator.
Outside IR35: the limited company structure
Outside-IR35 contracts are typically worked through a Personal Service Company (PSC), usually your own limited company. The tax efficiency comes from the ability to pay yourself a combination of a low salary (just above the National Insurance threshold) and dividends, which are taxed at a lower rate than employment income.
In 2026, the dividend tax-free allowance has been reduced to £500 (down from £2,000 in 2022), which reduces the efficiency slightly compared to previous years. But the salary/dividend split is still significantly more efficient than PAYE at comparable income levels, particularly above £50,270 where higher-rate Income Tax applies.
You will also need a good contractor accountant. Expect to pay £80–£150/month for a specialist. This is a cost of the structure, and it pays for itself many times over if you are consistently outside IR35.
Practical steps before taking any contract
- 1
Get a contract review. Before signing, have a specialist IR35 contract reviewer (not your general accountant) assess your contract. Services like Qdos or IPSE offer reviews from £99. A bad contract can pull an otherwise outside-IR35 role inside.
- 2
Document your working practices. The written contract matters, but so does how the engagement actually works in practice. HMRC investigates working practices, not just paper. Keep records showing you operate independently: use of substitutes, working for multiple clients, absence of integration into the client's team structure.
- 3
Do not ignore a blanket inside determination. Many large clients issue blanket inside-IR35 determinations without assessing individual roles. You have the right to dispute these. If the working practices genuinely support outside status, a challenge through the client's status disagreement process is worth pursuing, particularly on longer engagements.
- 4
Consider IR35 insurance. For outside-IR35 contracts, specialist IR35 insurance covers investigation costs and any tax liability if HMRC challenges your status. Annual premiums from reputable providers run £300–£600. For higher-rate contractors, this is a reasonable cost of operating compliantly.
Is contracting worth it in 2026?
For senior UK software engineers with specialist skills, contracting outside IR35 remains one of the most effective ways to increase take-home income materially. Day rates for Senior and Lead engineers in London range from £550 to £800; Principal and Architect-level contractors can command £900–£1,200/day in specialist areas like cloud security, AI/ML infrastructure, and distributed systems.
The landscape is more complicated than it was pre-2021, and the risk of an incorrect inside determination is real. But for engineers who manage their IR35 status professionally, with good contracts, documented working practices, and specialist support, the financial upside relative to permanent employment remains significant. Use our UK take-home calculator to model the difference for your specific situation.