Inside IR35 vs Outside IR35 — A Contractor's Plain-English Guide for 2026/27

If a single phrase decides your monthly take-home as a UK contractor, it's "inside IR35" vs "outside IR35". The framework is HMRC's way of asking would you be an employee if you stripped out the limited company in the middle? Get the answer wrong and the difference is roughly 25–30% of your day rate — taxed at PAYE rates with no expenses, no salary/dividend split, no corporation tax shelter.

This guide is what we tell every new AIS client. It's not legal advice. It is the working framework we use day-to-day.

The one-paragraph version

IR35 is a set of UK tax rules that tests whether a contractor working through their own limited company is really operating like a business — or just an employee in a costume. Outside IR35 means HMRC treats you as a genuine business: you pay corporation tax on profit, take a low salary, top up with dividends, and your take-home rate is ~75–80% of day rate. Inside IR35 means HMRC treats you as a deemed employee: the fee gets taxed at PAYE rates as if it were salary, you lose the salary/dividend split, and your take-home drops to ~55–60% of day rate.

The rules around who decides changed in April 2017 (public sector) and April 2021 (medium and large private sector). It's now your client who makes the call, not you — except in the small-private-sector case, where it's still your limited company.

The three big tests

HMRC and tribunal cases keep returning to three primary status factors:

1. Personal service / substitution

Can you send a substitute? If yes, you're more likely to be a business. If the client's contract or working practices require you specifically, that's an employment marker.

"I tried to send my colleague Sarah and the client said no, only you can work on this project" — that single sentence will tilt an IR35 decision toward inside.

2. Mutuality of obligation (MoO)

Does the client have an obligation to give you work, and do you have an obligation to accept it? An employee has MoO. A genuine contractor doesn't — you can turn down extensions, and the client doesn't have to keep you busy between projects.

HMRC's position is that some MoO exists in every paid engagement. Tribunals tend to look at whether MoO extends beyond the current piece of work.

3. Control (direction, supervision, control)

Who decides how, when, where, and what gets done? Genuine contractors are typically engaged for an outcome — they choose the method, often the hours, and often the location. Employees are told.

The secondary factors (do not ignore these)

Tribunal decisions and HMRC determinations weigh these too:

• Financial risk: Do you carry it? Fixed-price work, rectification at own cost, professional indemnity insurance — all point outside.
• Part and parcel: Are you on the org chart, in the team Slack, invited to the Christmas party, listed in the company directory? All point inside.
• Equipment: Do you bring your own laptop, software, tools? Points outside.
• Right to provide your own staff: If the contract lets you sub-contract or bring helpers, points outside.
• Exclusivity: Are you free to work for other clients in parallel? Points outside.
• Length and pattern: Long, exclusive, full-time engagements look more like employment.

The CEST tool and its blind spots

HMRC's Check Employment Status for Tax (CEST) tool is the official questionnaire. It's free, anyone can run it, and clients often hide behind it.

CEST has documented weaknesses:

1. It doesn't ask about MoO at all. The tool was published with HMRC's view that MoO is present in every contract — a view many tribunals have rejected.
2. It gives "undetermined" in ~20% of runs. When that happens, the client is on their own — and most default to "inside" to be safe.
3. It's not legally binding. HMRC has reserved the right to challenge CEST results if the information entered was "inaccurate" — which they tend to discover only when it's gone the other way.

Use CEST as a first sweep and a paper trail, not as a definitive answer. Get a second opinion from a specialist (Qdos, Markel, Bauer & Cottrell) for any borderline call.

The Status Determination Statement (SDS)

If your end client is medium or large (per the Companies Act 2006 threshold), they must issue a Status Determination Statement before the contract starts. The SDS must:

• State the determination (inside or outside)
• Provide the reasoning
• Be communicated to you (the worker) and to the agency/fee-payer in writing

You have the right to dispute it. The client then has 45 days to respond with either the original determination + reasoning, or a new determination. If they don't respond within 45 days, the original SDS becomes invalid and the responsibility shifts to them as the fee-payer.

The blanket determination problem

In the run-up to April 2021, several banks and large IT clients issued blanket "inside IR35" determinations across all their contractors — regardless of working practices. This is technically a breach of the "reasonable care" requirement that HMRC put into the legislation.

What to do:

1. Request the SDS in writing. If they refuse, that's already a breach.
2. Ask for the reasoning. A reasoned SDS must include specifics about your role.
3. Run CEST yourself and compare to the SDS. If the answers differ, file a dispute.
4. Get an independent review (£100–300 from specialists). A written opinion is hard to ignore.
5. Consider walking. If a client won't engage on reasonable-care grounds, every other contract you take with them carries the same risk.

The financial picture — a £500/day contract

Let's compare. £500/day x 220 billable days = £110,000 gross.

Outside IR35 (Pro plan in AIS)

• Salary £9,100 (no NI, no tax)
• Dividends balance, optimised across higher-rate band
• Corporation tax ~19–25% on profit
• Take-home (after personal tax) ~£75,000–80,000/year
• Effective rate: ~68–73%

Inside IR35

• Fee taxed as deemed employment income at source
• No salary/dividend optimisation, no corporation tax shelter
• Employee NI + Income Tax deducted before you see it
• Take-home: ~£62,000–66,000/year
• Effective rate: ~56–60%

The difference is £13,000–18,000/year on the same £110k gross.

What "outside" actually looks like in practice

If you want to defend an outside determination at a tribunal, here's what your engagement should look like:

• Written contract with substitution clause, no MoO, project-based scope, fixed deliverables
• Working practices match the contract — this is critical, contracts that don't match reality are worth nothing
• Multiple clients in the same tax year, or evidence you're free to pitch for them
• Your own equipment, methods, hours
• Project-based invoicing — milestones, not timesheets, ideally
• Professional indemnity insurance — typically £1–2m cover, ~£200/year
• Some financial risk — fixed-price work, rectification at own cost

How AIS handles it

On /tax/ir35, AIS gives you the 12-question status assessment with weighted scoring across all six categories (Control, Substitution, MoO, Financial Risk, Part & Parcel, Business Entity). Run it before signing every new contract.

The output is a written SDS-style document you can hand to the client as evidence of due diligence — which itself is one of the factors HMRC's "reasonable care" test looks for.

Next steps

• Run the IR35 assessment before your next contract
• Read Optimal Director Salary for 2026/27 — only relevant outside IR35
• Read UK Corporation Tax Rates for 2026/27
• Or start a free trial