What question is IR35 actually asking?
Strip away the jargon and the rules ask something surprisingly human. Imagine your limited company vanished overnight and you carried on doing exactly the same work, at the same desk, for the same client. Would you look like an employee?
Picture two contractors, both on £400 a day. One works where the client tells her, on the hours the client sets, on whatever task the client hands her next, just like the permanent staff beside her. The other agreed a fixed piece of work, does it where and when she likes, and could send a qualified substitute if she broke her leg. The rules exist for people who look like the first contractor: working through their own company, but in a way that would be employment if the company were not in between. HMRC's aim is that people in that position pay broadly the same Income Tax and National Insurance as an employee would.
The two-word labels you keep hearing just describe the answer. "Inside IR35" means the work counts as employment for tax. "Outside IR35" means it does not, and your company is taxed as a normal business. Whether either example above matches your contract is exactly the judgement this page does not make.
Who decides? It depends on your client
This is the practical bit, because the person responsible for the decision changes with the client.
Public sector clients, and medium or large private sector clients, decide for you. They must work out whether your contract sits inside or outside the rules and tell you the outcome.
Small private sector clients are different: the responsibility stays with your company. If your client is a small business outside the public sector, it is your company, not the client, that must decide whether the rules apply to the contract.
So what counts as small? HMRC's guidance for clients says a company client is medium or large if it meets two or more of these three conditions: annual turnover of more than £10.2 million, a balance sheet total of more than £5.1 million, or more than 50 employees. Below that, it is small. You do not have to guess: if you ask, a small client has to confirm its size.
What actually happens to the money when a contract is inside?
When a contract is inside the rules, the money changes shape before it ever reaches your company. Whoever pays your company, called the deemed employer or fee-payer, must take Income Tax and employee National Insurance off the fee first, and pay employer National Insurance to HMRC on top. Your company then receives what is left.
In other words, on an inside contract your £400 day rate does not land in the company bank account whole. It arrives with tax already taken off, much like a salary does.
What does that mean for your company's tax return?
Here is the honest hand-off, and it is the one paragraph on this page to read slowly. Money that arrives with Income Tax and National Insurance already taken off is not ordinary trading income any more, and it is not simply taxed again as company profit. Showing it correctly in the company's accounts and on the tax return is genuinely accountant territory, and we are not going to pretend otherwise. If your company had inside-the-rules income during the year, have an accountant prepare that year's return.
If all of your company's work sat outside the rules, none of this applies to your books: your invoices are normal business income and your return is a normal company tax return.
What is CEST?
HMRC has a free online tool called Check Employment Status for Tax, or CEST. You answer questions about how the work is really done and it gives you HMRC's view of the employment status for that engagement. Anyone involved can use it: you, your client, or an agency. The part worth knowing is HMRC's own commitment: it will stand by all results given by the tool, as long as the information you give remains accurate and is in accordance with its guidance. Answer honestly, keep a copy of the result, and it is HMRC's own view you are holding.