- Inside IR35 means you’re taxed more like an employee, while outside IR35 means you operate as a genuine business, which usually gives you more flexibility and higher take-home pay.
- Your IR35 status depends on how the contract works in practice, not just what the paperwork says. HMRC looks closely at control, substitution, and mutuality of obligation.
- Each contract must be assessed separately, and if a client decides you’re inside IR35, that decision can directly affect your tax, your rate, and whether the role is still worth taking.
Inside vs outside IR35 decides whether you are taxed like an employee or as a genuine business, and that can make a major difference to your take-home pay, tax planning, and contract options. If you are unsure where you stand, getting support from a contractor accountant can help you review contracts properly and avoid costly mistakes.
In simple terms, an inside IR35 contract means PAYE-style tax rules apply, while an outside IR35 contract usually lets you work through your limited company with more flexibility. The challenge is that status is not based on labels alone. It depends on who controls the work, whether you can send a substitute, and how the engagement works day to day.
This guide explains the difference between inside and outside IR35, who decides your status, and what each outcome means for your money. It will also help you spot the warning signs of a risky contract before you accept it.
Inside vs outside IR35: A quick definition
A quick look at what each status means and why it matters for contractors.
Inside IR35
You’re treated as an employee for tax purposes. That means income tax and National Insurance are deducted through PAYE, usually by the fee-payer or an umbrella company. Your take-home pay is lower, and you have less flexibility in how income is structured.
Outside IR35
You’re operating as a genuine business. You can pay yourself a mix of salary and dividends, claim allowable expenses, and manage your own tax through your limited company. You carry more financial risk, but also benefit from greater independence and higher potential take-home pay.
Inside vs outside IR35 side-by-side comparison
The key differences between inside and outside IR35 come down to how you’re taxed, who controls your work, and whether you’re seen as a genuine business.
| Factor | Inside IR35 | Outside IR35 |
| Tax treatment | Income Tax and employee NICs deducted via PAYE | Corporation Tax on profits, salary and dividends for flexibility |
| Who decides | Client for medium/large companies and public sector | Contractor decides if the client is a small business |
| Status Determination Statement (SDS) | Client must issue with reasons for decision | Should confirm “outside” with clear reasoning |
| Control | Client decides how, when, and where you work | You choose method, timing, and often location |
| Personal service | You must do the work yourself | Genuine right to substitution allowed |
| Mutuality of obligations | Expectation of ongoing work and acceptance | No obligation for either side to continue beyond contract |
| Financial risk | Low, client usually bears risk | Higher, you fix defects and bear business risks |
| Integration | Embedded into client’s organisation and processes | Independent business, separate systems and branding |
The strongest defence in an IR35 enquiry is simple: make sure your contract and your day-to-day working practices tell the same story.
Who decides your IR35 status?
Who decides your IR35 status depends on the type of client you are working with. This matters because the party responsible for the decision is also responsible for applying the rules correctly.
If you work with a public sector client or a medium or large private sector client
The client will usually decide your IR35 status. They should assess the engagement and issue a Status Determination Statement (SDS) explaining whether the contract is inside or outside IR35 and why.
That means you should check:
- whether the client has actually issued an SDS
- whether the reasoning reflects the real working arrangement
- whether the contract and day-to-day working practices match the decision
If you work with a small private sector client
The responsibility will usually sit with you as the contractor. In that case, you need to assess whether the engagement falls inside or outside IR35 and make sure your tax treatment reflects the reality of the contract.
That means you should review:
- the written contract
- how much control the client has over the work
- whether substitution is genuinely possible
- whether the engagement looks like a project or an employment-style role
Why this matters in practice
You should never assume the status written into a contract is automatically correct. A role may be labelled outside IR35, or the client may decide it is inside IR35, but the real question is whether the facts support that position.
If the client gives you an SDS, read it carefully rather than treating it as a formality. Warning signs include:
- vague reasoning
- blanket assessments across multiple contractors
- a decision that does not match how the work is actually carried out
Where the reasoning looks weak, it is worth raising the issue before the contract starts rather than accepting the label at face value.
How HMRC tests IR35 status
HMRC does not decide IR35 status based on one clause in the contract. It looks at the full working relationship and asks whether, in practice, the engagement looks more like employment or a genuine business-to-business arrangement.
The three main tests are:
- control
- substitution
- mutuality of obligation
HMRC may also look at wider signs that you are in business on your own account.
Control
Control looks at how much say the client has over the work.
HMRC will consider things like:
- who decides how the work is done
- who decides when the work is done
- who decides where the work is done
- how closely you are supervised or managed
A contract is more likely to fall inside IR35 if the client treats you like an employee, sets fixed working patterns, and closely directs how the work should be done.
A contract is more likely to fall outside IR35 if you are responsible for delivering an agreed outcome and have more freedom over how you complete the work.
Substitution
Substitution looks at whether you can send someone else to carry out the work.
A genuine right of substitution can support an outside IR35 position because it suggests the client is hiring a business to deliver a service, rather than hiring you personally.
For substitution to carry weight, it should be:
- written clearly into the contract
- realistic in practice
- acceptable to the client in genuine circumstances
A substitution clause will not help much if it exists on paper but would never be accepted in real life.
Mutuality of obligation
Mutuality of obligation looks at whether:
- the client is expected to keep offering work
- you are expected to keep accepting that work
An arrangement is more likely to point towards inside IR35 if there is an ongoing expectation that the client will keep providing work and you will keep doing whatever is assigned.
An arrangement is more likely to support outside IR35 if the contract is for a defined project, specific deliverables, or a fixed piece of work with a clear end point.
Other factors HMRC may consider
HMRC can also look at whether you appear to be running a real business.
That can include:
- using your own equipment
- invoicing for work completed
- taking on financial risk
- correcting defective work in your own time
- working for more than one client
- operating with a clear project scope or statement of work
What inside vs outside IR35 means for your money
Knowing whether you are inside or outside IR35 is not just about labels. It directly affects your take-home pay, how you handle tax, and the level of risk you carry. Here is how each position plays out financially.
Inside IR35: the financial impact
- Taxed as an employee: Income Tax and employee NICs deducted at source.
- Lower take-home pay: Less flexibility than outside IR35, meaning reduced net income.
- Umbrella companies: Often used when clients or agencies won’t payroll contractors directly. The umbrella acts as your employer, charging a fee.
- Risk of backdated bills: If HMRC rules a contract should have been inside IR35, you could face unpaid tax, NICs, interest, and penalties.
Outside IR35: the financial impact
- Tax efficiency: Company pays Corporation Tax, and you draw a mix of salary and dividends for lower personal tax.
- Higher net income: Dividends aren’t subject to employee NICs, so more of your earnings stay with you.
- Business risks: You absorb costs for errors, delays, or rework, something employees do not face.
- Potential HMRC enquiry: Even if you’re outside IR35, investigations happen. Strong records help you defend your status.
Making contracts and working practices match
When HMRC looks at IR35, they focus on how you actually work, not just what the contract says. If your day-to-day role looks like employment, no amount of clever wording will protect you.
The safest approach is to make sure contracts and working practices line up to avoid penalties for IR35 non-compliance.
Key clauses to get right
- Substitution: Your right to provide a qualified substitute should be genuine and practical.
- Control: Contracts should avoid language that gives the client power over how, when, or where the work is done.
- Mutuality of obligations: Make it clear that the client is not obliged to provide continuous work, and you are not obliged to accept it.
- Liability and risk: State who is responsible for fixing defective work and at whose cost.
Confirmation of Arrangements
It helps to sign a short document with the client confirming how the contract works in practice. This can cover substitution, control, and project scope. Having the client’s agreement in writing strengthens your case if HMRC challenges your status.
Staying compliant with IR35
Compliance is not just about getting your IR35 status right at the start of a contract. You need to maintain evidence and review your position regularly to stay protected if HMRC investigates.
Keeping accurate and organised records is easier if you follow in the steps of our guide to bookkeeping for contractors.
Keep strong records
- Copies of all contracts and any updates.
- Signed Confirmation of Arrangements from the client.
- Evidence of substitution rights or actual substitutions.
- Invoices, project sign-offs, and correspondence showing you work as a business.
Use IR35 assessment tools carefully
HMRC provides the Check Employment Status for Tax (CEST) tool, but it has limits. Many experts say it oversimplifies certain tests, especially mutuality of obligations. If you use CEST, keep the reasoning behind the answers as evidence. Independent contract reviews are often a safer bet.
Reassess when things change
Your IR35 position can shift if your working practices or contract terms change. Always reassess when you start a new engagement or if your role with an existing client evolves.
Insurance and protection against IR35 investigations
Even with careful planning, HMRC can still open an enquiry into your status. IR35 insurance can give contractors peace of mind by covering professional defence costs and, in some cases, tax liabilities.
What IR35 insurance usually covers
- Professional fees for legal and tax experts during an enquiry.
- Backdated tax, interest, and penalties if HMRC rules against you.
- Contract review services that flag risky clauses before you sign.
When it is worth considering
If most of your work is for medium or large clients, you face greater exposure because they make the determination. Insurance may also be useful if you have long-term contracts, or if your working practices could be open to interpretation.
IR35: Developments to Watch
While the IR35 rules have been stable since the 2021 reforms, contractors still need to keep a close watch on several key areas to maintain compliance.
Key Areas to Monitor
- Small Business Exemption: This exemption remains in place. If your client is a small private sector business, you are still responsible for assessing and deciding your own IR35 status.
- Set-off Mechanism: HMRC previously consulted on a mechanism to allow tax already paid by a worker to be offset if off-payroll rules were applied incorrectly. This is a significant development designed to reduce the risk of double taxation. This set-off mechanism has since been finalised and introduced, applying to liabilities assessed from April 2024. You can find more detail about the Off-payroll working rules (IR35) on GOV.UK.
Small business exemption threshold (future)
While the small business exemption is currently stable, important changes to the financial thresholds used to define a ‘small’ company are coming, which will impact IR35 responsibility for many businesses.
- The financial limits used in the Companies Act 2006 to define a small company are being increased for accounting periods beginning on or after 1 April 2025.
- The turnover limit will increase from £10.2m to £15m and the balance sheet total will increase from £5.1m to £7.5m. The employee limit of 50 remains unchanged.
- The earliest these changes will have a practical effect on IR35 off-payroll working responsibilities—by shifting liability back to the contractor—is the 6 April 2027 tax year, as the new thresholds must be met for two consecutive financial years.
- This change is expected to re-classify thousands of currently medium-sized companies as small, thereby increasing the number of contracts where the contractor is responsible for the IR35 determination.
Diligence is essential, as HMRC enquiries can take years and often hinge on subtle details of working practices. Even contractors with clear evidence of being outside IR35 have faced challenges.
Staying up to date with rule changes and case law, and keeping your finger on the pulse of wider changes such as Making Tax Digital, is the best way to avoid unexpected issues and keep your business compliant and running smoothly.
How Sleek helps you navigate IR35
Getting IR35 wrong can cost contractors thousands in back taxes, interest, and penalties. The safest approach is to understand your status clearly and keep your records straight. That is where Sleek comes in.
We review contracts to flag risks early, set up your limited company to operate cleanly outside IR35 where appropriate, and manage your bookkeeping and tax filings so you remain compliant year-round. If HMRC ever opens an enquiry, our experts are on hand to guide you through it.
With Sleek, you can focus on your contracts and clients while we handle the complexity of IR35 and tax.
Disclaimer: The preceding information is not legal advice. This content is aimed to provide general guidance. For more formal or legal advice, contact Sleek directly.
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FAQs on inside vs outside IR35
Is it better to be inside or outside IR35?
Neither is automatically better. Outside IR35 can mean more independence and higher take-home pay, but only if you genuinely operate as a business. Inside IR35 is correct if the role is effectively employment, and getting it wrong can lead to penalties.
Who decides my IR35 status?
For public sector and medium or large private sector clients, the client decides and must issue a Status Determination Statement (SDS). For small private sector clients, the contractor decides.
What is a Status Determination Statement (SDS)?
An SDS is a written statement from the client confirming whether a role is inside or outside IR35, along with the reasons for the decision. It must be passed to both the contractor and the party paying them.
Can HMRC challenge my IR35 status?
Yes. Even if a client has deemed you outside IR35, HMRC can still investigate. That is why contracts and working practices need to line up, with evidence kept to support your position.
What happens if I am wrongly classed as outside IR35?
If HMRC rules you should have been inside IR35, you may face backdated tax, NICs, interest, and penalties. In most cases, the fee-payer is liable, but disputes can arise over responsibility.
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Does the right of substitution always mean outside IR35?
Not always. A substitution clause must be genuine and workable in practice. If the client can easily block substitutions, it is unlikely to help your case.
Do I need IR35 insurance?
It is not compulsory, but IR35 insurance can cover professional fees and liabilities if HMRC challenges your status. It is worth considering if most of your income comes from contracts that could be interpreted as employment.
