Understanding inside vs outside IR35 is one of the most important aspects of working as a contractor in the UK. IR35 legislation determines whether HMRC views you as a genuine independent business or as someone effectively working as an employee for tax purposes.
For contractors, the outcome of IR35 status can significantly impact how much tax you pay, how you receive your income, and what financial responsibilities you have.
In this guide, we’ll explain what IR35 means, the difference between inside vs outside IR35, how status is determined, and what it means for self-employed contractors.
What is IR35 and Why Does It Affect Contractors?
IR35 regulations were introduced by HMRC to prevent ‘disguised employment’. This occurs when a contractor provides a service through a limited company but effectively works in the same way as a permanent employee.
The rules were designed to ensure that individuals who operate like employees pay similar levels of tax and National Insurance contributions.
For many IR35 contractors, the legislation changed significantly in recent years. In 2017, HMRC stated that responsibility for determining IR35 status lies with the client, not the contractor. This policy was rolled out across the public sector in 2017 and extended into the private sector in 2021.
However, in some cases, if you work with a small business or an overseas client, you may still be responsible for determining your own IR35 status. As the financial implications can be significant, understanding the difference between inside and outside IR35 is essential before starting any new contract.
Learn more about IR35 with our complete guide for contractors
Inside vs Outside IR35: What’s the Difference?
The difference ultimately comes down to how HMRC classifies your working relationship with the client.
If your engagement is inside IR35, HMRC treats you as an employee for tax purposes. This means the income from your contract is usually taxed through the PAYE system, just like a permanent employee’s salary. Income Tax and National Insurance contributions are deducted before you receive payment, which typically reduces your overall take-home pay.
Although you are treated as an employee for tax purposes, you do not automatically receive employment benefits such as holiday pay, sick pay or pension contributions from the client.
On the other hand, if a contract is outside IR35, you are considered to be operating as a genuine independent business. This allows you to continue working through your limited company, invoice clients for services, and manage your income using a combination of salary and dividends. Many contractors prefer working outside IR35 because it offers greater flexibility and can be more tax efficient.
However, each contract must be addressed individually. A contractor may work outside IR35 for one client and inside IR35 for another, depending on the work required for that specific client.
Three Key Factors for Working Out IR35 Status
Working out IR35 status involves considering both the contractual terms and the actual working relationship between the contractor and the client. HMRC typically focuses on three key factors when assessing whether a contractor is genuinely self-employed and whether they fall into inside or outside IR35 status.
Control
Control refers to the extent to which the client has authority over the contractor’s work. If the client controls how, when and where the work is carried out, this may indicate employment. Contractors working outside IR35 typically have greater freedom in how they deliver their services.
Right of Substitution
This factor determines whether the contractor must personally perform the work or can send a qualified substitute to complete the task. A genuine business relationship usually allows for substitution, which supports an outside IR35 determination.
Mutuality of Obligation
Mutuality of obligation refers to whether the client is required to provide ongoing work and whether the contractor must accept it. A continuous expectation of work can suggest an employment relationship.
These factors can be complex and open to interpretation; many contractors use an IR35 accountant or specialist financial advisor to review their contract and working arrangements.
Allowable Expenses Inside IR35
Even when working inside IR35, some expenses may still be claimable depending on your setup. Limited company contractors may be able to use the 5% flat-rate allowance, which allows you to deduct 5% of your contract income to cover general company running costs such as accountancy fees, insurance, training, admin, software and banking charges.
This simplifies expense claims by allowing contractors to deduct a percentage rather than calculating individual costs. For example, you can’t claim both the 5% allowance and the individual operating costs it covers.
Understanding how the flat-rate allowance works is important for contractors navigating the IR35 rules and expense claims.
Working Through an Inside IR35 Umbrella Company
Some contractors who accept inside IR35 contracts choose to work through an umbrella company rather than their own limited company.
An inside IR35 umbrella company setup can simplify the process because the umbrella company effectively becomes your employer. The company receives payment from the client, deducts tax and National Insurance through PAYE, and then pays you a salary.
This arrangement can be particularly useful for contractors who want to avoid the administrative responsibilities of running a limited company while still ensuring compliance with IR35 legislation.
However, umbrella companies are generally used for inside IR35 contracts, as outside IR35 engagements usually allow contractors to operate through their own business structure.
Risks of Incorrect IR35 Determinations
Getting an IR35 wrong can have serious financial consequences.
If HMRC determines that a contract was incorrectly classified as outside IR35, the contractor or the responsible party may be required to pay unpaid income tax, National Insurance contributions, interest and in some cases, significant penalties.
The liabilities can quickly escalate, particularly if several contracts are involved over multiple years. In some cases, the financial impact can run into tens of thousands of pounds.
Due to these risks, many IR35 contractors choose to have each contract professionally reviewed to ensure that both the contract working and working practices support the correct IR35 status.
Why Working With an IR35 Accountant Can Help
IR35 rules are complex and constantly changing, which is why many contractors choose to work with a specialist IR35 accountant.
An experienced accountant can help contractors review contracts, assess working practices, and ensure their business structure aligns with HMRC regulations. They can also provide guidance on tax planning, compliance, and record-keeping to reduce the risk of future disputes with HMRC.
For contractors managing multiple clients or contracts, professional advice can provide valuable reassurance that their business arrangements remain compliant.
Working with a specialist accountant also helps contractors stay informed about legislation changes that could affect tax obligations in the future.
Get Support With IR35 From SwiftBooks
Understanding the difference between inside vs outside IR35 can be complicated, particularly when contract terms, working practices and tax rules all need to be considered.
At SwiftBooks Accounting, we work closely with contractors to review contracts, explain IR35 obligations and ensure their accounts remain fully compliant with HMRC requirements.
Our team provides practical advice tailored to contractors, helping you to understand your tax position, manage your finances efficiently and avoid costly compliance mistakes.
If you need guidance on IR35 status or want expert support managing your contractor finances, speak to our team today to find out how we can help.