If you contract in the UK, your tax does not sort itself out the way it did when you were employed. This guide covers exactly what self assessment means for contractors, what you can claim, and what happens if you get it wrong.

Why self assessment for contractors is not the same as for employees

When you were employed, your employer handled PAYE and you rarely had to think about tax. As a contractor, you are responsible for calculating and reporting your own income, expenses, and tax liability to HMRC every year. That responsibility sits with you, regardless of how you work or how you get paid.

Contractors operating as sole traders or through a limited company face different rules, different allowable expenses, and different obligations under legislation such as IR35. Getting the wrong answer on any of these can mean an unexpected tax bill, penalties, or both. Understanding the basics before you file is not optional, it is how you avoid problems.

Where most contractors go wrong with self assessment

The most common errors in contractor self assessment are not dramatic. They are quiet mistakes that accumulate over time: missed deadlines, misclaimed expenses, and choosing the wrong structure for the income level you are actually earning.

Claiming expenses that do not qualify

HMRC allows sole traders and contractors to claim allowable business expenses including staff costs, subcontractor fees, employer National Insurance contributions, and relevant training. What it does not allow is personal expenditure dressed up as a business cost. Cases exist where contractors have faced bills of over £29,000 because an adviser incorrectly categorised expenses on their return. If you are unsure whether a cost qualifies, check it before you claim it. A full list of allowable staff and business expenses for the self-employed is available on GOV.UK.

Assuming a limited company always saves tax

Many contractors incorporate believing a limited company will automatically reduce their tax bill. At income levels between £40,000 and £50,000, research suggests there is generally no tax advantage to running a limited company over filing as a sole trader. The decision involves accountancy costs, administrative obligations, and IR35 exposure, not just the headline rate. Getting this wrong from the start means filing the right numbers in the wrong structure for years.

What to do: self assessment for contractors step by step

The self assessment process follows the same broad shape for most contractors, whether you operate as a sole trader or through a limited company. The difference is in the detail: what you report, what you offset, and what additional filings sit alongside your personal return.

1. Register for self assessment with HMRC if you have not already done so. Sole traders must register by 5 October following the end of the tax year in which they first became self-employed. If you missed this and have not yet registered, do it immediately. Late registration can lead to penalty proceedings even before you have filed a single return.
2. Gather your income records for the tax year (6 April to 5 April). This means invoices raised, payments received, bank statements, and any income from employment or other sources in the same year. Contractors often have multiple income streams in a transition year, and every one of them needs to appear on your return.
3. Identify and record your allowable expenses before you file. This includes costs that are wholly and exclusively for business purposes: professional indemnity insurance, relevant software subscriptions, home office costs on an apportioned basis, mileage at HMRC approved rates, and legitimate staff or subcontractor costs. Do not include personal costs, even partially, unless they meet the HMRC test for mixed-use apportionment.

Once your income and expenses are reconciled, you can calculate your taxable profit and file your return via HMRC’s online portal. The deadline for online filing is 31 January following the end of the tax year, and payment of any tax owed is due on the same date. If your bill is above £1,000, HMRC will also require payments on account for the following year, due in January and July.

Costs and what to expect when filing as a contractor

The cost of getting self assessment wrong almost always exceeds the cost of getting it right. Late filing incurs an automatic £100 penalty, with further daily and percentage-based penalties after three and six months. An incorrectly submitted return can trigger an HMRC enquiry, which takes time and can result in additional tax, interest, and penalties. At STZ Accounting, self assessment is handled as part of a fixed-price package so you know exactly what you are paying before any work starts.

How to get your contractor self assessment sorted today

You do not need to resolve everything in one sitting. The most important thing is to take one concrete step today rather than leaving this open. If your return is already overdue, the priority is to stop the penalty clock, not to achieve a perfect record from day one.

• Check whether you are registered for self assessment on the HMRC portal. If you are not registered and you have been contracting for more than one tax year, register today and note the date. This is the starting point for everything else.
• Open a folder, physical or digital, and start collecting every invoice, bank statement, and expense receipt from the current tax year. Categorise them as income, business expense, or personal. This single action will save hours when it comes to filing.