Self-Assessment Tax Return Guide UK

What Is Self-Assessment?

Self-Assessment is HMRC's system for collecting Income Tax from people whose tax is not automatically deducted at source. If you are employed and paid through PAYE, your employer handles your tax. But if you have other sources of income — self-employment, rental property, investments, or high earnings — you are responsible for reporting that income to HMRC yourself through a Self-Assessment tax return.

The system works on a tax year basis, running from 6 April to 5 April the following year. After each tax year ends, you must file a return declaring all your income and any tax reliefs or allowable expenses you are claiming. HMRC then calculates how much tax you owe (or whether you are due a refund). You are responsible for paying any tax due by the relevant deadline.

Who Needs to File a Self-Assessment Tax Return?

You must file a Self-Assessment return if any of the following apply to you:

• Self-employed sole trader with income above £1,000 (the trading allowance)
• Rental income above £1,000 (the property allowance) from UK or overseas property
• Total income over £150,000 in the tax year (even if all taxed through PAYE)
• Capital gains above the annual exempt amount that require reporting
• Untaxed income such as tips, commission, or foreign income
• Company directors (unless for a non-profit organisation with no pay or benefits)
• Partners in a business partnership

You may also need to file if you need to claim tax relief on pension contributions, charitable donations (Gift Aid), or if you receive the High Income Child Benefit Charge (income over £60,000). If you are unsure, HMRC has an online tool to check whether you need to file: GOV.UK — Check if you need to send a tax return.

Key Dates and Deadlines

Missing a deadline means automatic penalties, so these dates are critical. The following deadlines apply each year:

What You Can Claim: Allowable Expenses

If you are self-employed, you can deduct allowable business expenses from your income before calculating tax. This reduces your taxable profit and therefore your tax bill. Expenses must be wholly and exclusively for business purposes. Common allowable expenses include:

• Office costs: stationery, printing, postage, computer software, and a proportion of home office costs (heating, lighting, broadband)
• Travel: fuel, parking, train and bus fares, vehicle insurance, and breakdown cover for business journeys (not commuting)
• Clothing: uniforms, protective clothing, and costumes for performers (not everyday clothing)
• Staff costs: salaries, subcontractor costs, employer NI contributions
• Stock and materials: raw materials, goods purchased for resale
• Professional services: accountant fees, solicitor fees, professional subscriptions and memberships
• Phone and internet: business proportion of mobile phone and broadband costs
• Insurance: professional indemnity, public liability, and business contents insurance

If you work from home, you can either claim the actual proportion of household costs used for business, or use HMRC's simplified expenses flat rate (£10/month for 25–50 hours, £18/month for 51–100 hours, £26/month for 101+ hours of business use). For vehicles, you can use simplified mileage rates (45p per mile for the first 10,000 miles, 25p thereafter) instead of claiming actual costs. Source: GOV.UK — Expenses if you are self-employed.

Payments on Account

If your Self-Assessment tax bill is more than £1,000 and less than 80% of your tax is collected at source (e.g. through PAYE), HMRC will require you to make payments on account. These are advance payments towards your next year's tax bill, calculated at 50% of your previous year's tax liability each.

The first payment on account is due on 31 January (at the same time as your balancing payment for the previous year), and the second is due on 31 July. This means that in January, you could be paying both your remaining tax for the previous year and your first advance payment for the current year — which can be a significant cash flow hit if you are not prepared for it.

If you know your income will be lower next year — for example, you are winding down your business or had an unusually profitable year — you can apply to reduce your payments on account through your HMRC online account. However, if you reduce them too much and still owe tax, HMRC will charge interest on the shortfall.

Common Mistakes and Late Filing Penalties

The most common mistakes include missing the filing deadline, failing to declare all income sources, not claiming legitimate expenses, and incorrect figures. HMRC's penalty regime for late filing escalates quickly:

Other common errors include forgetting to include bank interest (now mostly covered by the Personal Savings Allowance, but still relevant for higher earners), not reporting dividends above the £500 dividend allowance, and failing to claim higher-rate pension tax relief. Keeping accurate records throughout the year — rather than scrambling in January — is the single best way to avoid mistakes.

Making Tax Digital for Income Tax

From April 2026, HMRC's Making Tax Digital (MTD) for Income Tax Self-Assessment programme requires self-employed individuals and landlords with annual gross income over £50,000 to keep digital records and submit quarterly updates to HMRC using compatible software. Those with income over £30,000 will follow from April 2027.

MTD represents a significant change to how Self-Assessment works in practice. Instead of filing one annual return, you will need to submit summary updates every quarter and a final declaration at the end of the year. If this applies to you, it is worth getting set up well in advance. Read our Making Tax Digital guide for full details on what you need to do.

How an Accountant Can Help

While you can file your Self-Assessment return yourself using HMRC's online portal, many people find it worthwhile to use an accountant. A good accountant will:

• Ensure you claim all legitimate expenses and reliefs, potentially saving more than their fee
• File your return accurately and on time, avoiding penalties
• Advise on tax-efficient structures, pension contributions, and dividend planning
• Handle HMRC correspondence and enquiries on your behalf
• Help you prepare for MTD and set up compatible bookkeeping software

Accountant fees for a straightforward Self-Assessment return typically range from £150 to £500, depending on complexity. For self-employed people with more complex affairs, the cost may be higher but often pays for itself through tax savings and peace of mind. You can search for an accountant near you on our find an accountant page.