What Is a Self Assessment Tax Return? Plain-English Guide
August 24, 2026
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August 24, 2026
A self-assessment tax return is a form you submit to HMRC each year to report income that hasn’t been taxed through PAYE and to calculate how much tax you owe (or how much you’re owed back). It covers a single tax year, running from 6 April to 5 April, and must be filed by 31 January the following year if you’re filing online.
If you’re employed and your only income comes through a regular salary, your employer handles your tax through Pay As You Earn. You never see it. It’s deducted before your wages hit your bank account. Self assessment exists for everything PAYE doesn’t cover.
Around 12 million people in the UK file one every year. If you think you might be one of them, our guide on who needs to complete a self assessment return covers every scenario.
Think of it as a summary of your financial year. You’re telling HMRC about every pound that came in and every legitimate expense that went out, so they can work out your tax bill.
The return covers:
Income from self-employment: If you’re a sole trader, freelancer, or contractor, you report your total business income (turnover) and your allowable business expenses. HMRC taxes you on the profit.
Rental income: Income from letting out property, minus allowable landlord expenses like mortgage interest (at the restricted rate), repairs, insurance, and letting agent fees.
Dividends: If you’re a company director or hold shares that pay dividends above the £500 allowance.
Savings interest: Bank and building society interest above your Personal Savings Allowance (£1,000 for basic rate taxpayers and £500 for higher rate).
Capital gains: Profits from selling assets like shares, cryptocurrency, or a second property, if the gains exceed the £3,000 annual exempt amount.
Foreign income: Income from overseas employment, investments, pensions, or property.
Employment income: If you’re also employed, your PAYE earnings still appear on the return. HMRC uses them to calculate your total income and make sure you’re in the right tax band.
The return also captures things that reduce your tax bill: pension contributions, Gift Aid donations, the marriage allowance, and various other reliefs. If you don’t declare these, you’re overpaying.
PAYE is automatic. Your employer calculates your tax, deducts it from your salary, and sends it to HMRC. You don’t do anything. The system is designed so that employed people with simple finances never need to interact with HMRC directly.
Self assessment is manual. You work out your income, report it to HMRC, and pay the tax yourself. HMRC’s online system does the actual tax calculation, but you supply the figures.
The two systems aren’t mutually exclusive. Plenty of people are on PAYE for their day job and file a self assessment return for their side income, rental property, or investments. Your employer handles the employed income. You handle the rest.
One important difference: PAYE spreads your tax across the year. Self assessment concentrates it into one or two lump-sum payments (31 January and potentially 31 July). That’s why budgeting matters so much for self-employed people.
The short list: sole traders and freelancers earning over £1,000, landlords with rental income above £1,000, company directors receiving dividends, anyone earning over £100,000 from any source, people with capital gains above £3,000, partners in business partnerships, people with significant foreign income and anyone claiming child benefit with a household income above £60,000.
The full breakdown, with every threshold and edge case, is in our dedicated guide: who needs to complete a self assessment tax return.
If HMRC has sent you a notice to file (a letter or notification in your online account), you must file regardless of whether you think you need to. The notice creates a legal obligation.
Self assessment comes with its own vocabulary. Here’s a plain-English glossary of the terms that confuse people most:
UTR (Unique Taxpayer Reference): A 10-digit number that identifies you in HMRC’s self assessment system. You get it when you register. It stays with you permanently. It’s not the same as your National Insurance number.
Government Gateway: HMRC’s online login system. You need a 12-digit user ID and a password. This is how you access the self assessment form online. This is not the same as your UTR.
Tax year: 6 April to 5 April. The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. Not the calendar year. Not your company’s financial year.
Allowable expenses: Costs you incurred wholly and exclusively for business purposes. These reduce your taxable profit. These include office supplies, travel, professional subscriptions, software, and a proportion of your home bills if you work from home.
Trading allowance: You can earn up to £1,000 of tax-free trading income each year. If your total self-employment income is under £1,000, you don’t need to declare it. If it’s over £1,000, you can choose to deduct the £1,000 allowance instead of claiming actual expenses (useful if your expenses are low).
Cash basis: An accounting method where you record income when you receive it and expenses when you pay them. Simpler than the accruals basis. Available to sole traders with turnover under £150,000.
Payments on account: Advance payments are made towards your next year’s tax bill. HMRC requires these if your bill is over £1,000 and less than 80% was deducted at source. Each payment is half of your current year’s liability.
SA100: The official name for the main self assessment form. You probably won’t see this term if you file online (HMRC just calls it “your tax return”), but it appears on paper returns and in official correspondence.
Here’s the entire cycle in 6 steps:
Not for most people. The HMRC online system is a guided form. It asks you questions about your income, shows you only the sections that apply, and does the tax calculation automatically. You don’t need to know which box is which or how the tax bands work. The system handles all of that.
A straightforward return (one self-employment income, a few expenses) takes 30 to 60 minutes if your records are ready. The complexity comes from having multiple income types, not from the form itself.
The things that trip people up aren’t usually the HMRC form. It’s the preparation: gathering records, working out which expenses are claimable, and understanding the deadlines. That’s where an accountant adds the most value.
If your finances are simple (one income source, a handful of expenses, no property or investments), you can absolutely do it yourself. HMRC built the system for exactly this situation.
An accountant becomes worthwhile when:
You have multiple income sources that interact in complex ways (self-employment plus rental plus dividends). You’re not sure what you can and can’t claim. You want someone to catch the reliefs and deductions you might miss. You’d rather spend the time on your business. Or maybe you’ve had a major life event this year, like selling a property, moving abroad, or starting or closing a business—that changes your tax position.
Tax Return Assist handles self assessment returns for self-employed individuals, landlords and small businesses across Essex and London. Their accountants review your records, file the return, and make sure you’re not paying a penny more than you need to.
Self assessment is just a way of telling HMRC what you earned and paying the right amount of tax. The form isn’t complicated. The preparation is what takes the most time.
If you’d rather hand it to a professional, call Tax Return Assist on 02039377911, email contact@taxreturnassist.co.uk, or get an instant quote. They’ll take it from records to submission, on time, every time.
Disclaimer: This article is for general information purposes only and does not constitute professional financial, tax, or legal advice. Tax rules change frequently, and individual circumstances vary. Always consult a qualified accountant or tax adviser before making decisions based on this content. Tax Return Assist accepts no liability for actions taken based on the information provided here.
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