Self Assessment Tax Return Explained (UK Guide for the Self-Employed & Landlords)

A Plain-English Guide for the Self-Employed, Landlords and Anyone with Untaxed Income

If you’re self-employed, a landlord, or earn income that isn’t taxed at source, you’ll almost certainly need to complete a Self Assessment tax return each year. The idea can feel daunting, especially the first time, but once you understand what’s involved, it’s a lot more straightforward than it seems. This guide walks you through everything you need to know in plain English, with practical tips to help you get it right and avoid costly penalties.

Self Assessment Tax Return — At a Glance

What is Self Assessment?

Self Assessment is HMRC’s system for collecting tax from people whose income isn’t automatically taxed through PAYE. When you’re employed, your employer deducts income tax and National Insurance from your wages before you receive them — you don’t need to do anything. But when you’re self-employed, a landlord, or have other untaxed income, HMRC has no way of knowing how much you’ve earned or what expenses you’ve had. Self-assessment is how you report that information and pay the right amount of tax.

Self Assessment exists because HMRC cannot automatically calculate tax where income is not reported by an employer or pension provider. The system places responsibility on the taxpayer to declare income accurately and pay the correct amount of tax each year.

You fill in a tax return each year covering the previous tax year (which runs from 6 April to 5 April), declare all your income and allowable expenses, and HMRC works out what you owe. You then pay it all by 31 January.

Who Needs to Complete a Self-Assessment Tax Return?

You need to file a Self Assessment tax return if any of the following apply to you during the tax year:

If you’re not sure whether you need to file, HMRC has an online tool at GOV.UK – Check if you need to send a Self Assessment tax return; it asks a few simple questions and gives you a definitive answer.

Who Does NOT Need to Complete Self Assessment?

You normally do not need to file a Self Assessment return if:

• All your income is taxed through PAYE employment
• Your savings and dividend income fall within HMRC allowances
• You are self-employed but earn £1,000 or less (Trading Allowance)
• HMRC has not issued you with a notice to file

If HMRC previously required a return but your circumstances have changed, you must ask HMRC to withdraw the requirement — simply ignoring it can lead to penalties.

How to Register for Self Assessment

Before you can file your first tax return, you need to register with HMRC. You must register by 5 October in the second year of trading. So if you started self-employment or received rental income for the first time during the 2025/26 tax year (which runs to 5 April 2026), you need to register by 5 October 2026.

If You’re Self-Employed

Register online through the GOV.UK using form CWF1. You’ll need your National Insurance number and the date you started your business. HMRC will then send you a Unique Taxpayer Reference (UTR) — a 10-digit number you’ll need to file your return. Allow up to 10 working days to receive it, and longer if it arrives by post.

If You’re Not Self-Employed (Landlord, Company Director, etc.)

Register using form SA1, also available on the GOV.UK. The process is the same — you’ll receive a UTR once registered.

⚠️ Don’t leave registration to the last minute. If you register close to the 5 October deadline and your UTR hasn’t arrived by 31 January, you’ll still be expected to file on time. HMRC won’t waive penalties because your UTR was late — so register as early as possible, ideally as soon as you become self-employed.

What Income Do You Need to Declare?

Your Self Assessment return asks you to declare all your income for the tax year, not just your self-employed earnings. This includes:

• Self-employment income — your total turnover, minus allowable expenses
• Rental income from property — gross rent received, minus allowable landlord expenses
• Employment income — wages from a PAYE job (HMRC uses this to check your tax code is correct)
• Savings interest — if it exceeds your Personal Savings Allowance (£500 for higher rate taxpayers, £1,000 for basic rate)
• Dividend income — if it exceeds the Dividend Allowance (£500 from April 2024 onwards)
• Capital gains — if you’ve sold property, shares, or other assets and made a profit above the Annual Exempt Amount
• Foreign income — any income earned outside the UK
• Pension contributions — recorded so HMRC can give you tax relief if applicable

What Expenses Can You Claim?

As a self-employed person or landlord, you can deduct allowable expenses from your income before tax is calculated. This reduces your taxable profit — and therefore your tax bill. HMRC’s rule is that expenses must be ‘wholly and exclusively’ for business purposes. If an expense has both business and personal use, only the business proportion can be claimed.

Here are the most common allowable expenses for sole traders:

Simplified Expenses vs Actual Costs

For some costs — such as use of home, vehicle mileage, and working from home — HMRC lets you use flat-rate ‘simplified expenses’ instead of calculating the exact business proportion. For example, you can claim 45p per mile for business driving in a car (first 10,000 miles) rather than working out the actual running costs. Simplified expenses are easier to calculate and are often generous enough to be worth using for most sole traders. You can find HMRC’s current simplified expense rates on GOV.UK.

See our guide on allowable expenses for further details.

What You Cannot Claim

It’s worth being clear on what HMRC will not accept as an allowable business expense:

• Your own wages or drawings, as a sole trader, what you take out of the business, is not a deductible expense
• Personal clothing — even if you wear it for work (unless it’s a uniform or protective clothing)
• Entertainment costs — client lunches and entertainment are generally not allowable
• Parking fines — HMRC does not allow penalties to be claimed as expenses
• Your own income tax or National Insurance — these are not business expenses

Payments on Account — The Part Everyone Forgets

Once your Self Assessment tax bill for a year exceeds £1,000, HMRC doesn’t just want you to pay that bill in January — it also wants advance payments towards the following year. These are called ‘payments on account’, and they can take first-time filers completely by surprise.

Here’s how it works: HMRC assumes your income next year will be roughly the same as this year. So it asks for two payments, each equal to half your current year’s tax bill, to be paid in advance:

• First payment on account: 31 January (the same day as your tax return and balancing payment)
• Second payment on account: 31 July

Can You Reduce Your Payments on Account?

Yes — if you expect to earn significantly less next year, you can apply to reduce your payments on account. You do this through your online HMRC account or by completing form SA303. However, if you reduce them too much and your actual bill is higher than expected, HMRC will charge interest on the shortfall, so only reduce them if you’re confident your income will be lower.

Start saving for tax from day one

The best habit you can build as a self-employed person is to set aside a percentage of every payment you receive into a separate savings pot — as soon as it arrives in your account. A rough guide is to save around 25–30% of your profit if you’re a basic-rate taxpayer, or 40–45% if you’re in the higher-rate band. This covers both your income tax and your National Insurance contributions. That way, January’s bill never comes as a shock.

How to Complete and Submit Your Tax Return

Step 1: Log In to HMRC Online Services

Go to GOV.UK and sign in to your Government Gateway account. You’ll need your UTR, your National Insurance number, and your Government Gateway user ID and password. If you’ve never set up online access, you’ll need to create a Government Gateway account first — this takes a few minutes, and you’ll need your UTR number.

Step 2: Check Which Supplementary Pages You Need

The Self Assessment return has a main form (SA100) plus additional supplementary pages depending on your income sources. The most common ones are:

• SA103 — for self-employment income (short version for simple businesses, full version if your turnover exceeds £85,000 or your accounts are more complex)
• SA105 — for UK property income (rental income)
• SA102 — for employment income from a PAYE job
• SA106 — for foreign income
• SA108 — for capital gains

HMRC’s online system guides you through which pages you need based on your answers, so you don’t need to figure this out in advance — it asks the right questions as you go.

Step 3: Gather Your Records

Before you start, have the following to hand:

• Your UTR and National Insurance number
• All income figures — invoices, bank statements, rental income records, P60 from any PAYE employment
• All business expenses — receipts, mileage logs, bank statements
• Any tax already paid — payments on account made during the year, CIS deductions if you work in construction
• P45 or P60 from any employers
• Pension contribution statements
• Bank interest statements (from your bank or building society)

Step 4: Fill In Your Return

Work through the return section by section. HMRC’s online system calculates your tax automatically as you enter figures — so you can see your estimated bill building up in real time. If anything is unclear, HMRC has help text next to each question — click the question mark icons.

Step 5: Review Before You Submit

Before you hit submit, review the figures carefully. Check that your income total looks right, your expenses are sensible, and the final tax figure matches what you were expecting. Once submitted, you can amend your return online within 12 months of the filing deadline if you spot a mistake.

Step 6: Pay Your Tax Bill

Once submitted, your bill will appear in your HMRC online account. You can pay by bank transfer, debit card, or through your bank’s BACS facility. Make sure your payment arrives by midnight on 31 January. Bank transfers can take a day or two to clear, so don’t leave paying until the last possible moment.

💡 Tip: File early — even if you can’t pay yet. If you can’t afford to pay your full tax bill right now, you can still file your return on time to avoid the late filing penalty, and then contact HMRC to set up a payment plan (called a ‘Time to Pay’ arrangement). HMRC is generally willing to help if you contact them proactively before the deadline.

What a Self Assessment Year Looks Like

Self Assessment Penalties — What Happens If You Miss the Deadline?

HMRC is strict about Self Assessment deadlines. Late filing penalties and late payment penalties are separate; you can be charged both if you file late and pay late.

Penalties apply automatically, even if you owe no tax at all, and even if the reason for missing the deadline seems reasonable. Here’s what the fines look like:

• 1 Day late – Automatic £100 fixed penalty
• 3 Months late – £10 per day for up to 90 days (maximum £900 on top of the £100)
• 6 Months late – A further 5% of the tax owed, or £300 — whichever is higher
• 12 Months late – Another 5% of the tax owed, or £300 — whichever is higher
• Tax paid late – 5% surcharge on unpaid tax at 30 days, 6 months, and 12 months — plus daily interest

Interest and penalties are also charged on any unpaid tax from the day after the payment deadline until the date it’s paid. The current HMRC late-payment penalty is 5% plus interest, and you have 30 days to pay the penalty.

Can You Appeal a Penalty?

Yes, you can appeal to HMRC if you have a ‘reasonable excuse’ for missing the deadline. HMRC accepts a limited range of reasons, including serious illness, a bereavement close to the deadline, or a system failure on HMRC’s own website. ‘I forgot’, ‘my accountant was late’, and ‘I didn’t know’ are generally not accepted. You have 30 days from the penalty notice to appeal.

I didn’t receive a reminder’ is not a reasonable excuse. HMRC does not have to remind you of the filing deadline — it is your responsibility to know when your return is due. Don’t rely on HMRC to chase you. Once you’re registered for Self Assessment, the annual deadline applies every year, whether or not you get a letter.

Common Self-Assessment Mistakes to Avoid

1. Forgetting payments on account is the most common shock for first-time filers. Remember that your January bill may be 150% of what you expected — your current year’s tax plus the first payment on account for next year.
2. Not keeping records throughout the year, trying to reconstruct a year’s worth of income and expenses in January from memory and a pile of receipts, is stressful and leads to errors. Keep records as you go — even a simple spreadsheet updated monthly saves hours of pain come January.
3. Claiming expenses that aren’t allowable. Claiming personal costs as business expenses — such as your regular clothing, personal phone bills, or home food shopping — is a common mistake that can trigger an HMRC enquiry. When in doubt, leave it out or ask an accountant.
4. Not declaring all income, HMRC has access to data from banks, employers, and third-party platforms (including eBay, Etsy, Airbnb, and Uber). If you have undeclared income, there’s a good chance HMRC will find it. It’s always better to declare everything and pay the right tax than to face a compliance investigation later.
5. Missing the registration deadline. If you become self-employed and forget to register by 5 October after your first year of trading, you can still register late — but you may face a penalty. Register as soon as you start earning untaxed income, not just before the filing deadline.

Top Tips for a Stress-Free Self Assessment

1. File early — don’t wait until January. You can file your tax return from 6 April, the moment the new tax year starts. Filing early means you know your tax bill months in advance, giving you time to budget and save up. You don’t pay any earlier — the payment date is still 31 January — but you avoid the January rush and any last-minute panics.
2. Keep a separate business bank account. Mixing personal and business money makes record-keeping a nightmare. A dedicated business account — even a free one — means your income and expenses are in one place, easy to reconcile at year’s end. Many sole traders use accounts from providers like Starling, Monzo Business, or Tide for this.
3. Update your records monthly, not once a year. Set aside an hour each month to update your income and expenses. Categorise your transactions, photograph receipts, and reconcile your bank account. This turns year-end into a straightforward review rather than a major project.
4. Use accounting software. Software like Sage UK, Xero or QuickBooks automates much of the bookkeeping work — importing bank transactions, categorising expenses, and producing summaries that make filling in your tax return much easier. Many also let you or your accountant file directly to HMRC. If you’re planning to join Making Tax Digital for Income Tax (mandatory from April 2026 for those earning over £50,000), you’ll need MTD-compatible software anyway — so it’s worth getting started now.
5. Don’t ignore HMRC letters. If HMRC writes to you — whether about a penalty, a query, or a notice to file — act on it promptly. Ignoring letters doesn’t make them go away and often makes the situation worse. If you’re unsure what a letter means, seek advice from an accountant or tax adviser.
6. Consider hiring an accountant. For many self-employed people, the cost of an accountant more than pays for itself. A good accountant will ensure you’re claiming all allowable expenses, avoid mistakes that trigger enquiries, and save you hours of stress. Many charge between £150 and £500 for a basic sole trader tax return, which is often deductible as a business expense itself.

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Common Questions About Self Assessment

Summary

Self-assessment doesn’t have to be stressful. The key is to stay on top of your records throughout the year, understand your deadlines, and not leave everything to January. Here’s a quick recap of the most important points:

• Register by 5 October if you’re newly self-employed or have untaxed income for the first time
• File your return online by 31 January — don’t wait for a reminder from HMRC
• Pay any tax owed by 31 January — the same deadline as the return
• Budget for payments on account if your bill exceeds £1,000 — January’s bill can be much higher than you expect
• Keep good records throughout the year — income, expenses, and receipts
• Claim all allowable expenses to reduce your tax bill legally
• File early if you can — you’ll know your bill months in advance and avoid the January rush
• Consider accounting software to make record-keeping and filing easier

If you’re already registered for Self Assessment and earning over £50,000 from self-employment or property, make sure you also read our guide to Making Tax Digital for Income Tax — it will affect how you report your income from April 2026 onwards.

Related Pages on Business Accounting Basics

• Making Tax Digital for Income Tax (MTD ITSA) — A Complete Guide
• Sole Trader Bookkeeping — Keeping Records the Right Way
• Allowable Business Expenses for the Self-Employed
• Payments on Account — What They Are and How to Budget for Them
• Best Accounting Software for Sole Traders UK

Last updated: February 2026 (reflecting latest HMRC guidance)