How to File Your First Self Assessment Tax Return in the UK (Simple Guide)

If you're self-employed, a sole trader, or earning untaxed income for the first time, Self Assessment is probably something you've been putting off thinking about. It sounds complicated. It sounds official. And if you miss the deadline, HMRC hits you with an automatic penalty even if you don't owe a penny in tax. But the actual process, once you know what you're doing, is straightforward. This guide walks you through everything from registering for the first time to submitting your return online, step by step.

What is Self Assessment?

Self Assessment is HMRC's system for collecting Income Tax on income that hasn't already been taxed at source. If you're employed and paid through PAYE, your employer handles your tax automatically. But if you're self-employed, earning rental income, or have other untaxed income, it's your responsibility to tell HMRC what you earned and pay the tax you owe. You do this through a Self Assessment tax return, a form (filed online or on paper) where you declare all your income, claim any allowable expenses and reliefs, and let HMRC calculate what you owe.

Do you need to file a Self Assessment tax return?

You must file a Self Assessment tax return if any of the following apply in the tax year:

• You were self-employed as a sole trader and earned more than £1,000
• You were a partner in a business partnership
• You earned untaxed income from renting out property
• You earned income from savings, investments, or dividends above certain thresholds
• Your total income was over £100,000
• You received Child Benefit, and you or your partner earned over £60,000 (High Income Child Benefit Charge)
• HMRC has written to you asking you to file a return

If you're not sure whether you need to file, HMRC has a free online checker at gov.uk that takes a few minutes to work through.

Key dates and deadlines for 2025/26

Getting the dates wrong is the most common and most avoidable mistake. Here are the key deadlines for the 2025/26 tax year (income earned between 6 April 2025 and 5 April 2026):

First-time filer tip: If this is your first Self Assessment, you must register with HMRC by 5 October following the end of the tax year. Don't leave this until January HMRC needs time to issue your Unique Taxpayer Reference (UTR) and activation code before you can file.

Almost everyone files online. Over 97% of tax returns are now submitted digitally, and it's faster, easier, and less prone to errors than paper.

What happens if you miss the deadline?

HMRC's penalty system is automatic and unforgiving:

• Up to 3 months late — £100 fixed penalty, even if you owe no tax
• 3 to 6 months late — Additional £10 per day, up to £900
• 6 months late — A further 5% of the tax due, or £300 — whichever is greater
• 12 months late — Another 5% or £300, whichever is greater
• Late payment — 5% of unpaid tax after 30 days, 6 months, and 12 months, plus daily interest from 1 February

If you genuinely cannot pay your full tax bill by 31 January, you may be able to set up a Time to Pay arrangement with HMRC online, available if you owe less than £30,000. This lets you spread payments monthly rather than facing penalties for non-payment.

Step 1 — Register for Self Assessment

If this is your first time filing, you need to register with HMRC before you can submit a return. The process differs slightly depending on your situation.

If you're self-employed or a sole trader: Register online at gov.uk/register-for-self-assessment. You'll need your National Insurance number and personal details.

If you're not self-employed but have other untaxed income: Use HMRC's online registration service and select the appropriate reason for registering.

Once registered, HMRC will send you a Unique Taxpayer Reference (UTR), an 11-digit number that identifies you for tax purposes. This usually arrives within 10 working days (longer if you're abroad). Keep it safe, you'll use it every year for the rest of your tax life.

You'll also need to set up a Government Gateway account if you don't already have one. This is your online login to access HMRC services.

Step 2 — Gather your information

Before you start filling in your return, collect everything you'll need. Having it all to hand before you log in makes the process much smoother.

For everyone:

• Your UTR number
• Your National Insurance number
• Bank account details (for any refund HMRC might owe you)

If you were employed at any point during the tax year:

• P60 (from your employer, showing your income and tax paid)
• P45 (if you left a job during the year)
• P11D (if you received any employee benefits like a company car)

If you're self-employed:

• Total income from self-employment (your invoices and sales records)
• Allowable business expenses (more on this below)
• Records of any assets you bought for the business

Other income sources:

• Bank statements showing savings interest
• Dividend statements if you hold shares
• Rental income records if you're a landlord
• Any other untaxed income received during the year

Good record-keeping throughout the year makes this step quick. Poor record-keeping makes it painful. If you're using BFSB to track your invoices and expenses, your income and expense figures are already in one place, which significantly reduces the time this step takes.

Step 3 — Know what expenses you can claim

If you're self-employed, you can deduct allowable business expenses from your income before calculating the tax you owe. This directly reduces your tax bill, so it's worth knowing what qualifies.

Common allowable expenses for sole traders include:

• Office costs — stationery, software subscriptions, postage
• Travel — business mileage (at HMRC's approved rate), train fares, parking for business purposes (not commuting)
• Stock and materials — goods you buy to sell or use in your work
• Marketing — website costs, advertising, business cards
• Professional fees — accountant fees, legal costs related to your business
• Phone and internet — the business proportion of your bills
• Equipment — computers, tools, and other assets used for work
• Clothing — only specialist clothing like uniforms or protective gear, not everyday clothes
• Training — courses that develop skills relevant to your current work

Important: Expenses must be "wholly and exclusively" for business purposes to be allowable. Personal expenses don't qualify, and mixed-use items (like a phone you use for both work and personal calls) can only be claimed in proportion to business use.

If your income is under £90,000, you can use simplified expenses, flat-rate amounts HMRC provides for things like business use of your home and mileage, which saves you from calculating exact figures.

Step 4 — Complete and submit your return online

Log in to your HMRC online account at gov.uk using your Government Gateway credentials and your UTR.

The online return is split into sections. Work through them in order:

1. Personal details — your name, address, and NI number. Most of this will pre-fill.

2. Tailor your return — HMRC asks what types of income you had during the year. Tick the relevant boxes (self-employment, employment, rental income, etc.). This determines which supplementary pages appear.

3. Self-employment income — enter your total income and total allowable expenses. The system calculates your taxable profit automatically.

4. Other income — enter employment income (from your P60), savings interest, dividends, rental income, and anything else that applies.

5. Reliefs and allowances — pension contributions, Gift Aid donations, and other reliefs that reduce your tax bill.

6. Review and submit — HMRC shows you the calculated tax figure before you submit. Check everything carefully. Once submitted, you can still amend your return within 12 months of the filing deadline, but it's much easier to get it right the first time.

The whole process typically takes one to two hours for a straightforward self-employment return, less if your records are well organised.

Step 5 — Pay your tax bill

Once you've submitted, you'll see exactly how much you owe. The payment deadline is 31 January — the same date as the online filing deadline.

You can pay by:

• Bank transfer (Faster Payments — usually clears same day)
• Direct debit through your HMRC account
• Online banking
• Debit or credit card (note: credit card payments incur a small fee)

Use your UTR number followed by 'K' as the payment reference so HMRC can match your payment to your account.

Important: Filing on time and paying late are treated separately. You can submit your return by 31 January and still be charged interest if the payment doesn't clear on time. Aim to have both done before the deadline.

Payments on account: What they are and why they catch people out

If your Self Assessment tax bill is more than £1,000, HMRC requires you to make payments on account — advance payments towards next year's tax bill, made in two instalments.

• First payment on account: 31 January (same day as your current bill)
• Second payment on account: 31 July

Each payment is half of your previous year's tax bill. This means in your first year with a tax bill over £1,000, you could owe up to 150% of what you expected — your current year's tax plus the first advance payment.

This surprises a lot of first-time filers. The practical fix is to set aside a portion of your income throughout the year — typically 20 to 25% of your self-employed profit — into a separate savings account earmarked for tax. That way the January bill is never a shock.

Making Tax Digital: What's coming

If you're a sole trader or landlord with income over £50,000, you'll be required to use Making Tax Digital (MTD) for Income Tax from 6 April 2026. This means keeping digital records and submitting quarterly summaries of your income and expenses to HMRC throughout the year, rather than a single annual return.

If your income is between £30,000 and £50,000, MTD applies from April 2027.

This is a significant change to how Self Assessment works. If it applies to you, it's worth starting to use MTD-compatible software now so the transition is smooth. Built For Small Business is designed to support this — keeping your income and expense records digitally throughout the year.

Common mistakes to avoid

Registering too late. You must register by 5 October following the end of the tax year. If you register in November or December expecting to file in January, HMRC may not get your UTR to you in time.

Missing the payment deadline. Filing on time doesn't mean you've paid on time. Both need to happen by 31 January.

Forgetting payments on account. If your bill exceeds £1,000, you'll owe more than just the current year's tax on 31 January. Factor this in.

Not claiming all allowable expenses. Under-claiming is as much of a problem as over-claiming — you end up paying more tax than you need to. Keep receipts and records of all business-related costs throughout the year.

Mixing personal and business finances. Using a separate bank account for your business makes your income and expense records much cleaner and reduces the risk of missing something at tax time.

Quick reference checklist

Before you submit your return, make sure you have:

• UTR number and Government Gateway login ready
• National Insurance number
• Total self-employment income for the tax year
• All allowable business expenses totalled
• P60 if you were also employed during the year
• Records of any other untaxed income
• Bank details for any potential refund
• Payment method ready for 31 January

Keep your records organised with Built For Small Business

The biggest headache with Self Assessment isn't the filing, it's finding all your numbers when January comes around. Built For Small Business helps UK sole traders and small businesses track income and expenses throughout the year, so when it's time to file, everything is already in one place.

It's completely free to use, no subscription, no trial period. Start tracking your business finances for free at builtforsmallbusiness.com

This guide is for general information purposes only and does not constitute tax or legal advice. Tax rules can change, and your circumstances may vary. If you're unsure about your obligations, we recommend speaking with a qualified accountant or tax adviser.

Last updated: April 2026