How to Fill Self Assessment Tax Return & Avoid HMRC Mistakes

If you are self-employed, a business owner, or earn income outside of PAYE, you are required to complete a Self Assessment tax return each year. In simple terms, how to fill a Self Assessment tax return means telling HMRC how much you earned, what your allowable expenses were, and how much tax you owe for that tax year. It sounds daunting, but once you break it down into steps, it is far more manageable than it looks. This guide will walk you through the entire process.

What is a Self Assessment Tax Return?

A Self Assessment tax return is a form, officially called an SA100, that you submit to HMRC every year. It tells HMRC about your income and financial situation so they can work out how much Income Tax and National Insurance (Class 4) you owe.

You do not get a bill in the post. Instead, HMRC relies on you to report what you have earned. That is why it is called “Self Assessment.”

The tax year in the UK runs from 6 April to 5 April the following year. For example, the 2025/26 tax year runs from 6 April 2025 to 5 April 2026.

Who Needs to Fill in a Self Assessment Tax Return?

Before you start filling anything in, make sure you actually need to submit a return. You must register for Self Assessment if, in the last tax year, you:

• Were self-employed as a sole trader and earned more than £1,000
• Were a partner in a business partnership
• Had untaxed income (e.g., rental income, freelance work, investments)
• Earned more than £100,000 in employment income
• Received Child Benefit and your (or your partner’s) income was over £60,000
• Had income from abroad
• Were a company director (with untaxed income or dividends)

If you are not sure whether you need to submit, you can use HMRC’s online checker.

Can I Do My Own Self Assessment Tax Return in the UK?

Yes, absolutely. You can file your own Self Assessment tax return directly through HMRC’s online portal. Millions of people in the UK do this every year without the help of an accountant.

That said, there are situations where it is worth getting professional help:

• You have multiple income sources (employment + freelance + rental, for example)
• You are not sure which expenses are allowable
• You made losses that you want to carry forward or offset
• You have foreign income or overseas assets
• Your business accounts are complex

If your situation is straightforward, you are a sole trader with one income stream and basic expenses, and filing it yourself is very achievable. The HMRC portal walks you through each section and only shows you the parts that are relevant to your situation.

Quick Tip: Even if you file your own return, it is a good idea to have an accountant check it the first time. A single mistake can cost you more in penalties than the accountant’s fee.

How to Do Your Tax Return Step by Step

Here is a clear, step-by-step guide on how to fill in a Self Assessment tax return in the UK.

Step 1 – Register for Self Assessment

If you have never filed a Self Assessment tax return before, you need to register with HMRC first.

• Self-employed/sole trader: Register at the HMRC Website
• Not self-employed but have other income: Use the same link and select the relevant reason

Once you register, HMRC will post you a Unique Taxpayer Reference (UTR) number within 10 working days. You cannot file without this.

Registration deadlines:

Step 2 – Set Up Your HMRC Online Account (Government Gateway)

To file online, you need a Government Gateway user ID and password.

1. Go to the HMRC SATR portal.
2. Click “Create sign-in details” if you do not have an account
3. Follow the steps to verify your identity using your National Insurance number, passport, or other documents
4. Link your UTR number to your account

Once set up, you can log in anytime to start, save, and submit your return.

Step 3 – Gather All Your Financial Records

Before you start filling in the form, collect all the documents and figures you will need. This saves a lot of back-and-forth.

If you are self-employed, you will need:

• Total income (sales/turnover) for the tax year
• Records of all allowable business expenses
• Bank statements (business and personal, if mixed)
• Mileage records (if claiming vehicle expenses)
• Receipts for equipment, software, subscriptions, etc.
• Records of any other income (rental, savings interest, dividends)
• Your P60 or P45 if you also had employment income
• Pension contributions (personal, not employer)
• Gift Aid donations

Tip: If your annual turnover is below £85,000, you only need to enter your total income and total expenses, not a full breakdown. This is known as the three-line account.

Step 4 – Log In and Start Your Return

1. Log in to your HMRC Government Gateway account
2. Click “Complete your tax return”
3. Select the correct tax year
4. Work through each section. HMRC’s system only shows you sections that apply to you

Step 5 – Fill in the Main SA100 Form

The main form has several sections. Here is what each one asks for:

Personal Details

• Your name, address, National Insurance number, and UTR

Income Section

You will declare all income here, broken down by type:

• Self-employment income (you will also need to complete the SA103 supplementary pages)
• Employment income (from a P60 or P45)
• UK property income (requires SA105 pages)
• Foreign income (requires SA106 pages)
• Dividends and savings interest
• Pension income
• Capital gains (requires SA108 pages if you sold assets)

Self-Employment Pages (SA103)

This is the section most sole traders spend the most time on.

You will enter:

• Business name and description – for example, “Freelance Graphic Designer” or “Plumbing Services”
• Business income (turnover) – total amount you invoiced or received in the tax year
• Allowable expenses – costs you incurred wholly and exclusively for business

Common allowable expenses for sole traders:

Important: You cannot claim personal expenses even if you mix them with business ones. Only the business proportion is allowable.

If you work from home, you can claim a flat rate through the simplified expenses method:

Tax Reliefs and Allowances

In this section, you will claim any reliefs you are entitled to:

• Personal Allowance: £12,570 for 2026 (HMRC applies this automatically)
• Trading Allowance: £1,000 if your self-employment income is below this
• Marriage Allowance: if your income is below £12,570 and your spouse’s is not
• Pension contributions: higher and additional rate taxpayers can claim further relief here

Step 6 – Check the Tax Calculation

Once you have filled in all your figures, HMRC’s system calculates your tax bill automatically. You will see a summary showing:

• Total income for the tax year
• Total allowable deductions
• Taxable income
• Income Tax owed
• Class 4 National Insurance owed (if self-employed)
• Payments on Account, advance payments toward next year’s tax bill (if your bill exceeds £1,000)

Read through this summary carefully. If something looks wrong, go back and check the figures you entered.

Step 7 – Submit and Pay

Once you are happy with everything:

1. Click “Submit your return”
2. HMRC will confirm receipt and give you a submission reference number, save this
3. Pay your tax bill by 31 January (this is also the filing deadline for online returns)

Payment options:

• Online or telephone banking (use HMRC’s bank account details and your UTR as reference)
• Debit card through your HMRC online account
• Direct Debit (set up in advance)
• CHAPS or BACS transfer

Key Deadlines to Remember:

Is It Easy to Fill in a Self Assessment Tax Return?

For most sole traders and freelancers with a single income stream, yes, it is straightforward once you know what to expect. HMRC’s online system is fairly user-friendly and guides you through each section.

The parts that trip people up most often are:

• Working out allowable expenses — especially the home office proportion or vehicle use
• Payments on Account — many first-time filers are surprised to learn they owe an extra 50% on top of their current year’s bill as a deposit toward next year
• Missing income — forgetting to include bank interest, dividends, or foreign income
• Mixing up cash flow and profit — your turnover is what you invoiced, not what landed in your bank account (unless you use cash basis accounting)

If you find yourself confused at any point, HMRC’s helpline is on 0300 200 3310 (Monday to Friday, 8 am to 6 pm). There is also useful guidance on GOV.UK for each section of the form.

What Are Common Self Assessment Mistakes?

These are the mistakes HMRC sees most often — and they are all avoidable.

Mistake 1 -Missing the Deadline

The most common mistake of all. If you miss the 31 January deadline, you get an automatic £100 fine — even if you owe no tax. After 3 months, additional daily penalties of £10 per day (up to £900) kick in. After 6 months, a further 5% of the tax owed (or £300, whichever is higher) is added.

Fix: Set a reminder in October, gather your records in November, and file in December. Do not wait until January.

Mistake 2 – Claiming Disallowable Expenses

A lot of people claim things they should not, everyday clothing, personal meals, holidays dressed up as business travel, or their home broadband, when only a small part is used for work.

HMRC can and does open enquiries into tax returns where expenses look unusually high compared to turnover.

Fix: Only claim what is “wholly and exclusively” for business. If something is partly personal, only claim the business portion.

Mistake 3 – Forgetting Income Sources

Bank interest, small freelance jobs paid in cash, rental income, stock dividends, all of these count as taxable income and must be declared, even if they seem minor.

Fix: Go through your bank statements for the full tax year before you start filling in your return.

Mistake 4 – Not Keeping Records

HMRC can ask you to prove any figure on your tax return for up to five years after the filing deadline for self-employed people (four years for employed people with non-complex affairs). If you cannot show receipts or invoices, you could face a penalty.

Fix: Keep all business records — digital or paper — for at least five years after 31 January following the end of the tax year.

Mistake 5 – Getting Payments on Account Wrong

If your tax bill is over £1,000 for the first time, HMRC will ask you to make two “Payments on Account” — advance payments for next year’s bill, each equal to 50% of the current year’s bill. Many people are unprepared for this and end up short.

Fix: When you see your tax calculation, factor in the Payments on Account so you are not caught off guard. If your income is expected to drop significantly, you can apply to reduce these payments online.

Mistake 6 – Using the Wrong Accounting Method

There are two ways to account for your income and expenses:

• Traditional accounting (accruals basis): Income is counted when you invoice, expenses when they are incurred
• Cash basis: Income is counted when you receive payment, expenses when you actually pay them

Most small businesses with a turnover below £150,000 benefit from cash basis as it reflects real money in and out. But mixing the two, or switching without adjusting figures, causes errors.

Fix: Decide which basis you use and stick to it consistently.

Mistake 7 – Ignoring the Class 2 and Class 4 NI Contributions

Self-employed people pay Class 4 National Insurance on their profits (6% on profits between £12,570 and £50,270, and 2% above that for 2024/25). This is calculated automatically by HMRC’s system, but you need to make sure your profit figure is correct in the first place.

Note: Class 2 NI was abolished from April 2024, so you no longer need to pay the flat weekly rate.

Do I Need an Accountant for Self Assessment?

Not always. If your affairs are simple, you can file yourself using HMRC’s free online service.

However, hiring an accountant makes sense if:

• You are not confident about what you can and cannot claim
• You want to make sure you are not overpaying or underpaying
• You have multiple income sources or complex business finances
• You are behind on previous years’ returns
• You want someone to deal with HMRC on your behalf if they enquire into your return

A good accountant will often save you more in tax than their fee costs — and give you peace of mind.

What Happens If I Make a Mistake on My Tax Return?

Do not panic. You can amend your return online up to 12 months after the original filing deadline. For the 2024/25 return (filed by 31 January 2026), you can amend it until 31 January 2027.

To amend:

1. Log in to your HMRC account
2. Go to “Self Assessment”
3. Select the tax year
4. Click “Make corrections”

If HMRC believes the error was deliberate, they may investigate further. But genuine mistakes corrected promptly are treated very differently to deliberate tax avoidance.

How to Fill Self Assessment Tax Return – Quick Summary Checklist

Here is a quick checklist you can work through:

Check if you need to file (use HMRC’s checker)

Register for Self Assessment and get your UTR

Set up or log in to your Government Gateway account

Gather all income and expense records for the tax year

Log in to HMRC and start your return

Fill in all income sources (self-employment, employment, rental, etc.)

Enter allowable business expenses

Claim any reliefs and allowances you are entitled to

Review the tax calculation — check for surprises like Payments on Account

Submit by 31 January

Pay your tax bill by 31 January

Save your confirmation reference number

Keep all records for at least five years

Need Help With Your Self Assessment Tax Return?

If filling in your Self Assessment feels overwhelming, or you simply do not have the time, we can take it off your hands. At Cheap Tax Returns, we offer affordable, professional Self Assessment filing for sole traders, freelancers, and business owners across the UK.

We handle everything from start to finish, make sure you claim every allowance you are entitled to, and ensure your return is filed correctly and on time.

Get in touch today and leave the paperwork to us.